A B C D E F G H I J K L M N O P Q R S T U V W X Y Z 

Active Setpoint

The term Active Setpoint is used to describe the currently selected setpoint of a controller. Some controllers can use the main Local Setpoint or an Alternative Setpoint. The alternative setpoint can be another local setpoint or a remote setpoint sent from an external device. Only one setpoint can be active at a time. On controllers that incorporate a Profiler / Programmer feature, the active setpoint value is controlled by the profiler function when a profile program is active.

Also refer to: Alternative Setpoint; Controller Mode; Effective Setpoint; Local Setpoints; Profiler Mode; Remote Setpoint; Setpoint; and Setpoint Selection.

Actual Setpoint

- Refer to Effective Setpoint.

Alarm Inhibit

Alarm Inhibit prevents unwanted process or deviation alarm activation at power-up or when the controller setpoint is changed. The alarm activation is inhibited until a ‘Safe’ condition is present. The alarm operates normally from that point onwards. E.g. if inhibited, a low alarm will not activate at power-up, until the process has first risen above the alarm point and then fallen back below. The inhibit function is sometimes called output Hold on CAL controllers.

Also refer to: Alarm/Limit Types and Alarm Operation.

Alarm/Limit Hysteresis

An adjustable band through which the process variable must pass before an alarm/limit will change state. This Hysteresis is only applicable to alarm/limits based on the Process Value or Control Deviation, as illustrated below. The band is always on the “safe” side of an alarm/limit point, e.g. a high alarms hysteresis band is below the high alarm value, and a low alarms hysteresis is above the low alarm value.

Also refer to: Alarm/Limit Types; Alarm Operation; Control Deviation and Process Variable.

 Alarm Hysteresis Image


Alarm/Limit Operation

Process and Control Deviation Alarm/Limit types are illustrated, together with the action of any associated outputs. Direct is on when the alarm is active, reverse is on when the alarm is inactive.

Also refer to: Alarm/Limit Hysteresis; Alarm Inhibit; Alarm/Limit Types; Latching Output; Logically Combined Outputs and Loop Control Alarm.

  Alarm Hysteresis Image


Alarm/Limit Types

There are four basic alarm/limit types, Process*, Control Deviation, Rate of Signal Change and Event Based alarm/Limits.

*Process alarms called Full Scale Alarms on some CAL brand controllers.

Process Alarm/Limits are based on the absolute value of the Process Variable. If the PV rises above a Process High limit value, or falls below a Process Low limit value, the alarm/limit will become active.

Deviation Alarm/Limits are based on the value of the Control Deviation error. If the PV is more than the High Deviation limit value above setpoint, or more than the Low Deviation limit value below setpoint, the alarm/limit will become active.

Rate Of Signal Change Alarms are based on the rate of change of the PV. If the rate of change is greater than the alarm value for longer that the Minimum Duration time, the alarm will activate.

Event based alarms/limits activate when the condition for that limit type is true. These can be Signal Break, Low Memory or Loop Control Alarms.

Also refer to: Alarm Operation; Control Deviation; Loop Control Alarm; Process Variable; Rate Of Change Alarm; and Setpoint.

Alternative Setpoint

Some controllers can have several setpoint sources. Usually there is a local setpoint and one or more alternative setpoints. The alternative setpoints can be additional local setpoints or a remote setpoint input from auxiliary analogue input. Selection of the setpoint source can be via the instrument menu or a digital input. Only one setpoint can be chosen as the active at a time.

Also refer to: Active Setpoint; Auxiliary Input; and Remote Setpoints.

Auto Pre-Tune

When the auto pre-tune is enabled, a pre-tune activation is attempted at every power-up (standard Pre-Tune activation rules apply). Auto pre-tune is useful when the controlled process varies significantly each time it is run. Auto pre-tune ensures that it is tuned correctly each time the process is started. If available, self-tune may also be engaged to fine-tune the controller.

Also refer to: Pre-Tune; Self-Tune; PID and Tuning.

Automatic Reset

- Refer to Integral Action

Automatic Tuning

The automatic adjustment of a PID controllers tuning terms (proportional bands, integral time and derivative time). Several automatic tuning methods are possible depending on the controller model. These methods involve the instrument analysing either naturally occurring (i.e. Self-tune), or artificially induced disturbances (pre-tune/start-up tune or tune at setpoint) in the process, to establish its thermal characteristics and time constants. These are used to calculate the appropriate tuning terms.

Also refer to: Controller; Derivative Action; Integral Action; PID; Pre-Tune; Primary Proportional Band; Self-Tune; Secondary Proportional Band; Start-up Tune and Tune At Setpoint.

Auxiliary Input

An extra analogue input that can be used in addition to the main process input. This can provide functions such as remote setpoints, heater current measurement or valve position indication. Typical signal types are mA, mV, VDC or Potentiometer. These signals are scaled to represent the desired input in the appropriate engineering units. For example, a 4 to 20mA signal might be scaled so that 4mA equals 0.0%RH, and 20mA equals 100.0%RH.

Also refer to: Heater Current Input; Input Type; Linear Input; mADC; mVDC; Remote Setpoint and VDC.

Band Alarm

- Refer to Alarm/Limit Types

Bar Graph

An instrument display type that can show parameters such as PID Power Output, Control Deviation, %Memory Used in a graphical form. Bar-graphs are uni-directional or bi-directional depending on the information to be displayed.

Also refer to: Control Deviation; Data Recorder; Operation Mode; Main Menu and PID


Sometimes referred to as “Offset”, “Manual Reset” or “Working Point. This is used to manually bias proportional output(s) to compensate for control deviation errors due to process load variations. If the process variable settles below setpoint use a higher Bias value to remove the error, if the process variable settles above the setpoint use a lower Bias value. This problem is most common with proportional only or PD control because integral action automatically performs this function when using PI or PID control.

Lower Bias values will also help to reduce overshoot at process start up.

Bias is expressed as a percentage of output power. It is not applicable if the primary output is set to on-off control.

Also refer to: Control Deviation; Integral Action; Reset; Offset; ON/OFF Control; PI Control; PID; Proportional Control; Primary Proportional Band; Process Variable; and Setpoint.

Bumpless Transfer

A method used to prevent sudden changes to the correcting variable, when switching between automatic PI or PID control and Manual control modes. During a transition from PI or PID to Manual control, the initial Manual Power value is set to the previous automatic mode value. The operator then adjusts the power output value as required.

During a transition from Manual control to PI or PID, the initial automatic value is set to the previous Manual mode value. The correcting variable level will gradually adjusted by the control algorithm at a rate dependant on the integral action resulting from the Integral Time Constant value.

Since integral action is essential to Bumpless Transfer, this feature is not available if Integral is turned off.

Also refer to: Correcting Variable; Integral Action; Manual Mode; PI and PID.


Adjustment of an input or output circuit, in order to achieve the best possible accuracy.

Caution: Use of accurate reference signals is vital for correct calibration, and great care must be taken to calibrate correctly.

Also refer to: Full Scale Calibration, Single Point Calibration, Two Point Calibration and Zero Point Calibration.


Cascade Control

Applications with two or more capacities (such as heated jackets) are inherently difficult for a single instrument to control, due to large overshoots and unacceptable lags. The solution is to cascade two or more controllers (each with its own input) in series to form a single regulating device. The product setpoint temperature is set on the master controller. This is compared to the product temperature, and the master loops PID power level is fed to the slave controller as its setpoint. Cascade control can be implemented in a single multi-loop controller where master PID power given to the slave directly, or two discrete instruments can be wired together, in which case the master power (a mA or VDC signal) is wired to the auxiliary input of the slave controller as a remote setpoint input This scaled to suit any expected temperature. The slave loops natural response time should ideally be at least 5 times faster than the master. 

In the example, the maximum input represents 400ºC, thus restricting the jacket temperature. At start-up the master compares the product temperature (ambient) to its setpoint (300ºC) and gives maximum output. This sets the maximum (400ºC) setpoint on the slave, which is compared to the jacket temperature (ambient) giving maximum heater output.

As the jacket temperature rises, the slave’s heater output falls. The product temperature also rises at a rate dependant on the transfer lag between the jacket and product. This causes the master’s PID output to decrease, reducing the ‘jacket’ setpoint on the slave, effectively reducing the output to the heater. This continues until the system becomes balanced.

When tuning a cascade system, first set the master to manual mode. Tune the slave controller using proportional control only (Integral & Derivative are not normally required); then return the master to automatic PID mode before tuning the master. The result is quicker, smoother control with minimum overshoot and the ability to cope with load changes, whilst keeping the jacket temperature within acceptable tolerances.

Also refer to: Auxiliary Input; Control Loop/Zone; Derivative Action; Integral Action; mADC; Manual Mode; Master & Slave; Proportional Control; PID; Remote Setpoint; Setpoint; Tuning and VDC.

Chart Recorder

Chart recorders monitor and log information about a process. Typically this will be temperature, pressure or RH etc; but any parameter that can be converted to a suitable electrical signal can be recorded. The data is recorded on a moving strip-chart or circular chart with coloured pens to give a graphical view of the data over time. Limited annotation of the data may also be possible, such a adding time/date or scaling information. Chart recorders do not always have a real time clock.

Also refer to: Circular Recorder; Data Recorder; Paperless Recorder; Real Time Clock and Strip Chart Recorder

Circular Recorder

A type of chart recorder where the pens write on to a circular chart that revolves once over a fixed time period (e.g. 1 day, 1 week etc). The pen sweeps across the chart in an arc to give a graphical view of the data over time. The data compressed towards the centre of the chart due to the relative slower linear movement of the paper in the centre compared to the outer edge. The 10 or 12 inch diameter circular charts are more easily filed or attached reports when compared to torn off pieces of strip charts.

Also refer to: Chart Recorder; Data Recorder; Paperless Recorder; and Strip Chart Recorder

Communications Write Enable

A security setting that enables/disables changing parameter values via the serial communications option on an instrument. When disabled, all communications with the instrument are “read-only”.

Also refer to: Serial Communications.

Configuration Menu

A selection of menus or sub-menus, where the user can adjust major instrument settings. Typical menus include settings for Inputs; Control; Outputs; Alarms; Communications; Recorder; Clock; Display and Lock Codes. Configuration is usually protected by a password/unlock code, a physical switch/link-jumper or special key-press sequence.

Also refer to: Lock Codes.


- Refer to Relay

Continuous Control

Continuous control is used to produce the correcting variable on controllers using linear output(s). It typically has 4 to 20mA, 0 to 20mA, 0 to 5V, 0 to 10V or 2 to 10V DC outputs for proportional control in PI, PD or PID modes. On-off control cannot be used with linear outputs.

Continuous control is sometime called Current Proportioning.

Also refer to: Correcting Variable; Linear Output; On-Off Control; PD; PI; PID; Proportional Control; and Time Proportional Control.

Control Deviation

Control Deviation is the difference or “error” between the process variable value and the effective setpoint. The deviation error is equal to PV – SP. An excessive deviation warning can be given by using a deviation alarm. A PI or PID controller with correctly tuned integral action will adjust its output until the deviation error is eliminated. Without integral action, manual adjustment of the biasing may be required.

Also refer to: Alarm/Limit Types; Bias; Effective Setpoint; Integral Action; PI Control; PID Control; Process Variable; Setpoint and Tuning.

Control Action

Control Action refers to the primary power output direction. Reverse action is typically used with heating applications as it increases the correcting variable as the process variable falls within the proportional band. Direct action increases the correcting variable as the process variable rises within the proportional band. This would be suitable for cooling in a chiller, or perhaps a dehumidifying application.

The action of a secondary output is usually the opposite of the primary output. This would be seen in a heat & cool or similar application.

Also refer to: Control Type; and Correcting Variable.

Control Enable/Disable

The Primary & Secondary control outputs can be temporarily turned off on some instruments by disabling control. All other functions continue as normal. Depending on the instrument type, the control enable/disable function may be accessed from the configuration menu, the normal operation mode or via a digital input.

Also refer to: Configuration Menu; Digital Input; Operation Mode and PID.

Control Loop/Zone

A control loop is part of a process whose value must be maintained at a specific level. Most often this will be controlling of a temperature, but can be virtually any process that can be measured and adjusted. The key components of a control loop are: 1) the process or product itself; 2) a measuring device so the controller knows the current value; 3) a controller; and 4) a control actuator that the controller can adjust to increase or decrease the process value. Each component connects to the next in a “loop”, hence the name.

In a simple heating application, these might be: 1) an oven; 2) a thermocouple; 3) a PID controller; and 4) electric heater elements.

Multi-loop/multi-zone control refers to two or more control loops in a single process or application. Each zone has control loop components and may operate completely independently from the other zones, although if the zones are connected in the application, they may influence each other (e.g. the heating zones in a plastics extrusion machine where heat from each zone passes into its neighbours).

Also refer to: Controller; PID; Process Variable; and Thermocouple.

Control Type

This defines if a controller has Single (unidirectional) or Dual (bidirectional) control outputs. Single control uses a Primary output only. This can drive the process variable in one direction (e.g. heat only, cool only, increase humidity etc). Dual control uses both Primary and Secondary outputs which can force the process variable to increase or decrease (e.g. heat & cool, humidify and dehumidify etc).

Also refer to: Control Action; PID; Primary Proportional Band; Process Variable; and Secondary Proportional Band.


An instrument that controls a process at a given target setpoint value. A correcting variable is applied to move the process up/down until the control deviation error is eliminated (process variable = Setpoint). The controller uses proportional (P, PI, PD o PID) or on-off control methods.

Also refer to: Correcting Variable; Limit Controller; On-Off Control; PD Control; PI Control; PID; Process Variable; Proportional Control; and Setpoint.

Controller Mode

The normal operating mode when profiling is not fitted or it is not being used.

Also refer to: Controller.

Correcting Variable

The output from a controller that is used to adjust the process variable value up or down, as it attempts to remove any control deviation error. The correcting variable can be proportional to the error, or in on-off mode.

The correcting variable is commonly referred to as the controller output power.

Also refer to: Control Deviation; On-Off Control; PID; Process Variable and Proportional Control.


Central Processing Unit. In a controller, this refers to the onboard microprocessor that controls the functions of the instrument, such as input measurement, the control algorithm, alarm operation and the display.

Also refer to: Controller.

Current Proportioning Control

- Refer to Continuous Control

Cycle Time

For time proportioning outputs, the cycle time is used to define the time over which the controller averages the on vs. off time, in order to provide the required correcting variable. Ideally the cycle time should at least 1/20th of the integral time, but longer times may have to be used with electromechanical control devices (e.g. relays/contactors, solenoid valves etc). Shorter cycle times give better control, but these type of devices can suffer excess were and reduced life when used with short cycle times.

Cycle times for dual control type, the primary and secondary control outputs can be independently adjusted.

Also refer to: Correcting Variable; Control Type; PID; Proportional Control; Relay; Solenoid Valve and Time Proportioning.

Data Logger

- Refer to Data Recorder.

Data Recorder

A Data Recorder/logger option is available on some controllers. This can record process values, setpoints, alarms and profile events etc over time, eliminating the need for separate loggers, paperless or chart recorders. Recordings can be transferred to a PC for analysis via to a memory stick or serial communications. Controllers with a recording option include a real time clock with battery back-up to maintain the correct time when the power is turned off.

Also refer to: Chart Recorder; Controller; Paperless Recorder and Real Time Clock


- Refer to On-Off Differential and Overlap/Deadband.

Derivative Time

- Refer to Derivative Action

Derivative Action

Derivative action defines how a controller responds to the rate of change in the process variable. The correcting variable is decreased if the PV is rising, or increased if the PV is falling. A longer derivative time constant will cause the controller to react more to a given movement in the process. It is normally set to off when controlling a modulating value to premature wear due to the constant small adjustments it can make to the valve position. To achieve optimal control, the derivative action must be tuned to match the conditions in the application. This parameter is not available if primary control output is set to on-off.

Also refer to: Correcting Variable; Modulating Valve; On-Off Control; PD Control; PID and Process Variable.

Deviation Alarm

- Refer to Alarm/Limit Types



DeviceNet is an industrial automation communications field bus, based on CAN technology. It is managed by the Open DeviceNet Vendors Association (ODVA) in North America.

Also refer to: Serial Communications.

Digital Input

A digital input that can be driven to one of two states (active or inactive) via a logic output/voltage or a by opening/closing a contact. They are often used to set a device into different states, possibly in a logical combination with other inputs or instrument events. Typical uses are auto/manual control or setpoint selection, control enable/disable, profile select/control, recorder start/stop, engaging tuning, limit reset, profile selection etc.

Also refer to: Control Enable; Data Recorder; Logic Output; Manual Mode; Profiling and Setpoint Selection.

Direct Acting Alarm

- Refer to Alarm/Limit Operation.

Direct Acting Control

- Refer to Control Action.

Display Resolution

For digital displays, this is the maximum number of digits that can be displayed and/or the maximum number of decimal places.

For graphical displays this is expressed as the number of dots wide x the number of dots wide, or sometimes in dots per inch (dpi).

Also refer to: LSD


A dwell (sometimes called a “soak”) maintains the value of the previous segment for a defined time. The profiler programs usually contain one or more dwells, and controllers with ramp & soak or ramping setpoint features will dwell at the end of the ramp phase.

Also refer to: Controller; Profiler; Profile Segments; Setpoint Ramping and Soak

Effective Setpoint

The Effective setpoint is the current instantaneous value of the setpoint being used for control, after taking into account factors such as profiles, ramps, offsets or any mathematical operations that may be applied. For example, when a profile is running, the setpoint is controlled by the profiler function, and the effective setpoint will be defined by the progress of the profile. Similarly when a setpoint ramp is used, the effective setpoint will move towards the target setpoint as it rises or falls at the defined ramp-rate.

Effective setpoint should not be confused with “active setpoint” or “selected setpoint” although these parameters may be used to alter a controller’s effective setpoint.

Also refer to: Active Setpoint; Profiler; Setpoint; Setpoint Ramping and Setpoint Selection.

Engineering Units

Process variable values and setpoints are usually scaled and displayed in engineering units so that the value shown directly relates to the process. For example a 4-20mA signal might be scaled to represent 0.0 to 14.0pH from a pH sensor or transducer. The displayed value would be 7.0 when the signal was at its mid point (12mA). Some products can display the name of the engineering unit beside the value (for example 0.7ph). Typical engineering units are °C; °F; K; bar; %; %RH; pH or psi.

Also refer to: Input Span; mADC; Process Input; Transducer and Process Variable.


A networking technology for local area networks (LANs). Used to link computers and other equipment in order to control or share data and control such devices. If available, Ethernet communications allows controllers, recorders etc to connect to a master device over a wired Ethernet LAN. Depending on the model, the supported protocols over Ethernet are Modbus TCP or EtherNet IP.

Also refer to: EtherNet IP; Modbus TCP and Serial Communications

EtherNet IP

EtherNet/IP is an industrial Ethernet field bus solution use in manufacturing automation. It is managed by the ODVA in North America.

Also refer to: Ethernet and Serial Communications.

Full Scale Alarm

- Refer to Alarm/Limit Types

Full Scale Calibration Point

The input “gain” adjustment. This should be made at a point towards the top of input range. A know signal is applied to the input, and the displayed reading adjusted to match. If the full range of the input isn’t needed for the application, calibration can be carried out at the highest required value. This will improve the accuracy over the span used, but above this level, the accuracy will be impaired. It may even be outside of the instrument specification.

Caution: Use of accurate reference signals is vital for correct calibration of the input.

Also refer to: Calibration; Two Point Calibration and Zero Calibration Point

Heater Current Input

The current required by electrical heaters is usually too large to measure directly. Instead; the industry standard method is to use a current transformer (CT). The heater power conductor is passed through the centre of the CT once, or sometimes multiple times, to form the primary “winding”. The CT secondary winding supplies a proportional stepped-down current for the controller to read.

Also refer to: Auxiliary Input.


An instrument that measures and displays process values. Often, alarm outputs are available that will activate at preset process values, as are features such as serial communication or analogue retransmission, but control features are not provided.

Also refer to: Alarms; Controller; Process Variable; retransmission and Serial Communications

Input Filtering

An input filter is used to remove extraneous impulses affecting the process value, such as electrical noise or random eddy currents in liquid flow. The filtered value is used for all PV dependent functions (display, control, alarm etc). Use input filtering with care as it will also slow the response to genuine process changes if the filter time constant is too long.

Also refer to: Alarms; Controller and Process Variable.

Input Range

The overall process input range for the input type selected. This range can be scaled to reduce the usable input span of the instrument to match the application.

Also refer to: Input Span and Process Input.

Input Scaling

- Refer to Input Span

Input Span

An instruments measuring and display limits. This may be the full range for the input type selected, but input scaling may reduce this to match the needs of the application.

The scaled span value is used on some instruments for calculations relating to the span of the instrument. For example some controllers express their proportional bands as a percentage of span. For a span of -10.0°C to +40.0°C, a 10% proportional band would be 5°C wide.

For linear input signals, scaling defines the displayed values in engineering units for when the input is at minimum and maximum. If 4 to 20mA represents 0 to 14pH, the lower scaling point relates to 4mA so the associated scaling value would be 0, and the upper scaling point relates to 20mA so its scaling value would be 14.

Also refer to: Engineering Units; Input Range; mADC; Primary Proportional Band; and Secondary Proportional Band.

Input Type

The type of signal to be applied to a process or auxiliary input. Typical input types are thermocouples (J, K, C, R, S, T, B, L & N), RTDs (PT100 or thermistor) or linear (mV, mA or VDC), but these will vary with the model type.

It is important that an input is correctly setup to match the signal applied, otherwise inaccurate values or error messages will be displayed.

Also refer to: Auxiliary Input; Input Range; Linear Inputs; Process Input; Process Variable; RTD; Thermistor and Thermocouple

Integral Action

Integral action is sometimes known as “Reset” or “Automatic Reset” because it automatically compensates for process load variations by biasing the control output when a deviation error occurs. Proportional outputs are biased over time until the error is reduced to zero. To achieve optimal control, the integral action must be tuned to match the conditions in the application. When tuning a controller, decreasing the integral time constant increases the integral action to remove the error more quickly but if set too short, instability will be introduced into the process.

This parameter is not available if primary control output is set to on-off.

Also refer to: Control Deviation; On-Off Control; PI Control; PID; Primary Proportional Band; Secondary Proportional Band; and Tuning.

Integral Time

- Refer to Integral Action

Latching Alarm/Limit Output

A type of output that once activated, has to be reset before it will deactivate. Depending on the instrument in use, the reset signal can be given via the keypad/menu, via a digital input or over a communications link. If the instrument does not have the option for latching outputs, external latching relays can be fitted as slaves to internal (non-latching) relays.

Also refer to: Digital Input; Relay and Serial Communications


Light Emitting Diode. Red or Green 7-segment LEDs are used to numeric values and simple parameter legends on most controllers and indicators. Single LEDs are used as indicator lights alarm, automatic tuning or manual mode status.

Also refer to: Alarms; Automatic Tuning and Manual Mode.

Linear Input

A mVDC, mADC or voltage signal used to represent the value of the process variable. This can be any variable that can be converted into a suitable electrical signal with a transducer. Common examples are Humidity, pressure, pH or temperature.

Where available, auxiliary inputs may also use linear input signals.

Also refer to: Auxiliary Input; mVDC; mADC; Process Variable; Transducer and VDC.

Linear Output

A mVDC, mADC or voltage signal used to provide a continuous proportional control or retransmit output signal.

Also refer to: mVDC; mADC; Proportional Control; Retransmit Output and VDC


- Refer to Alarm Operation or Limits Controllers.

Limit Controller

A device that can protect a process by shutting it down at a preset “exceed condition”. Limit controllers work independently of the normal process controller, in order to prevent damage to equipment and products, or in some cases to prevent a hazardous condition. A fail-safe latching relay is fitted, which is reset by the operator once the process is returned to an acceptable condition.

Limit controllers are highly recommended for any process that could be damaged or become hazardous under fault conditions. Depending on the process and local regulations, they may need approval by a standards authority such as Factory Mutual in the USA, or comply with a safety directive such as EN14597 in Europe. Ensure that you select a product that has the required standards/approvals for you application.

Also refer to: Controller and Latching Relay.

Local Setpoints

Local setpoints are target setpoint values stored within a controller. All controllers have at least one setpoint, but some can have alternative and/or remote setpoints available. Local setpoints are normally entered by from the front keypad, but can also be set via a serial communications link. The setpoint value can be adjusted within the instruments input span, but may be further limited by setpoint limits.

Also refer to: Alternative Setpoint; Auxiliary Input; Controller; Remote Setpoint; Serial Communications; Setpoint; Setpoint Limits; and Setpoint Select.

Lock Codes

- Refer to Password

Logic Output

A logic output typically provides a 5, 10, 12 or 24VDC pulse either as a logic or digital input to another instrument, or to the signal input terminals of a solid state relay. When used as a control output, the pulses are time-proportioned to achieve proportional control.

Also refer to: Controller; Digital Input; Logic Output; PLC; Proportional Control; Time Proportioning Control; Relay; Solid State Relay; Triac and VDC.

Logically Combined Outputs

Some instruments allow a logical (OR / AND) combination of factors (alarms/limits, events, digital inputs etc) to be used to turn outputs on or off. The following table explains the concept of logical OR & AND outputs with direct and reverse action.

Examples Of Logical Alarm Outputs

 Logical OR: Alarm 1 OR Alarm 2

 Direct Acting

































 Logical AND: Alarm 1 AND Alarm 2

 Direct Acting
































Also refer to: Alarm/Limit Operation; Digital Inputs and Profile Events

Loop Control Alarm

A loop alarm detects faults in the control feedback loop by continuously monitoring process response to the control output(s). When used, the controller starts an internal timer when the PID control output reaches saturation (0% or 100% power for single control type, -100% or +100% for dual control type). Thereafter, if the output has not caused the process variable to be corrected by a predetermined amount 'V' after time 'T' has elapsed, the alarm becomes active. When the process variable starts to move in the correct direction or when if PID output is no longer at saturation, the alarm is deactivated.

Depending on the model, the loop alarm time 'T' can be automatic (twice the Integral Time value) or set to a user defined value. Correct operation with the automatic loop alarm time depends upon reasonably accurate PID tuning. A user defined value is required for On-Off control, and the timer starts as soon as an output turns on.

The value of 'V' is dependent upon the controller and input type. Typical values of V for temperature inputs are 2°C or 3°F, and for Linear inputs, V = 10 x LSD.

The loop alarm is automatically disabled in manual control mode or during execution of automatic tuning. Upon exit from manual mode or after completion of the automatic tuning, the loop alarm is re-enabled.

Also refer to: Alarm/Limit Types; Automatic Tuning; Control Loop/Zone; Control Type; Linear Input; LSD; Manual Mode; On-Off Control; PID and Process Variable.


Least Significant Digit is the smallest incremental value that can be shown at an instruments defined display resolution.

Also refer to: Display Resolution.


This stands for milliamp DC. It is used in reference to linear DC milliamp inputs and the linear DC milliamp outputs on controllers, indicators and recorders. Typically, these will be 0 to 20mA or 4 to 20mA.

Also refer to: Input Type; Linear Input and Linear Output

Main Menu

An instruments top-level menu. This should allow access to other menus (configuration, setup etc) as well as the operation mode. Instruments generally require a special key-press sequence or password to gain access their menus.

Also refer to: Configuration Menu; Operation Mode and Password.

Manual Mode

PID controllers generally operate in automatic mode, but most can be switched into manual mode when required.

In manual mode, most controllers operate as follows: The setpoint value is replaced by a % output power value. This value is adjusted using the keypad between 0% and 100% for controllers using single control type, or -100% to +100% for controllers using dual control type.

Depending on the model, auto/manual mode may selected via a dedicated button, from a menu selection, or from a digital input if one is available for this function. Switching between automatic and manual modes is achieved using “bumpless transfer”.

It is possible to use a controller as a “Manual Station. In this application, it is permanently in manual mode.

Caution: Manual Mode should be used with care. In this mode, the PID algorithm is not in control of the process. The output level has to be set correctly by the operator, who must monitor the process for correct operation. Manual power is not restricted by any power output limits.

Also refer to: Bumpless Transfer; Controller; Control Type; Operation Mode; PID; and Power Output Limits.

Manual Reset

- Refer to Bias and Reset

Master & Slave Controllers

The terms “master” and “slave” are used to describe the controllers in multi-zone applications where one instrument controls the setpoint of another. In a simple Setpoint Master/Slave application, the master controller transmits its setpoint to the slaves via serial communications, or retransmits it as an analogue DC linear output signal. If serial comms are used, the master controller must be able to act as a communications master device and the slave must have compatible communications. If DC linear retransmission is use, the slave controller must have a matching a remote setpoint input. It is common to apply an offset to the slave zones, so that each is slightly higher or lower than the master. This required a slave setpoint offset parameter, or for linear retransmission a remote setpoint offset.

Another Master & Slave example is Cascade Control. This is where the slave setpoint is set using the master controllers PID power output.

The terms Master and Slave are also used in a different context in relation to serial communications, where the master reads information from, or writes values to the other (slave) devices on the network, but the slave never initiates the data transfer.

Also refer to: Cascade Control; Control Loop/Zone; Linear Output; Retransmit Output; Remote Setpoint; Serial Communications and Setpoint.

Minimum Duration Of Change

A form of alarm hysteresis that is unique to a “rate of change alarm”. It is the minimum time that the rate of change in the process variable must be above the alarm threshold, before the alarm will change state (from on to off, or from off to on). If the duration is less than this time, the alarm will not activate no matter how fast the rate of rise.

Also refer to: Alarm/Limit Hysteresis; Alarm/Limit Types and Rate Of Change Alarm.

Modbus RTU

Modbus RTU is a common type of serial communications protocol on instruments fitted with RS485 or RS232 communications.

Modbus RTU is a master/slave protocol and only the master can initiate communications. Each slave is given a unique address on the network, and the messages from the master contain the address of the intended slave. Only this slave will act on the command, even though other devices might receive it (an exception is specific broadcast messages sent to address 0 which are acted upon by all slaves but not acknowledged).

The messages can instruct the slave to change a value in one of its memory registers, or ask it to send back one or more values contained in the registers. The Modbus RTU message format includes a cyclic redundancy checksum (CRC) to ensure that the message arrives undamaged.

Some instrument can act as a Slave only, while others can be master or slave. A typical use for a controller with Modbus master communications it to continuously send its setpoint value using broadcast messages to other slave controllers.

Also refer to: Master & Slave; Modbus TCP; RS485 and Serial Communications.

Modbus TCP

Modbus TCP is a version of the Modbus protocol for Ethernet networks. The data model and function calls used by Modbus TCP and Modbus RTU are identical. The encapsulation is different with the Modbus message “wrapped up” in a TCP/IP packet, and a checksum is not required ensure that the message arrives intact.

A master device initiates the communications, and a slave instrument only acts on the message if it has been sent to its own IP address.

Also refer to: Ethernet and Modbus RTU.

Modulating Valve

A valve that can be positioned anywhere between fully closed and fully open by means of an incorporated drive motor. A typical application is temperature control in a furnace using gas burners. In most cases a controller directly energises the ‘open’ and ‘close’ motor windings using three point stepping control via two relay outputs, but some modulating valves have positioning circuitry that will accept a linear (mA or VDC) control signal. This circuitry energises the motor until the valve is positioned proportionally to the signal. In either case, the controller would normally use PI control.

Also refer to: Linear Output; PI Control and Three Point Stepping Control.

Multi-Loop/Multi-Zone Control

- Refer to Control Loop/Zone

Multi-Point Scaling

When a process input is connected to a linear signal, multi-point scaling is sometimes required to correct for non-linearity of the signal. The user can scale the input vs. the displayed value at a number of “breakpoints”. It is advisable to concentrate the breakpoints where there is the greatest non-linearity, or the area of particular interest in the application.

Also refer to: Linear Input and Process Input.


This stands for millivolt DC. It is used in reference to the linear DC millivolt inputs or outputs on controllers, indicators and recorders. Typically, these will be 0 to 50mV or 10 to 50mV.

Also refer to: Auxiliary Input; Input Range; Linear Input; Linear Outputs; mADC; and VDC


Depending on the instrument and the context it is use, “Offset” may mean one of the following:

The control output Bias / Working Point.

Process variable offset. A zero point calibration adjustment applied to the process input.

A constant value applied to any variable parameter. E.g. a slave controller’s setpoint input from the master may have an offset applied to it in a multi-zone application.

Also refer to: Bias; Master & Slave; Process Input; Single Point Calibration and Zero Point Calibration

On-Off Control

In on-off mode, a controller’s output(s) are turned on / off as the process variable crosses the setpoint, in a manner similar to a central-heating thermostat. There is usually some adjustment of the on and off switching hysteresis either side of the setpoint, to allow for a differential or dead-band. This helps to avoid rapid switching of the output on faster processes.

Some oscillation of the process variable is inevitable when using this type of control, and a larger hysteresis will increase the amplitude of these oscillations.

In dual control applications, it is usually possible to setup a combination of control methods, where the primary control output uses PID and the secondary output uses on-off control.

On-off control cannot be implemented with linear (analogue) outputs.

Also refer to: Control Type; On-Off Differential; Linear Output; PID; Process Variable and Setpoint.

On-Off Differential

A switching differential, centred about the setpoint, when using on-off control. Relay ‘chatter’ can be eliminated by proper adjustment of this parameter, but too large a value may increase process variable oscillation to unacceptable levels. On-off differential is also known as hysteresis or dead-band.

Also refer to: On-Off Control; Process Variable; Relay and Setpoint

 On-Off Hysteresis

- Refer to On-Off Differential

Operation Mode

This is usually the mode entered at instrument power-up or it can be selected from the instruments main menu. It is the mode used during most normal operation.

Also refer to: Main Menu.


In dual control, overlap/deadband defines the portion of the primary and secondary proportional bands over which outputs are both active (Overlap), or neither is active (Deadband). It is adjustable in the range -20% to +20% of the sum of the two proportional bands. Entering positive values causes an overlap, while negative values cause a deadband.

Overlap/deadband is not applicable if the primary output is set for on-off control.

If the secondary output is set for on-off, it has the effect of moving the on-off differential band of the secondary output to create the overlap or deadband.

When overlap/deadband is turned off, the edge of the secondary output differential band coincides with the point at which the primary output reached zero.

The effect of the Overlap/Deadband parameter is shown in the following table

 Overlap/Deadband Image

Also refer to: Control Type; On-Off Differential, On-Off Control, Primary Proportional Band and Secondary Proportional Band.

Paperless Recorder

Paperless recorders monitor and log information about a process. Typically this will be temperature, pressure or RH etc; but any parameter that can be converted to a suitable electrical signal can be recorded. A paperless recorder is differentiated from a data logger by its full colour display. This allows the recorder to mimic the look and function of a traditional chart recorder, showing a graphical view of the data over time. Other views, such as bar graphs, process views and reports may also be available. Paperless recorders usually offer on-screen analysis of the data, but recordings can also be transferred to a PC for analysis and storage. Paperless Recorders include a battery backed-up real time clock.

Also refer to: Chart Recorder; Data Recorder and Real Time Clock


A password or code number is frequently required when entering the configuration/setup modes on instrumentation or other menus where alteration of the parameters by unauthorised persons may adversely affect the operation.

Usually the password consists of a number with 4 or more digits. The correct code must be entered to gain access. Usually a lock can be turned off, giving unlimited access to the menu. This should only be done in secure locations accessible to trusted personnel only.

Also refer to: Configuration Mode.


PD Control

Proportional and Derivative control (PD), combines proportional control with derivative action. It is similar to PID control, but without the integral action.

Also refer to: Derivative Action; Integral Action; PID Control and Proportional Control.


PI Control

Proportional and Integral control (PI) is most often used for modulating valves or motor control. It combines proportional control with integral action. It is similar to PID control, but without application of derivative action.

Also refer to: Derivative Action; Integral Action; Modulating Valve; PID Control and Proportional Control.


PID Control

PID control maintains accurate and stable levels in processes (e.g. controlling temperature) by combining Proportional control with Integral and Derivative action. Proportional control avoids the oscillation characteristic of on-off control by continuously adjusting the correcting variable output(s) to keep the process stable. Integral action eliminates control deviation errors over time, and derivative action counters rapid process movements. To achieve optimal control, the PID three terms must be tuned to match the conditions in the application.

Also refer to: Control Deviation; Controller; Correcting Variable; Derivative Action; Integral Action; On-Off Control; PD Control; PI Control; Process Variable; Proportional Control; Setpoint and Tuning.


PID Controller

- Refer to Controller



This stands for Programmable Logic Controller. A microprocessor based device used in machine control. It is particularly suited to sequential control applications using digital I/O. Some PLC’s are capable of basic PID control, but tend to be expensive and often give inferior control quality. PLCs are frequently linked via serial communications to PID controllers.

Also refer to: Controller; PID and Serial Communications.



Pre-tune is an automatic controller tuning method that artificially disturbs the process start-up pattern, so that the controller can calculate the PID terms prior to the setpoint being reached. During pre-tune, full primary power is applied until the process value has moved approximately halfway to the setpoint. At that point, power is removed (or full secondary power is applied for dual control), thereby introducing an oscillation. Once the oscillation peak has passed, the pre-tune algorithm calculates the optimum proportional band(s), integral time and derivative time. The pre-tune process is shown in the following diagram.

When pre-tune is completed, the PID control output power is applied using the newly calculated values. Using pre-tune reduces the possibility of setpoint overshoot when the controller is new or if the application has been changed.

Pre-Tune Image

Pre-tune will not engage if either primary or secondary outputs on a controller are set for on-off control, during setpoint/profile ramping or if the process variable is less than 5% of the input span from the setpoint. As a single-shot operation, pre-tune will automatically disengage once complete, but can be configured to run at every power up by enabling “Auto Pre-Tune”.

Also refer to: Automatic Tuning; Auto Pre-Tune; Control Type; Derivative Action; On-Off Control; Input Span; Integral Action; PID; Primary Proportional Band; Process Variable; Profiler; Secondary Proportional Band; Setpoint; Setpoint Ramping; Start-up Tune; Tune At Setpoint and Tuning.

Power Loss Recovery Action

- Refer to Profile Recovery Method

Power Output Limits

Used to limit the power levels of primary and/or secondary control outputs. Normally a controller’s algorithm can set the PID outputs to any value between 0 and 100%. If this is undesirable in a particular application, limits may be available to restrict the primary or secondary power levels.

Power limits must be used with caution because the instrument will not be able to control the process if the limits prevent the outputs from reaching the value needed to maintain the desired setpoint. It is not possible to limit the power in on-off control mode.

Also refer to: Control Type; On-Off Control; PID and Setpoint.

Primary Proportional Band

The portion of a controllers input span over which the primary output power level is proportional to the process variable value. Applicable if the control type is single or dual. For dual control a secondary proportional band is used for the second output. To achieve optimal proportional, PD, PI or PID control, the primary proportional band must be tuned to match the conditions in the application

The control action can be direct or reverse acting. With direct action, the correcting variable decreases as the process variable falls within the proportional band, reverse action acts in the opposite way.

Also refer to: Controller; Control Action; Control Type; Input Span; PD Control; PI Control, PID Control; Proportional Control; Process Variable; Secondary Proportional Band; and Tuning.


Process High Alarm

- Refer to Alarm/Limit Types


Process Input

An instruments main input, used to monitor the value process it is connected to. This value is known as the Process Variable or PV. Some instruments have input circuits that are “Universal” and can be configured to work with a wide variety of input types, while others may be limited to one type. The most common input types are: thermocouples or PT100s for temperature, and DC linear mV, voltage or mA signals. A DC linear signal can represent virtually any physical parameter as long as it can be converted into a suitable voltage/current via a transducer. Common examples are %RH, pH, pressure and temperature. The input can usually be scaled for the range and engineering units required (e.g. 0.0 to 100.0%RH, or 0 to 1000 PSI).

Also refer to: Engineering Units; Input Span; mA; mV; PV Offset; Process Variable; Transducer and VDC.


Process Low Alarm

- Refer to Alarm/Limit Types


Process Variable (PV)

Process Variable is the parameter that is to be indicated and/or controlled. It is monitored by the main process input of the instrument. Common types are Thermocouples, PT100 or other RTD temperature probes; %RH, pressure, level, flow etc from transducers that convert these parameters into DC linear input signals (e.g. 4 to 20mA). Linear signals can usually be scaled into engineering units.

Also refer to: Engineering Units; Input Span; Linear Input; Process Input; RTD; Thermocouple and Transducer.


Process Variable Offset

- Refer to Zero Calibration Point


Profibus DP

Profibus is a standard for field bus communication promoted by Siemens. Two variants are commonly used today.

Profibus PA (Process Automation) is used in process automation applications and is suitable for explosion/hazardous areas limiting current it supplied to field instruments so that explosive conditions cannot happen under error conditions.

Profibus DP (Decentralized Peripherals) is used in factory automation to provide a higher speed network for process signals to sensors and actuators often via a centralized PLC. It uses the same protocol as PA allowing the two network types to be connected via a coupler.

Also refer to: PLC and Serial Communications.


Profile Events

Events are outputs that can be activated during the profile segments of a profile controller. Some profilers have just one event output, while others can have several, each of which can be defined to be active or inactive independently for the duration of the segments. At the end of a program, they can usually be selected so that active event outputs stay on until the unit is powered down or a new profile runs.

Also refer to: Controller; Profile Segments and Profiler.


Profile Header

A profile header contains critical information about a profile, such a how it starts / stops, the power loss recovery action, or if it should repeat multiple times when run etc. Some profilers have a single header applicable for all profiles, while others have individual settings for each.

Also refer to: Profiler; and Profile Recovery Method


Profile Segments

Profile Segments can be ramps, dwells or steps. Additional special segments may also be possible, such as holds, ends, loops or joins. A step segment increases or decreases the setpoint value instantaneously, a ramp increase/decreases the setpoint over a set time, or at a defined rate. A dwell (sometimes called a “soak”) maintains the value of the previous segment for a defined time.

Hold segments maintain the value of the previous segment until the profiler is given an instruction to move to the next segment. A loop sends the profile back to a previous segment to run that part of the profile more than once. A join ends the current profile and start another one. An end segment stops the current profile.

Profilers usually have options for one or more segment event outputs.

Also refer to: Effective Setpoint; Profile Events; Profiler and Setpoint Ramping.



A profiler (sometimes called a programmer) controls the value of the effective setpoint over time; increasing, decreasing or holding its value as required. This is used in applications where the rate of rise or fall of the process variable must be closely controlled, or where a value must be maintained for a period before moving to the next value. The sequence is called a profile or program, and each step is called a segment. Profilers generally have more than one program available and the number of possible segments possible in each program can vary. Most profilers can have event outputs which can be activate/deactivate for individual segments according to the requirements of the process.

Also refer to: Effective Setpoint; Profile Events; Profile Header; Profile Segments; and Profiler Mode.


Profiler Mode

This mode is available on profile controllers. It is the mode entered when a profile is running. In this mode, the effective setpoint is determined by the program that is running.

Also refer to: Controller Mode; Effective Setpoint; Profiler; Profile Segments; Profiler and Setpoint.

Profile Recovery Method

If there is a power cut while a profiler is running a program, the instrument will use the defined profile recovery method once the power has been restored. Depending on the model, possible options may include:

  • Aborting the profile and maintaining the profile value from the time the power failed.
  • Aborting the profile and using the controller setpoint value.
  • Aborting the profile with the control outputs off.
  • Restarting the profile again from the beginning.
  • Continuing the profile from the point it had reached when the power failed.


The parameter for setting the recovery method is usually in the profile header.

Also refer to: Profile Header; Profile Segments; Profiler and Setpoint.



- Refer to Profiler.


Proportional Control

In proportional control, a controller’s algorithm can adjust the correcting variable anywhere between 0 and 100%, so that it is exactly as required to keep the process stable and on setpoint.

If the control type is dual, both primary & secondary outputs are available, each of which can give proportional control. When the proportional band(s) are correctly tuned, the process variable is maintained at a steady value, avoiding the oscillation characteristic of on-off control. Proportional control is most commonly used in conjunction with integral and derivative action to give PI. PD or PID control. If integral action isn’t used, the biasing of the proportional bands may need resetting periodically to remove any control deviation.

Also refer to: Bias; Controller; Control Deviation; Control Type; Correcting Variable; Derivative Action; Integral Action; On-Off Control; PD; PI; PID; Primary Proportional Band; Process Variable; Secondary Proportional Band; Setpoint and Tuning.



- Refer to RTD



- Refer to Profile Segments and Setpoint Ramping



- Refer to Derivative Action.


Rate Of Change Alarm

An alarm based on the rate of change in the measured process variable. If the PV changes at a rate greater than the defined rate of change alarm level, the alarm will be activated provided that the rate of change is higher than this level for longer than the minimum duration of change time. If the duration is less than this time, the alarm will not activate no matter how fast the rate of rise is.

Also refer to: Alarm Operation; Alarm/Limit Types; Minimum Duration Of Change and Process Variable.


Real Time Clock

An internal clock present in all data loggers and paperless chart recorders which allows them to keep track of the day, date & time even when powered down. Some controllers, profilers and paper chart recorders also have this feature. At powered down, an integrated battery can keep the clock running for several years. Some RTC’s use large capacitors instead of batteries. This is useful in applications where batteries are forbidden, but reduces the permitted power-down time to a few days at most.

Also refer to: Chart Recorder; Controller; Data Recorder; Paperless Recorder and Profilers.


Recorder Option

- Refer to Data Recorder.



An electromechanical switch operated by a solenoid coil. Relays are commonly fitted in controllers as alarm/limit or time proportioning control outputs. The limited current capacity and switching cycles of internal relays means that they are usually connected to larger external slave relays/contactors with much larger switching capability. These are easier and cheaper to replaced than an complete controller once worn out. A suitably rated RC snubber should be connected to relays to protect nearby equipment from the effects of noise generated as they switch. The reliability and long-term cost savings of solid state relays make them more suitable than relays in many applications.

Also refer to: Controllers; Latching Relay; Solid-State-Relay and Time Proportioning Control.


Remote Setpoint (RSP)

An analogue RSP signal can be used to adjust the value of the effective setpoint, as an alternative to using manually entered local setpoints. A controller with remote setpoint has a 2nd auxiliary input that can accept a mA or VDC signal (or in some cases potentiometer or mV signal) which can be scaled and used as the setpoint source. The remote setpoint value is constrained by the input range and any setpoint limits. Typical uses for remote setpoint inputs are multi-zone setpoint and master & slave applications and on cascade control slaves.

Also refer to: Alternative Setpoint; Auxiliary Input; Cascade Control; Controller; Control Loop/zone Effective Setpoint; Linear Input; Local Setpoints; Master & Slave; mADC; mVDC; Setpoint; Setpoint Select; and VDC.



Depending on the instrument and the context it is use, “Reset” may mean one of the following:

  • To reset configuration settings back to their default values, as set when the instrument was first supplied.
  • An alternative name for integral action in a PI or PID controller.
  • Manual reset, to bias the control proportional band in a proportional controller.

Also refer to: Bias; Integral Action; PI; PID and Proportional Control.


Retransmit Output

A linear VDC or mADC output signal, proportional to the process variable or setpoint, for use by slave controllers or with external devices such as a data recorders or PLCs. A retransmit output can be scaled to the portion of the input / setpoint span that the user is interested in.

The retransmit minimum scale point defines the value of the process variable or setpoint, at which the output will be at its minimum value. The retransmit maximum scale point is the corresponding point at which the output will be at its maximum value. E.g. for a 0 to 5V output, it is the PV or SP values the user wants to correspond with to 0V and 5V respectively.

Also refer to: Data Recorder; Input Span; Linear Output; mADC; Master & Slave; PLC; Process Variable; Setpoint and VDC.


Reset To Defaults

Some instruments have a menu option that returns all of its settings back to their factory defaults. Alternatively, the product manual may list the default values which the user can choose to enter into the configuration. Resetting should be used with great care, as the action cannot be undone. It is advisable to record the current settings before proceeding with a reset. Following the reset, the instrument will have to be re-configured to match the needs of the application.

Also refer to: Configuration Menu.


Reverse Acting Alarm

- Refer to Alarm/Limit Operation.


Reverse Acting Control

- Refer to Control Action



RS232 is the standard most widely used for interfacing peripherals to PC’s and is designed for serial communications with single instrument up to distances of 15 metres, in a low electrical noise environment. Connection is via a screened two core cable where the voltage signal on each line is referenced to the screen which is grounded.

RS232 only defines the physical layer electrical specification, not the protocol that is transmitted across it. Various protocols are possible including simple ASCII communications and Modbus RTU.

Many modern PC’s do not have RS232 ports, but USB to RS232 converters are available for use with computers that lack this type of port, as are convertors for USB to RS485 and RS232 to RS485.

Also refer to: Modbus RTU; RS485; Serial Communications and USB.



RS485 (also known as EIA-485) is a two-wire, half-duplex serial communications link. It is the standard most commonly used for industrial applications due to its high noise immunity and multi-drop capability. Up to 128 devices can communicate via RS485, over distances up to 1200 metres using inexpensive twisted pair wires. Data speeds can be as high as 35 Mbit/s over 10 m and 100 kbit/s at 1200 m, but many devices do not support such high data rates. RS485 uses differential signals (the voltage difference between the wires) to convey data. One polarity indicates a logic 1, the reverse polarity indicates logic 0. The applied voltages can be between +12 V and -7 volts, but the difference of potential need only be >0.2 volts for valid operation.

It is recommended that the wires be connected as series of point-to-point (multi-dropped) nodes (not in a star or ring format), with 120ohm termination resistors connected across the wires at the two ends of the network. Without termination resistors, reflections of the signals can cause data corruption, and electrical noise sensitivity is increased. The master device should normally provide powered resistors, to bias the wires to known voltages when they are not being driven by any device. Without biasing resistors, the data lines float and noise can be interpreted as data when actually all devices are silent.

RS485 only defines the physical layer electrical specification, not the protocol that is transmitted across it. Various protocols are possible, but the most common protocol for industrial instrumentation is Modbus RTU. HMIs PLCs and other instruments frequently have RS485 ports but PC’s do not. To connect an RS485 network to a PC USB to RS485 or RS232 to RS485 convertor will be required

Also refer to: Modbus RTU; RS232; Serial Communications and USB.



- Refer to Real Time Clock.



A resistance temperature detector. The resistance of an RTD sensor changes as its temperature changes. The use pure, often precious metals in their construction makes them more expensive than thermocouples and thermistors, but they have the benefit of being very accurate and stable over their usable range.

Many instruments support PT100, a type of RTD made using thin platinum wire with a resistance of 100? at 0°C. PT1000 (platinum 1K? at 100°C) and NI120 (nickel, 120? at 0°C) are also relatively common. These all have positive temperature coefficients (PTC), meaning their resistance increases when their temperature rises.

Also refer to: Input Range; Process Input; Thermistor and Thermocouple.

Scale Lower Limit

- Refer to Input Span.


Scale Upper Limit

- Refer to Input Span.


Secondary Proportional Band

When a controller is used in dual control mode, this is the portion of the input span over which the secondary output power level is proportional to the process variable value. Normally the secondary proportional begins at the end of the primary band, but it is possible to have an overlap or deadband between them.

To achieve optimal proportional, PD, PI or PID control, both primary and secondary proportional bands must be tuned to match the conditions in the application.

The control action for the secondary output is always the opposite of the primary control action. If the primary control action is reverse, the secondary action will be direct so the correcting variable decreases as the process variable falls within the secondary proportional band.

Also refer to: Controller; Control Action; Control Type; Input Span; Overlap/Deadband; PD Control; PI Control, PID Control; Process Variable; Proportional Control; Primary Proportional Band and Tuning.



Self-Tune is a type of automatic tuning algorithm that continuously optimises PID terms while a controller is operating. It uses a pattern recognition algorithm, which monitors the control deviation. The diagram shows a typical application involving a process start up, setpoint change and load disturbance.

Self-Tune Image

The deviation signal is shown shaded, and overshoots have been exaggerated for clarity. The Self-Tune algorithm observes one complete deviation oscillation before calculating a new set of PID values. Successive deviation oscillations cause the values to be recalculated so that the controller converges on optimal control. When the controller is switched off, these PID terms are stored, and are used as starting values at the next switch on. The stored values may not always be ideal, if for instance the controller is brand new or the application has changed. In these cases, the user can utilise pre-tune to establish new initial values. Self-tune will then fine-tune these values as it monitors any control deviation.

Use of continuous self-tuning is not always appropriate. For example applications which are frequently subjected to artificial load disturbances, for example where an oven door is likely to be frequently left open for extended periods, can lead to incorrect terms being calculated. In addition, because self-tune tunes for PID control, it is not recommended for applications, that require PD, PI or proportional only control.

Self-tune cannot be engaged if on controllers set for On-Off Control.

Also refer to: Automatic Tuning; Control Deviation; Controller; On-Off Control; PD; Pre-Tune; Proportional Control; PI; PID; Setpoint and Tuning.

Sensor Break Pre-Set Power

If there is break on a controller process or auxiliary input it will not be able to control the process. Most controllers respond by turning of their control outputs, but some models can be configured to set their outputs to a pre-defined safe power level. For example a small amount of heat may be applied to prevent a product solidifying in the machine.

Also refer to: Auxiliary Input; Process Input; Signal Break Detection.


Serial Communications

A option on many instrument model that allows other devices such as PCs, PLCs, master controller or other communications device to access an instruments parameters. Depending on the model, the communication options/protocols can include RS485 (Modbus RTU), Profibus DP, DeviceNet or Ethernet (Modbus TCP / EtherNet IP). Some devices have USB ports for configuration or data transfer.

Also refer to: DeviceNet; Ethernet; EtherNet IP; Master & Slave; Modbus RTU; Modbus TCP; PLC; Profibus DP; RS485 and USB.



The target value at which a controller attempts to maintain the process variable, by adjusting its control output power (the correcting variable). Controllers have a Local Setpoint and sometimes remote or other alternative setpoints. Setpoint values are limited by the instrument input range and any setpoint limits.

Also refer to: Alternative Setpoint; Auxiliary Input; Controller; Correcting Variable; Local Setpoints; Process Variable; Remote Setpoint and Setpoint Limits.


Setpoint Limits

The minimum and maximum permissible value for a controller’s setpoint. Setpoints are always limited by the input span, but some models have the facility to restrict the setpoint further. Such limits should be set to keep the setpoint above/below any value that might cause damage to the process.

Also refer to: Input Span and Setpoint.


Setpoint Ramping

Many controllers have the facility to ramp their effective setpoint towards the final target value at a predefined rate. When the setpoint reaches the top of the ramp, a “soak period” begins where the setpoint is maintained at this value.

A deviation alarm is often used with this feature to check that the process is closely following the ramp.

Ramping protects a process from rapid changes in the setpoint and the resulting thermal shock as the controller tries to force the process variable to follow. This is especially useful if there is a power-cut, because it guides the rise back to the target setpoint when power is restored. For example, if you set the ramp rate to 600°/hr and the setpoint to 400°C, and the current temperature is 100°C at power-on, the effective setpoint starts a 100° and rises towards 400° at 600°C per hour. A similar process occurs when switching back to automatic mode from manual control.

The exact implementation of setpoint ramping varies with the controller model. Some implement a ramp whenever the setpoint value is adjusted, others only do so when the active setpoint is changed (e.g. from local setpoint 1 to local setpoint 2). Also some models have an adjustable time for the soak, after which the control outputs are disabled. Others have an indefinite soak following the ramp.

Also refer to: Active Setpoint; Alarm/Limit Types; Controller; Control Deviation; Effective Setpoint; Manual Mode; Process Variable; Setpoint and Setpoint Selection.


Setpoint Selection

If a controller has more than one setpoint source available, the user can choose whether the main or alternative setpoint will be active. Usually the choice is made via a setpoint select parameter in the menu; or via a digital input or by a command given over a serial communications link.

Also refer to: Active Setpoint; Alternative Setpoint; Digital Input; Serial Communications and Setpoint.


Signal Break Detection

A controller cannot correctly control a process if it is unable measure the process variable. In most cases the instrument can detect a loss input signal, and within a few seconds it displays an error message such “Open” or “Input Fail” and takes the appropriate actions such activating alarms or turning outputs off.

Some models have fixed actions under signal fail conditions, but it is often possible to choose its behaviour as break high (act as if the process had gone high) or break low (act as if the process had gone low, or even going to preset output values.

Similar actions are required if an auxiliary input signal is lost.

Instruments can detect a break with thermocouple or RTD inputs and with non-zero based DC linear inputs (e.g. 2 to10V or 4 to 20mA), but cannot detect a break if the result lies within the normal input range. E.g. a break in a 0 to 20mA signal results in 0mA input, which is still a valid value.

Also refer to: Alarm/Limit Types; Auxiliary Input; Input Range; Linear Input; mADC; RTD; Sensor Break Pre-Set Power; Thermocouple and VDC


Single Point Calibration

Single point calibration consists of a single offset value applied to an input. It affects the readings equally across the entire input span, raising or lowering the apparent value by the value entered. No adjustment is made for controller gain errors using this method.

Also refer to: Calibration; Offset; and Two Point Calibration



- Refer to Dwell.


Solid State Relay (SSR)

An external device manufactured using two Silicone Controlled Rectifiers in reverse parallel. They can be used to replace mechanical relays in most AC power applications. Some types of SSRs can switch DC, but most cannot. As a solid-state device, an SSR does not suffer from contact degradation when switching electrical current. Much faster switching cycle times are also possible, leading to superior control. SSRs require a logic signal from a controller, PLC or other device, which typically can be between 4 and 30VDC in its high state. When the logic signal is high, it causes conduction of current from the line supply through the SSR to the load.

Also refer to: Controller; Cycle Time; Logic Output; PLC; Relay; Triac and VDC.


Solid State Relay Driver / SSd

- Refer to Logic Output



- Refer to Profile Segments


Strip Chart Recorder

A type of chart recorder where a long, vertically moving chart that passes under one or more pens moving horizontally in proportion to the recorded parameter. The combined effect is to draw a graph that reflects a process over time. The active part of chart is typically 100mm or 250mm wide. It is common for the pens to be able make simple dot-matix annotations alongside the continuous trace, so that time and scaling information is also recorded. Far more data can be recorded onto a strip chart than is possible with circular recorders, but these types of recorder are being replaced by paperless recorders or data loggers in many applications.

Also refer to: Chart Recorder; Circular Recorder; Data Recorder and Paperless Recorder.


Solenoid Valve

An electromechanical device, used to control the flow of gases or liquids. It has just two states, open or closed. Usually a spring holds the valve closed until a current is passed through the solenoid coil forcing it open. Unlike modulating valves, a standard process controller with time-proportioned or on-off can be used to control a solenoid valve.

A typical application might be a burner, where a bypass supplies some gas at all times, but not enough to heat the process more than a nominal amount (low flame). A controller output opens the solenoid valve when the process requires additional heat (high flame).

Also refer to: Modulating Valves; On-Off Control and Time Proportioning Control.


Start-up Tune

A form of automatic tuning of the PID terms available on most controllers. This type of tuning is usually carried when the controller is at or close to ambient conditions. The controller drives the process part-way towards the target setpoint (typically 50-75% from the start value to setpoint), then induces a temporary disturbance by turning the control outputs off for a short period. The process response is analysed and the correct tuning terms are calculated.

This type of automatic tuning requires sufficient distance between the process variable and the setpoint when it is used and can cause an over-shoot of the setpoint during the test. If this is a problem for the application, consider using tune at setpoint if it is available.

Also refer to: Automatic Tuning; Controller; PID; Process Variable; Setpoint; Tune At Setpoint and Tuning.


A thermistors resistance varies significantly with temperature, making them useful as temperature sensors, particularly where the measured span will be narrow. Each thermistor sensor type has different resistance vs temperature curve and is often extremely non-linear so the connected instrument must be matched to it in order for the process variable to be measured accurately.

Thermistors differ from resistance temperature detectors (RTD) in that the material used in their construction is generally a ceramic or polymer not a pure metal.

Also refer to: Input Span; RTD and Process Variable



A temperature sensor made up of two different metals. They convert the temperature difference between their cold junction (the measuring instrument) and the hot junction, into a small signal of just a few microvolts per °C. Thermocouples are cheap and interchangeable, but the wires and connectors used must be matched to the metals used in their construction. They can measure a wide range of temperatures; with some thermocouples able to withstand very high temperatures such as furnaces. The main limitation of thermocouples is accuracy especially with the higher temperature types.

Each thermocouple type is identified by the colour of insulation on the two conductors, and where possible, the colour of the outer sheath according to the current IEC 584-3 international standard, although it is still common to find previous standards in use, especially in existing applications. The common types are shown below. Note: * = Wire is magnetic


Also refer to: Input Type and Process Input.

Three Point Stepping Control

Modulating valves require a special “three point stepping” control algorithm if the valve motor is directly driven by the controller. Three point stepping provides an output to move the valve further open or closed whenever there is a control deviation error. When the deviation error reaches zero, further action is not required unless the process conditions change. This type of controller is often called a valve motor drive controller.

Some modulating valves have valve positioning circuitry. These require a continuous DC linear output from the controller instead of a 3-point stepping drive to the motor.

Also refer to: Controller; Control Deviation; Linear Output; Modulating Valve; and Valves.


Time Proportioning Control

Time proportioning is utilised when proportional control is required from a logic output (e.g. relays, triacs or SSR driver outputs) where continuous control is not possible. Time proportioning control is accomplished by cycling the output on and off during the prescribed cycle time, whenever the process variable is within the proportional band(s). The control algorithm determines the ratio of time (on vs. off) to achieve the level of the correcting variable required to remove the control deviation error. For example,a 32 second cycle time, 25% power demand would result in the output turning on for 8 seconds, then off to 24 seconds. This type of output might be used with relays / contactors, solid state relays or solenoid valves.

Also refer to: Control Deviation; Correcting Variable; Continuous Control; Cycle Time; Primary Proportional Band; Proportional Control; Relay; Secondary Proportional Band; Solenoid Valve; SSR and Triac.



Any device which converts energy can be considered a transducer, but the term is commonly applied to sensor or detector that converts a physical parameter in to an electrical signal. This signal can be measured by indicators, controllers, recorders and other instrumentation. Common types include (but are not limited to) temperature, humidity, pH, pressure or flow.

Also refer to: Controller; Data Recorders and Indicator.