The document discusses different types of signals used in distributed control systems, including analog input (AI), analog output (AO), digital input (DI), and digital output (DO) signals. It also describes resistance temperature detectors (RTDs) and thermocouples (TCs), explaining their advantages and disadvantages. Finally, it lists the specific signal types used to control motors, motor operated valves, pneumatic valves, and solenoid valves.

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Followingare the differentkindsof signalsusedin the DistributedControl Systems (DCS)
1) AI – AnalogInputSignal.
2) AO – AnalogOutputSignal.
3) DI – Digital InputSignal.
4) DO – Digital OutputSignal.
5) RTD Signals – Resistance Thermal Detector.
6) TC – ThermoCouple signals.
AI signal in4-20 mille amps will be receivedfromthe fieldTransmitters and instrumentslike,
a) PT - Pressure Transmitters.
b) FT - FlowTransmitters.
c) LT - Level Transmitters.
d) DPT - Differential Pressure Transmitters.
e) TT - Temperature Transmitters.
f) PT - PositionTransmitters
g) Speedindication.
h) VT - VibrationTransmitters
i) EccentricityTransmitter.
j) KW indication and
k) Currentindicationetc. andall local informationtothe DCSPanels
AO signals (Commands) in 4 – 20 mille amps will be supplied from the DCS panels to I / P Convertors
mountedon the control valvesafter beingprocessedby the ControllerUnitsof the DCS.
Example:Opencommandforanyvalve or damper.
DI Signals in either 230 Volts AC or DC will be received by the DCS Panels from the limit switches or
contactors of the field instruments, the source of the AC/DC supply being supplied to the field
instrumentsthrough the correspondingDCS Panels.
Example:
a) Valve ordamperOpenFeedBack.
b) Valve ordamperClose FeedBack.

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c) Feedbackon the positionof the Local PushButton.
d) Motor ON/OFFFeedBack.
e) Informationonthe MCC Disturbance etc.
DO Signals in either AC or DC will be supplied to the field equipment through the DCS Panels, the
supplysource beingfrom an external switch gear or otherinterconnectingpanelsetc.
a) Valve/damper openCommand.
b) Valve/damper Close Command.
c) Valve IntermediatestopCommand.
d) Electrical Motor Start Command.
e) Electrical Motor StopCommand
RTD – Resistance Temperature detector is a high precision wire wound resistor whose resistance
varies with temperature. By measuring the resistance of the metal, its temperature can be
determined.
Several different pure metals such as platinum, nickel and copper can be used in the manufacture of an
RTD. A typical RTD probe contains of a coil of very fine metal wire, allowing for a large resistance change
without a large space requirement. Usually platinum RTDs are used as process temperature monitors
because of theiraccuracy and linearity.
Advantages of RTDs:
1) The response time compared to the thermocouple is very fast in the order of a fraction of a
second.
2) An RTD will notexperience anydriftproblemsince itisnotself powered.
3) Withinitsrange it ismore accurate and has highersensitivitythanathermocouple.
4) In an installationwhere longleadsare requiredthe RTDdoesnot require anyextension cable.
Disadvantages of RTDs:
1) Since the metal used for an RTD must be in its purest form, they are much more expensive than
the thermocouples.
2) In general anRTD is notcapable of measuringaswide atemperature range asa thermocouple.
3) A powersupplyfailurecancause erroneousreadings.

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4) Very small changes in the resistance are being measured by the RTD. Hence all connections
mustbe tightand free of corrosionwhich otherwisecreate errors.
Failure Modes:
1) An open circuit in the RTD or in the wiring between the RTD and the bridge will cause a high
temperature reading.
2) Loss of poweror a short withinthe RTD will cause alow temperature reading.
Thermo Couple (T/C):
A thermocouple consists of two pieces of dissimilar metals with their ends joined together by twisting,
soldering or welding. When heat is applied to the junction a voltage in the range of mille – volts (mV) is
generated.
Advantages of Thermocouples:
1) Thermo couples are mostly used on Transformers. The hot junction is inside the transformer oil
and the cold junction at the meter mounted on the outside. With this simple and rugged
installation, the meter directly reads the temperature rise of oil above the ambient temperature
of the location.
2) In general thermocouples are used exclusively around the turbine hall because of their rugged
constructionandlowcost.
3) A thermocouple iscapable of measuringawidertemperaturerange thananRTD.
Number & the Type of signals used for the operation MOV, Pneumatic Valve
and for Motors:
For Motor Operated Valve (MOV):
Serial No Type of Command Type of Signal Remarks
1 Open Command DO
2 Close Command DO
3 Stop Command DO
4 Open Feed Back DI

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5 Close Feed Back DI
6 MCC Disturbance/Trip DI
7 Local/Remote selection DI
For PneumaticOperated Valve:
Serial No Type of Command Type of signal Remarks
1 % openingcommand AO
2 PositionTransmitter AI
For the Solenoidvalve
1 Open Command DO
2 Close Command DO
3 Open Feed Back DI
4 Close Feed Back DI
For Electrical Motors:
Serial No Type of command Type of signal Remarks
1 Start Command DO
2 StopCommand DO
3 On Feed Back DI
4 MCC Disturbance Trip DI
5 Local/Remote Feed Back DI
6 Motor Speed Indication AI Usedfor the ID, FD and PA Fans
7 Motor Load in KW Indication AI Usedfor the ID, FD and PA Fans
8 Motor Currentindication(Amps) AI Usedfor the ID, FD and PA Fans
Note:
1) 0.50 mm single pair cable shall be used from all the transmitters and local instruments to the
JunctionBoxes.
2) 0.50 mm multiple paircable shall be usedfromthe JunctionBoxestothe DCSpanels.