• Step 3 – The CPU executes internal test by combining the instruction from Step 1 with the information obtained from step 2. • Step 4 – The CPU checks the user program memory if next instruction is another examine instruction . – If it is another examine instruction, the CPU checks if it is logically ANDded or ORed with the previous instruction. – If the next instruction is logically ANDed with the previous one, then both instructions must produce continuity for the rung to maintain continuity. – If the next instruction is logically ORed with the previous one, then it is sufficient for either instruction to provide continuity in order for the rung to maintain continuity. • Step 5 – The CPU determines if the overall rung conditions are true or false , and then it fetches the output energize instruction from the user program memory, so it could determine which output address/es to affect.
Processor Section • Step 6 – The CPU now knows the rung condition and output address/es to be affected, and it sends the proper digital signal to the output image file, which then passes the digital signal to the associated output terminal. • After completing the execution of an instruction rung, it fetches and then executes the next sequential instruction from the user program memory. • Below is a figure which shows the repetitive process being done by the PLC as long as it is in the RUN mode . Input Scan Output Scan Last Program Instruction User Program Instruction First Program Instruction