Hi Gerard,

I used to work in the industry where PLCs were heavily used. There is a very basic difference between PLCs and microcontrollers. PLCs are a bunch of relays that can be connected in various ways useful to you. They dont have a CPU built in while Microcontrollers are basically CPU, a memory unit, I/O unit integrated into one chip. 

The great thing about PLCs are as follows:

1. you can reconfigure/reconnect inputs and outputs to produce certain kind of behaviours.

2. 100s of input pins are available having to worry minimally about communication protocols etc

Disadvantages:

1. Quite big

2. Infrastructure costs are high

This is where the microcontrollers come in handy. They are little computers all in one chip.

Advantages:

1. They have a small form factor

2. Great for rapid prototyping and testing SoC based systems.

3. Availablility of complicated and fast communication protocols that opens up a wide variety of fields that PLC cannot get into. For example: tiny robots like thymios, epucks.

I am attaching a quora post that talks in more detail.

https://www.quora.com/What-is-the-difference-between-a-microcontroller-and-a-programmable-logic-controller-PLC

Hi Joshua

Thanks for your answer.

It means that it is wrong to consider that PLCs ship CPUs in the same way as the μCs

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Concerning the answer of Zeashan,

Is it just because µCs do not have I/O interfaces that they are not suited for industrial control applications ? 

Knowing that nowadays μCs are equipped with very fast CPUs, what factors would prevent the use of μCs in industrial environments?

Apart from the fact that the PLC has a sophisticated I/O interface and even programming environments, what should be added to the μC to serve as a PLC?

I have a computer background and I would like to understand.

Thanks VM