Table of Contents
Wirings in PLC & DCS Systems
Wiring of PLC & DCS systems consists of various types of inputs and outputs. They are
- Analog Inputs
- Analog Outputs
- Digital Inputs and
- Digital outputs.
1. Analog Input
- The analog input card of the PLC and DCS is usually connected to transmitters such pressure transmitters (PT), level transmitters (LT), temperature transmitters (TT),Flow transmitters (FT), vibration transmitters, control valve opening feedback, running drive’s current, etc.
2. Analog output
- The analogue output card of the PLC and DCS is connected to a control valve’s opening command, as well as the load change of a drive using a VFD
3. Digital Input
- Pressure switches, level switches, temperature switches, flow switches, vibration switches, on-off feedback of valves, run indications of any drives, etc. are some of the devices connected to the digital input card of the PLC and DCS.
4. Digital Output
- For opening and closing the on-off valve, starting any drive, or stopping any drive, commands are sent using the digital output card of the PLC and DCS
PLC and DCS wiring diagrams with field instrumentation
Digital Input (DI) with PLC/DCS wiring
The digital input signals are designed with different combination like:
- Single & Two wire connection
- DI With two wire loop/line monitoring
- Digital input with dry & wet contact
Two-Wire Connection with Input Card
- A two-wire connection for a digital input to the PLC is shown in the above image.
- A digital contact in the field is seen using the PLC’s digital input card. The digital input card detects variations in voltage whenever the contact in the field changes.
- If any switch’s field contact is open, the channel on the digital input card won’t get any voltage back.
- The voltage is transferred back to the channel of the digital input card when the same contact closes.
- Example: Field instruments like proximity and limit switches, valve and motor run feedback
Two Wire Connection with Line (Loop) Monitoring
Line(Loop) Monitoring
- The process of continuously checking the status of digital input signals in a PLC or DCS system is known as line monitoring. It entails examining the quality and integrity of the signals obtained from numerous connected devices or sensors.
- Line monitoring is crucial for assuring the dependability and correctness of digital input signals in both single-wire and two-wire connections.
- It enables any irregularities, such as signal loss, noise interference, or wiring failures, to be detected by the PLC or DCS system, allowing it to respond appropriately based on the monitored state.
- To maintain the integrity and functionality of the entire system, these steps may involve generating alarms, starting fault diagnostics, or starting particular control procedures.
When Line/Loop Monitoring is preferred?
- This configuration is used when additional safety measures are required to detect digital input shorts and opens.
- At the terminations of field instruments and, occasionally, at marshalling cabinets, two resistors are typically utilized.
- The concept of change in resistance underlies this. Both resistors R1 and R2 are in line while the circuit is open, so we will have some loop resistance, with value of X ohms.
- Resistor R1 turns to zero during a short circuit, giving us another loop resistance, with value of Y ohms.
DI with Single-wire connection
- In a single-wire connection, various digital input signals share a single ground or reference wire.
- The shared ground wire is linked to each digital input signal before it is returned to the PLC or DCS system.
- To identify the state of each individual digital input signal in a single-wire connection, line monitoring often entails measuring the voltage level or current flow on the shared ground wire.
Why is this line/loop configuration necessary?
- If the normally open contact is used and the parameter is high, the contact will close. The plant should malfunction if it detects closed contact.
- If the cable is damaged then in this situation it will never trip the plant because the digital input card will always be in open contact. Therefore, for a two-wire connection with a loop monitoring design, the resistors employed in the circuit will be relevant.
- On the digital input card, a raw value rather than an open contact will always be present. Therefore, once the cable breaks, the digital input card will experience a malfunction because its raw value will be zero. Therefore, these resistors will make it easier to find the cable break.
DI with Wet contact type connection
- When we require an additional power source for the field equipment, motor control centers, etc., we employ this setup. In marshalling cabinets, we typically arrange the appropriate wiring in accordance with the needs of the application.
- We refer to this as a WET Contact since we are using the extra power source in the marshalling cabinet (or at the control room side).
- When the field instrument is in the open state, the relay won’t be activated, and since the DI card won’t have a return signal, it will be regarded as being in the off state. The relay will be activated when the field instrument is in the closed state, which causes the DI card to receive a return signal and be considered to be in the ON state.
- The interrogation voltage is the name of this external relay power supply. The panel’s interrogation voltage ranges from 24 VDC to 110 VDC, among others.
DI with Dry contact type connection
- We refer to this as a DRY Contact because there is NO extra power source being used in the marshalling cabinet or at the control room side.
- When the field instrument is in the open state, the relay won’t be activated, and since the DI card won’t have a return signal, it will be regarded as being in the off state.
- The relay will be activated when the field instrument is in the closed state, which causes the DI card to receive a return signal and be considered to be in the ON state. Example Level Switch, Pressure Switch, Feedback from valve, Running motor
Digital output with PLC and DCS wiring
The digital outputs signals are designed with different combination like:
- Single & Two wire connection
- DO With two wire loop/line monitoring
- Digital output with dry & wet contact
- Interfacing Digital output with control system
DO with Two-wire connection
- When the DO command is delivered, the corresponding voltage will be sent to the field instrument, energizing it.
- The output voltage is 24 Volt which is sent from DO card module.
- When the DO command is executed again, the corresponding voltage to the field instrument is cut off, and it is de-energized.
- For safety purposes, Barriers can be used following the DO card.The wiring diagrams does not depict barriers.
DO with relay – Wet contact type connection
- In this type of digital output connector, there is a relay connected between the field instrument and the DO card.
- Relay coil energized by DO card.
- The field instrument is powered by an additional power source (24 VDC, 110 VDC, 230 VAC, etc.) in the marshalling cabinet (on the control room side).
- This arrangement can control high voltage operating instrument.
- Example:Motor, On–Off Device
- The system vendor must confirm the line monitoring for digital output cards up to relay coil and/or up to field.
- This configuration is not used
Interfacing of DO signal with relay to other control system
- It is utilized across a variety of industries for the purpose of interfacing with control systems provided by other parties.
Analog Input with PLC and DCS wiring
AI with Two-Wire Connection with Input Card
- PT, TT, FT, LT, and other two-wire transmitters are connected using this setup. For added security, we may utilize barriers after the AI card.
- 24 VDC voltage is supply voltage produced from AI card
- 4 – 20 mA loop current is used to field instrument side.
- Loop current varies when there is change in physical quantity of field instruments.
- The change in loop current is sensed with AI card connected to PLC and DCS
Interfacing of AI card with other control system
- PLC AI is interfaced to DCS AO
- DCS AI is interfaced to PLC AO
PLC AI to DCS AO
Both side of same configuration will not work. Therefore, PLC AI is sinking passive configuration and DCS AO is sourcing active configuration.
DCS AI is interfaced to PLC AO
As discussed above same configuration in both side will not work. Therefore, PLC AO is sourcing active configuration and DCS AI is sinkingpassive configuration.
Analog Output with PLC and DCS wiring
- The analogue input card performs the opposite function as the analogue output card.
- In response to a PLC or DCS command, the card generates 4 to 20 mA. This current is provided directly to the field device, typically a control valve.
