The processor module is at the heart of the PLC system. It contains a board with one or more microprocessors that executes the operating system programs, the programs developed by the user, the memory where these programs are stored, as well as all of the components necessary for the operation. The processor module, as implemented by the operating system, provides a cycle that is composed of the following sequence of operations:

1. Updates the input memory area with the values ​​coming from the physical inputs
2. Executes the user program by operating on the values ​​of the memory and always keeping the results in the memory
3. Executes the system management programs, for example, for diagnostics
4. Writes the physical outputs of the values that are ​​stored in the reserved memory area

The preceding cycle optimizes the communication of the input and output modules and guarantees that the stored values ​​of the inputs remain unchanged during the execution of the programs. The reading of the inputs and the writing of the outputs is managed entirely by the operating system and not by the control program. The most common real-time operating systems are Microware OS-9, VxWorks, QNX, and Linux.

An important parameter of the processor module is the scan time. This is defined as the time interval that elapses between two consecutive activations of the same portion of the program application, thus comprising the time necessary to update the inputs and outputs. The scan time must not be confused with the response time of the system, which is defined as the maximum time interval between the detection of a certain event and the execution of the related programmed action. The response time also takes into account the delays introduced by the input and output modules.