Chapter 6 Applied Instructions Example 2: 1. The instruction CJ between the instruction MC and the instruction MCR can be used in the five conditions below. (a) The execution of the program jumps from the part of the program outside one MC/MCR loop to the part of the program outside another MC/MCR loop. (b) The execution of the program jumps from the part of the program outside the MC/MCR loop to the part of the program inside the MC/MCR loop. (c) The execution of the program jumps from the part of the program inside the MC/MCR loop to the part of the program inside the MC/MCR loop.
(d) The execution of the program jumps from the part of the program inside the MC/MCR loop to the part of the program outside the MC/MCR loop. (e) The execution of the program jumps from the part of the program inside one the MC/MCR loop to the part of the program inside another the MC/MCR loop. 2. When the instruction MC is executed, the previous state of the switch contact is put onto the top of the stack inside the PLC. The stack is controlled by the PLC, and can not be changed by users. When the instruction MCR is executed, the previous state of the switch contact is popped from the top of the stack.
Under the conditions listed in (b), (d), and (e) above, the number of times the items are pushed onto the stack may be different from the number of times the items are popped from the stack. When this situation occurs, at most 32 items can be pushed onto the stack, and the items can be popped form the stack until the stack is empty. Therefore, when CJ or CJP is used with MC and MCR, users have to be careful of the pushing of the item onto the stack and the popping of the item from the stack. 6-137