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Wiring Diagrams of PLC and DCS Systems – DI, DO, AI, AO

In this article, we are sharing the basic concepts of PLC and DCS control systems Wiring Diagrams for Digital Input (DI), Digital Output (DO), Analog Input (AI), and Analog Output (AO) signals.

Note that these diagrams are without a Barrier or isolator, fuses, and surge protector for keeping it very simple and understandable.

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Wiring Diagrams of PLC and DCS Systems

The below list shows the basic types of wiring connections available for DI, DO, AI, AO Signals:

Digital Input (DI) Signals

  • Single-wire Connection
  • Two-wire connection
  • Two-wire connection with Line Monitoring
  • DI with Relay – Wet Contact
  • DI with Relay – Dry Contact

Digital Output (DO) Signals

  • Two-wire connection
  • DO with Relay – Wet Contact
  • DO with Relay – Dry Contact
  • Interfacing with Other Control Systems
  • DO with Line Monitoring

Analog Input (AI) Signals

  • Single-wire connection
  • Two-wire connection
  • Three-wire connection
  • Interfacing with other control systems

Analog Output (AO) Signals

  • Two-wire connection

Digital Input Signals to PLC / DCS

Input type with respect to system

Terminal type/connection, Shield is not shown for simplicity.

Two-Wire Connection with Input Card

Digital Input Without Relay and Dry Contact

The two-wire connection is used to connect field instruments like limit switches, proximity switches, motor run feedback, Pump start permissive, etc.

Two-Wire Connection with Line Monitoring

Digital Input With Line Monitoring

When we need additional protection measures to identify the short circuit and open circuit of digital inputs then we use this configuration. Generally, a pair of resistors are used at the terminations of field instruments or in some cases at marshalling cabinets.

This works on the principle of change in resistance. During the Open circuit, both resistors R1 & R2 in line, so we will get some loop resistance, say “x” ohms. During Short Circuit, resistor R1 becomes zero and we will get some other loop resistance, say “y” ohms. The input cards are designed such that it will identify the signal status and differentiates these conditions, alerts the system engineers with alarms.

DI Card with Single Wire Connection

Digital Input Card With Single Wire Connection

DI With Relay – Wet Contact (For Coil)

Digital Input Card with Relay and Wet Contact

This configuration is used when we need an additional power supply for the field instruments, motor control centers, etc.

Generally, in marshalling cabinets, we make the necessary wiring arrangements as per the application requirements. We are using the additional power supply in the marshallaling cabinet (or at the control room side), so we are calling this a WET Contact.

When the field instrument is in Open state then the relay will not be energized thus DI card doesn’t have the return signal, so it will consider as OFF state.

When the field instrument is in Close state then the relay will be energized thus DI card has the return signal, so it will consider as ON state.

This external relay power supply called as interrogation voltage.

The interrogation voltage from the panel can be 24 VDC, 110VDC, etc.

What is the Interrogation Voltage?

Interrogation voltage is a term used in relation to digital inputs of PLC or DCS control systems. When there is some activity in the digital inputs like limit switch activation, then there will be a change in input state from open to close or close to open depends on the type of contact used in the limit switch.

Now we need to interrogate (identify) the state of these contacts (open/close) using a voltage. So we send a voltage (12VDC, 24VDC, 110 VDC, etc) to the limit switch and receives the signal back when the limit switch is in close state. The PLC reads these signals and takes the action as per its logic. This voltage is called as interrogation voltage.

DI With Relay – Dry Contact (For Coil)

Digital Input Dry Contact

We are NOT using any additional power supply in the marshallaling cabinet (or at control room side), so we are calling this a DRY Contact.

When the field instrument is in Open state then the relay will not be energized thus DI card don’t have the return signal, so it will consider as OFF state.

When the field instrument is in Close state then the relay will be energized thus DI card have the return signal, so it will consider as ON state.

Example: Motor Run Feedback.

The interrogation voltage from voltage can be 24 VDC, 110 VDC, etc.

Digital Output Signals from PLC / DCS

Output type with respect to system terminal type/connection.

The shield is not shown for simplicity reason.

Two-wire connection with Output Card

Digital Output With Two wire Connection

This configuration is used to control the digital instruments like Solenoid Valves.

We may use Barriers after the DO card for safety reasons. Barriers are not shown in the diagrams.

When the DO command is given then the respective voltage will go the field instrument and it will be energized.

When the DO command is taken back then the respective voltage will be stopped to the field instrument and it will be De-energized.

DO With Relay – Wet Contact

Digital Output Card With Relay and Wet Contact

We use an additional power supply (24 VDC, 110 VDC, 230 VAC, etc) in the marshalling cabinet (at the control room side) to power the field instrument.

This configuration generally used for hooters, beacons, high-power solenoid valves, etc.

DO With Relay – Dry Contact

Digital Output with Dry Contact

This configuration generally not preferred.

Interfacing with Other Control Systems (DCS/PLC/ESD)

Interfacing with PLC or DCS Control Systems Wiring

This configuration widely used in industries for interfacing with third-party control systems.

DO with Line Monitoring

The line monitoring for digital output cards also possible but upto relay coil and/or upto field shall be confirmed by the system vendor.

Analog Input Signals to PLC / DCS

Shields are not shown for simplicity reasons.

The 4-wire instruments require an additional power supply.

Two-wire Connection with AI Card

Two-wire Connection with AI Card

This configuration is used to connect two-wire transmitters like PT, TT, FT, LT, etc.

We may use barriers also after the AI card for safety precautions. These are not shown in the diagrams.

AI Card with Single Wire Connection

AI Card with Single Wire Connection

For Interfacing with Other Control Systems

DCS AO to PLC AI

DCS AO to PLC AI

Both side’s Active will not work.

PLC AO to DCS AI

PLC AO to DCS AI

Both sides Active will not work

Both sides Passive will not work.

Analog Output Signals

DCS Analog Output Wiring

Author: Jatin Katrodiya

Join the Discussion! Share your thoughts about the basic wiring diagrams using the comments section.

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5 comments

Srinu March 10, 2020 at 1:41 pm

DI or discrete/digital input is the type of PLC input which used to read the status of digital input devices. The Digital input device only has 2 statuses, ON or OFF. ON state mean PLC DI point received status in form of electrical power (can be 24 VDC, 120 VAC, etc.) and OFF state mean PLC DI point received no power from devices. An example of a Digital input device is a switch.

DO or discrete/digital output is the type of PLC output which used to control digital output devices. Digital output devices only have 2 status, ON or OFF. ON command mean PLC DO point send command in form of electrical power (24 VDC, 120 VAC, etc.) and OFF command means PLC DO send no power to devices. An example of digital output devices is a relay.

AI or analog input is the type of PLC input which used to read the value from the sensor. AI receives value from the sensor in the form of electrical current (4-20 ma) or electrical voltage (1-5 V). ADC (Analog to Digital Converter) inside PLC will convert current or voltage to a digital value in the form of 16 or 12-bit word data to further process. An example of a sensor is the temperature sensor. The temperature sensor will measure temperature and send measurement value to the AI channel in the form of current or voltage.

AO or analog output is the type of PLC output which used to control analog devices. AO sends a command to the device in the form of electrical current (4 – 20 mA) or electrical voltage. DAC (Digital to Analog Converter) inside PLC will convert a digital value (12 bit or 16-bit word data resulted from PLC calculation) to current or voltage and send it to a controlled device through the AO channel. An example of an AO device is the control valve, PLC could control opening percentage of the control valve with send 4 – 20 mA current which represents 0 – 100% open command.

Reply
Vikash kumar pandey March 11, 2020 at 11:08 pm

Nice concept

Reply
P Maruthi Prasad June 6, 2020 at 7:01 pm

Excellent explanation in very simple way possible. Thank you very much for valuable information.

Reply
Raj Narayan July 20, 2020 at 8:22 pm

Sir very nice information

Reply
h.YK November 17, 2020 at 1:05 pm

Yes, following is missed:
“Analog Input (AI) -Three-wire connection”

Reply

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Wiring Diagrams of PLC and DCS Systems - DI, DO, AI, AO

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