The various PLC blocks and their functions are as follows :Â
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 :
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Analog i/p section
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Digital i/p section
2.  CPU (Central-Processing-Unit) :
A CPU The Central processing unit for CPU contains the below mentioned blocks:
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Arithmetic-logic-unit (A.L.U)
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Programmed memory
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PIQ
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Timer
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Counter
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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 memoryâ€. 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 .
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 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 :
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Analog o/p section
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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.