Input Output Processing for PLC ( Programmable Logic Controller ) tutorial

This chapter continues the discussion of inputs and outputs from Chapter

2 and is a brief consideration of the processing of the signals from input

and output devices. The input/output (I/O) unit provides the interface

between the PLC controller and the outside world and must therefore

provide the necessary signal conditioning to get the signal to the required

level and also to isolate it from possible electrical hazards such as high

voltages. This chapter includes the forms of typical input/output modules

and, in an installation where sensors are some distance from the PLC

processing, their communication links to the PLC.

Input/output units
Input signals from sensors and the outputs required for actuating devices
can be:
1 Analogue, i.e. a signal whose size is related to the size of the quantity
being sensed.
2 Discrete, i.e. essentially just an on−off signal.
3 Digital, i.e. a sequence of pulses.
The CPU, however, must have an input of digital signals of a particular
size, normally 0 to 5 V. The output from the CPU is digital, normally 0 to
5 V. Thus there is generally a need to manipulate input and output signals
so that they are in the required form.
The input/output (I/O) units of PLCs are designed so that a range of
input signals can be changed into 5 V digital signals and so that a range of
outputs are available to drive external devices. It is this in-built facility to
enable a range of inputs and outputs to be handled which makes PLCs so
easy to use. The following is a brief indication of the basic circuits used
for input and output units. In the case of rack instruments they are
mounted on cards which can be plugged into the racks and so the
input/output characteristics of the PLC can thus be changed by changing
the cards. A single box form of PLC has input/output units incorporated
by the manufacturer.