PLC Architectures

In this topic we will learn what is the architecture and functional block diagram of PLC ?

Every PLC has three types of main modules.

1. Input Module

2. Output Module

3. CPU or Processor

So lets discuss about all these modules in details.

1. Input Module

Input modules in PLC are two type . 1. Digital Input and 2. Analog Inout

So digital Inputs are like ON -OFF switches , Proximity sensors, Toggle Switches , Push buttons or any digital types of inputs. 

In Analog input Pressure, Temperature, Flow, Humidity, Level different type of physical quantity measurement sensors with transmitters in standard analog input 4 to 20 ma or +_10 V.

2. Output module 

Same as Input , outputs are also two types Digital and Analog.

In Digital output Solenoid valves, Lights, Indicator, Hooter and analog output would be I to P converter, Variable Frequency Drive, Proportional Valve.

3. CPU or Processor 

CPU is the brain of this PLC , All logic are written in memory of the CPU and there are various part of the CPU as show in Diagram. CPU will execute the logic written in memory accordingly Signal received from input and update output.

Scan cycle of PLC

A PLC program is generally executed repeatedly as long as the controlled system is running. The status of physical input points is copied to an area of memory accessible to the processor, sometimes called the "I/O Image Table". The program is then run from its first instruction rung down to the last rung. It takes some time for the processor of the PLC to evaluate all the rungs and update the I/O image table with the status of outputs. This scan time may be a few milliseconds for a small program or on a fast processor, but older PLCs running very large programs could take much longer (say, up to 100 ms) to execute the program. If the scan time were too long, the response of the PLC to process conditions would be too slow to be useful.

Special-purpose I/O modules may be used where the scan time of the PLC is too long to allow predictable performance. Precision timing modules, or counter modules for use with shaft encoders, are used where the scan time would be too long to reliably count pulses or detect the sense of rotation of an encoder. The relatively slow PLC can still interpret the counted values to control a machine, but the accumulation of pulses is done by a dedicated module that is unaffected by the speed of the program execution