Software 4.1.1.1 The 4 phases of a PLC cycle Ш Update process image The status of the inputs is read and written into an in- ternal RAM range (operand range I00.00 ...). The program uses these values in the next cycle. Exception: Operations with byte input markers BIxx.xx immediate- ly read the inputs without waiting for the next update of the process image. Ш Process program Program processing always starts with the first line of the ORG module and ends with the last line of the ORG module (see example "structured programming"). The calculated values (assignments) are written into the process image memory.
Ш Update outputs The output markers are copied to the outputs only at the end of a complete program processing cycle. Thus, even if the outputs have been changed by the program several times, only the last status will be output to the relevant terminal. Exception: Assignments to byte output markers BOxx.xx immedia- tely write their result into the output memories without waiting for the process image to update the outputs. Ш Internal PLC action In certain cases, the CPU has to respond to requests that are required for self-testing or for communication with the programming PC. 37