Rockwell Automation 1772-LV Mini-PLC - 2/15 Programmable Controller (Series B) Programming and Operations - Manual (Page 67)

Advanced Instruction Set
Chapter 6
6Ć10
Program the first ZCL instruction with input conditions to begin the zone
(start fence).
Program the second ZCL instruction unconditionally to end the zone (end
fence).
Do not nest ZCL zones with other ZCL or MCR zones.
Each zone must be separate and complete.
Figure 6.4
ZCL Programming
ZCL
MCR
Start fence
When ZCL zone is
false all outputs
remain in their
last state
Unconditional end
fence
Function: When the rung becomes:
True
All output instructions within the zone act according to the logic conditions
preceding them.
False
All output instructions within the zone remain in their last state; regardless of
the I/O rack last state switch setting or changes in logic on the input side of
the rung. These same outputs may now be controlled by another zone
program. Only one zone may control a set of outputs at one time.
Purpose: Interrupts the program scan to update I/O data before the normal I/O
update sequence.
Immediate Update I/O
Instructions