A typical block diagram of PLC consists of five parts namely:

• Rack or chassis
• Power Supply Module
• Central Processing Unit (CPU)
• Input & Output Module
• Communication Interface Module

Rack or Chassis

In all PLC systems, the PLC rack or chassis forms the most important module and acts as a backbone to the system. PLCs are available in different shapes and size. When more complex control systems are involved, it requires larger PLC racks. Small sized PLC is equipped with fixed I/O pin configuration. So, they have gone for modular type rack PLC, which accepts different types of I/O modules with sliding and fit in concept. All I/O modules will be residing inside this rack/chassis.

Power Supply Module

This module is used to provide the required power to the whole PLC system. It converts the available AC power to DC power which is required by the CPU and I/O module. PLC generally works on 24V DC supply. Few PLC uses an isolated power supply.

CPU Module and Memory

CPU module has a central processor, ROM & RAM memory. ROM memory includes operating system, driver and application program. RAM memory is used to store programs and data. CPU is the brain of PLC with an octal or hexagonal microprocessor. Being a microprocessor based CPU, it replaces timers, relays, and counters. Two types of processors as a single bit or word processor can be incorporated with a PLC. One bit processor is used to perform logic functions. Whereas word processors are used for processing text, numerical data, controlling and recording data. CPU reads the input data from sensors, process it and finally sends the command to controlling devices. DC power source, as mentioned in the previous discussion is required voltage signals. CPU also contains other electrical parts to connect cables used by other units.

Input and Output Module

Have you ever thought about how to sense physical parameters like temperature, pressure, flow, etc? using PLC? Of course, PLC has an exclusive module for interfacing inputs and output, which is called an input & output module. Input devices can be either start and stop push buttons, switches, etc and output devices can be an electric heater, valves, relays, etc. I/O module helps to interface input and output devices with a microprocessor. The input module of PLC is explained in the below figure.

The input module of PLC does four main functions.

1. Input module interface receives the signal from process devices at 220 V AC
2. Converts the input signal to 5 V DC that can be used by PLC
3. Isolator block is used to isolate/prevent PLC from undergoing fluctuation
4. After which the signal is sent to the output end i.e the PLC

There are two main sections in the input module namely power section and logical section. Both the sections are electrically isolated from each other. Initially push button is closed. So, 220 V AC supply is given to the bridge circuit through the resistors R1 and R2. The bridge rectifier is used to convert the AC signal into DC and zener diode is used to provide low voltage supply to LED. When the light from LED falls on the phototransistor, it works in the conduction region. Finally, 5V DC supply is given to the processor.

The output module of PLC works similar to the input module but in the reverse process. It interfaces the output load and processor. So here the first section would be logic session and power section comes next. The working of the output module is shown in the below figure

So, here when the logic high signal is generated from the processor, LED will turn ON and allow the light to fall on a phototransistor. When the transistor goes to conduction region, it generates a pulse to the gate of the triac. Isolator block is used to isolate the logic section and control section.

Communication Interface Module

To transfer information between CPU and communication network, intelligent I/O modules are used. These communication modules help to connect with other PLCs and computers which are placed at a remote location.