As mentioned in my earlier posts, the i/p instructions are fed to i/p unit. To know about it in details, refer http://electricalfundablog.com/blog/instrumentation-control/process-control . Now, lets discuss about how Signal transmission happens to CPU. During the commencement of an instruction cycle, the Central Processing Unit collects each and every i/p instructions from the i/p unit and put it in the reprogrammable memory in the form of a ‘Process image’. It is therefore, this reprogrammable memory is also known as Process Image Input (PII).
As soon as the Central processing unit completes the storing of i/p instructions to PII, the program starts to execute layer by layer. Depending upon the program instructions, the Central processing unit then carry out logical/arithmetic functions from Process Image Input. It also calculates the timing and counting provided by timer/counter and understands/provides flag-states for indication purpose. What is a Flag-state?? It is just a binary representation in ‘1’ and ‘0’ which indicates a particular condition.
The calculation result is then put again in the registers ( CPU’s internal memory). The internal-memory is also known as Process Output Image (PIQ). After this operation, the Central processing unit then move the data from PIQ to the O/p component. Accordingly, the o/p unit then controls actual o/p mechanism. Thus, the signal transmission chain is finally completed.