The various PLC blocks and their functions are as follows : 

1.   In-put Unit :

It acts as an interfacing link between the Input instructions and the Central processing unit. The analog input generated by the sensor/transducer/limit-switch etc are fed to the input unit. The input unit then performs the conversion of this analog i/p into a digital i/p (called as Standard control-signal) for processing by PLC. The standard control signal delivered by i/p module could be 5/9 volts where as the analog i/p that it receives can be 230 V AC/115 V AC/ 24v DC. As per requirement, it also prioritise the instructions given from i/p unit and dispatch it to Central processing unit one by one.

Based on the characteristics of I/p feed, the Input section may be of following type :

  • Analog i/p section

  • Digital i/p section

Basic blocks of PLC system

2.   CPU (Central-Processing-Unit) :

A CPU The Central processing unit for CPU contains the below mentioned blocks:

  • Arithmetic-logic-unit (A.L.U)

  • Programmed memory

  • PIQ

  • Timer

  • Counter

  • Flag

The heart of CPU is its microprocessor/ micro-controller chip. The working of CPU is fully controlled by the instructions/program stored in “user program memory”. The user program directs and controls the CPU’s working. This program is prepared by the user based on the control logic required to for the control and automatic task .

  •  Arithmetic-Logic-Unit (A.L.U)

A.L.U is the most important part of Central processing unit. It carry out the below mentioned tasks:

  • Categorizes different i/p feeds.
  • Carry out the logical operations.
  • Carry out calculation of data.
  • Monitors the timer (and counter).
  • Monitors the flag states.
  • It puts the i/p feed in PII (Process-Input-Image) prior to commencement of each of the Program-cycles.
  • Post calculations and logical operations, It shifts the end-result to the PIQ (Process-Output-Image)

3.   Output-Unit :

The output-unit provides an interface acts as a link among Central Processing unit and the o/p elements. These o/p elements may be relay, actuator, solenoid-valve, damper etc. This o/p unit ,in actual, provides the o/p by controlling the o/p elements. When a Central processing unit provides the final processed signal to O/p unit, it then changes this signal into a suitable voltage-level which is required by the o/p element. You can understand the functionality of the o/p unit by just this fact only that a Central processing o/p which comes to the o/p unit may be just 5/9 volt, however, the o/p unit changes it to 115/230 volt AC or even 24volt DC.

Based on the characteristics of o/p elements, the O/p unit may be of following type :

  • Analog o/p section

  • Digital o/p section

4.  Bus-system :

The Bus-system provides a route for signal transmission. In a PLC based system, these Bus-systems does the transportation of signals among the Central processing unit and the I/p-O/p units. Based on the functionality of a Bus, It be categorised into 03 types :

  • Address-bus : It identifies / locate the place in the registers where the data is kept.
  • Data-bus: It transports data from each block to another in a PLC system
  • Control-bus: It transports the control/timing signal to each and every required units so that the processor’s functions can be synchronised.

5.   Standard Power- Supply system :

As obvious by its name, it fulfils the voltage requirement of all the units / blocks of PLC system.