Which components are essential to modular Programmable Logic Controllers?

In modular Programmable Logic Controllers (PLCs), the essential components are the power supply and the CPU. The power supply is crucial because it provides the necessary electrical energy to operate both the CPU and the various I/O modules within the PLC system. The CPU, or central processing unit, is the brain of the PLC; it processes input signals, executes control logic, and outputs commands to control devices. Without these two components, the PLC would not be able to function effectively, as there would be no means of processing data or performing control tasks.

The other choices represent components that can be part of an automated control system but are not essential to the operation of a modular PLC. Control valves and actuators are commonly used for controlling fluid flow and mechanical movements, while sensors and transducers are vital for monitoring conditions and providing input data to the PLC. Solenoids and relays are actuating elements used for switching and controlling circuits but are peripheral to the basic operation of a PLC. They rely on the CPU's commands to operate effectively, rather than forming the foundational components of the PLC itself.