Which statement best describes modular PLCs?

• A. They are built by selecting components and installing them into a rack or chassis.
• B. They come preassembled with fixed I/O.
• C. They cannot be expanded.
• D. They are identical to fixed PLCs in all aspects.

Answer: (A)

Modular PLCs are designed as a chassis-based system where you pick and combine separate modules—such as the CPU, input/output, communication, and any special function blocks—and slide them into a common rack or backplane. This arrangement lets you tailor the controller to exactly the I/O count, types, and features you need, and you can add or replace modules as the system grows or changes. That makes the statement about building the PLC by selecting components and installing them into a rack the best description of modular PLCs.

The other ideas don’t fit because modular PLCs are not limited to fixed, preassembled I/O; they are built to be expanded and reconfigured. They are not identical to fixed PLCs in all aspects, since modular systems offer scalability and customization that fixed units do not.