How field devices are wired to PLC?

During the wiring of PLC, the wire size is an important factor. One or more conductors can be accepted by the input-output terminal of a particular wire size we need to check that if the wire is the proper size and is correct gauge so that it can handle the maximum possible current. A programmable logic controller is an industrial computer control system that monitors the input device continuously and it takes a decision based upon the custom program to control the state of output devices. PLC generally consists of CPU input and output ports

A CPU is of two types one which has an input-output device and another without it


In PLC operation there are four basic steps which will take place in a repeating loop input scan, output scan, program scan, housekeeping

Input scan detects the input device state which is connected to the plc

Output scan it energizes and de-energizes all the output device that is connected to the PLC

Program scan it executes the user created program

Housekeeping it does communication with programming terminals internal diagnostics…



This is a method which is used to simplify the connections to each input-output module in this the wires are bundled by a tie wrap and the wires which are bundled are connected to a single module and then these wire will be routed through the duct with other bundles of wire which have same signal characteristics the input and output bundles carrying same signals will be kept in separated ducts to avoid interference.

If we need to connect a two wired discrete DC device to the PLC we need to check if the device is polarized or non-polarized if it is a polarized device its wires will be labeled one wire will be labeled positive and other will be negative. In this type of connection, the positive wire must be connected to the positive terminal of the PLC input and the negative to the negative terminal.

In order to connect a non-polarized device like simple push buttons and selector switches the device can be connected by either wire connected to the positive terminal of the PLC and other to the negative. image


Special caring needs for certain device connections these connections include leaky inputs, inductive loads, output fusing, and shielded cable.

Leaky inputs

There will be some leakage of current for some devices even when they are in the off state so in-order to rectify this we need to place a resistor across the input


To protect the devices from moderate overloads fusing is done if the output doesn’t have internal fuses then it must be installed externally



Shielding is done to reduce the signal coupling shielded cables must be used for the control lines to protect low-level signals from the electrostatic and magnetic coupling.

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