What is a PLC Rack or PLC Chassis?

Till now we can see What is a PLC System? How does PLC work? in my previous articles. We have also seen various free tutorials of the PLC system, such as,

In this article, we are going to see what is a plc rack or plc chassis?

What is Rack and Chassis in PLC System?

Generally, in the industries, we can find mainly two types of PLC system: Fixed PLC and Modular PLC.

PLC Rack

Modular PLC is mostly used in industries because it can provide flexibility to increase the IOs.


What is a PLC Rack?

The rack serves as the backbone of the modular PLC system that holds all the modules together such as CPU, Power Module, Communication Module, Input-Output Modules, etc. PLC Rack synchronizes both the power and communication signal to all the modules.

A PLC Rack can be of different sizes and shapes depending upon the requirement of the control system.


You can see the different modules are fitted in the same rack or chassis of the modular PLC system in the below diagram.

PLC Modules

There are multiple input and output modules are plugged together in the same rack of the modular PLC system. The power supply module is plugged into the plc rack which regulates the required amount of power to all the modules.

What are the types of PLC Rack?

Depending on the requirement, the PLC systems have racks of different sizes and shapes.

The one system which has a stackable style, in which Power Supply, CPU, IO Modules, etc. are separate. But they are all connected with connectors built to attach all the components together.

They all are connected in a continuous data bus throughout the system. This internal data bus is often referred to as the PLC backplane. This type of PLC backplane is often referred to as the “passive backplane”.

Rack and Chassis

Another option is a separate base unit that holds all the modules. Each module will seat in a particular slot in the base unit. In this type of system PLC backplane is already installed in the base unit. This type of PLC backplane is often referred to as the “active backplane”.

The base unit comes with different numbers of slots and some base unit has a built-in power supply connected with it.

Rack and Chassis


What is the difference when it comes to change the module in the active backplane and passive backplane?

The main question from the maintenance point of view is that How easy to change the module? In a stackable style or passive backplane PLC system, you need to break the data bus while changing the module.

But in an active backplane PLC system, it is very easy to change the module. It just simply slides out to remove the module and slide in to insert the module.

What is the purpose of the rack and chassis in the PLC System?

The main functions of the rack or chassis in the PLC system are

  • It distributes the power supply evenly.
  • It houses different input-output modules.
  • It acts as a communication link between the CPU and different modules.

You can read more articles about Electrical and you can also find books that boost your knowledge in the field of Instrumentation


Thanks for Reading!

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