The Central Processing Unit
The central processing unit is the brain of whole PLC. The CPU consist of a microprocessor memory chip and other integrated circuits to control logic monitoring and communications. The CPU has different operating modes. First is programming mode in which is accepts the downloaded logic from an PC and another is the run mode in which the CPU can execute the program and operate the process . since a PLC is a delicate controller it will only process this one program over and again. One cycle through the program is called a seen time which in voles reading the inputs from the other modules executing the logic based on these inputs and then updates the outputs accordingly. The memory in the CPU stores the program while also holding the status of the I/O and providing a means to store values.