Top 8 Differences between Modular PLC and Compact PLC

Everyone is aware of the Term PLCs. But when it comes to choosing a type of PLC for a particular application, people get stuck. Sometimes it is challenging to understand the difference between Modular PLC and Compact PLC.

PLCs can be classified based on their functions, capacity, memory, number of I/O ports, and several other parameters. However, PLC can be classified into the following two main categories:

1. Modular PLC
2. Compact PLC

Compact PLCs are also known as Unitary PLCs whereas Modular PLCs are also known as Rack-mounted PLCs.

In this article, we have discussed the differences between Modular and Compact PLC.

Difference between Modular PLC and Compact PLC

The difference between Modular PLC and Compact PLC is given below in tabular form.

Sr. No.	Modular PLC	Compact PLC
1	They are also known as Rack-mounted PLC	They are also known as Unitary PLC
2	It contains a range of modules that are slotted together to build up a system.	It contains all the features of a basic system in one compact unit.
3	Great flexibility in the choice of modules. Modules can be easily installed or removed without affecting other modules.	There is no choice of modules as it is a Unitary device. Modules cannot be added/removed.
4	Input Output terminals can be expanded.	They cannot be expanded.
5	More number of Inputs and outputs.	A limited number of Inputs and outputs.
6	Best suited for future expansion.	Not suitable for future expansion.
7	Costlier than Compact PLCs.	Comparatively cheaper than Modular PLCs.
8	Scanning time increases with an increase in input and output modules.	Less scanning time is required.
9	If any feature fails then only that part has to be changed, saving the cost.	If any feature fails then the whole unit has to be replaced.
10	These are big in size	These are small and compact
11	Applications: They are useful for applications where there would be lots of inputs and outputs because more I/O modules can be added if needed. E.g. Control process lines in manufacturing industries.	Applications: They can be used for any applications that do not require a lot of inputs or outputs. E.g. Car Park Barrier, Traffic signal control system.

Modular PLCs are widely used in the automation industries as more I/O modules are required for an automation plant/system. However, because of its flexibility in its configuration, its programming is sometimes more complex than that of compact PLC. Hence, for new engineers, it is advised to practice on compact PLC and then advance to Modular PLCs.

Components of PLC

There are three main components of the PLC system.

1. Central Processing Unit (CPU)
2. Power Supply
3. Input Output (I/O) Modules

Apart from these three components, the bus system is also considered a component of the PLC system.

Central Processing Unit (CPU)

CPU is a microcontroller-based circuitry. The CPU consist of the following blocks:

• Arithmetic Logic Unit (ALU)
• Program memory
• Process image memory (Internal memory of CPU)
• Internal timers and counters
• Flags

CPU performs the task necessary to fulfill the PLC functions. These tasks include scanning, I/O bus traffic control, program execution, peripheral and external device communication, special functions or data handling execution, and self-diagnostics.

Input Module

These modules act as an interface between the real-time status of the process variable and the CPU.

Analog input module: Typical input to these modules is 4-20 mA, 0-10 V. e.g. Pressure, flow, level Tx, RTD (ohm), Thermocouple (mV)

Digital input module: Typical input to these modules is 24V DC, 115V AC, and 230V AC. Eg. Switches, pushbuttons, relays, pump valves on-off status.

Output Module

These modules act as a link between the CPU and the output devices in the field.

Analog output module: Typical output from these modules is 4-20 mA, 0-10 V.
E.g. Control valve, speed, and vibration.

Digital output module: Typical output from these modules is 24V DC, 115V AC, and 230V AC.
E.g. Solenoid valves, lamps, actuators, dampers, and pump valve on-off control.

Power Supply

The power supply gives the voltage required for the electronics module (I/O logic signals, CPU, memory unit, and peripheral devices) of the PLC from the line supply. The power supply provides the isolation necessary to protect the solid-state devices from most high-voltage line spikes. As I/O is expanded, some PLCs may require additional power supplies in order to maintain proper power levels.

Bus System

It is the path for the transmission of the signal. The bus system is responsible for the signal exchange between the processor and I/O modules. The bus system comprises several single lines i.e. wires, tracks, etc.

Do you wish to start learning PLC programming? Start with a Single Push button ON-OFF ladder logic program.