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.