Rockwell Automation 1772-LS_LSP,D17726.8.6 PROG/OPER MANUAL-MINI PLC-2/05 - Manual (Page 62)

Chapter
7
7Ć1
Program Control Instructions
This chapter describes these program control instructions:
output override
immediate I/O update
Some applications need programming techniques designed to override a group
of non-retentive outputs or update I/O ahead of the usual I/O scan time. The
program control instructions satisfy this need.
The output override, or zone type instructions, operate similarly to a hardwired
master control relay in that they affect a group of outputs in the user program.
But these instructions are not a substitute for a hardwired master control relay,
which provides emergency I/O power shutdown.
The following table illustrates specific instructions for these categories:
Output Override Immediate Update I/O
Master Control Reset Immediate Input Update
Zone Control Last State Immediate Output Update
Master Control Reset and Zone Control Last State
Purpose: A Master Control Reset (MCR) establishes a zone in the user program
in which all non-retentive outputs are turned off simultaneously.
Important: Retentive instructions (-(U)-, -(L)-, -(RTO)-) should not be placed
within an MCR zone, because the MCR zone maintains retentive instructions in
the last active state when the start fence goes false.
A Zone Control Last State (ZCL) instruction allows control of one or a group of
outputs in more than one manner in the same program. It establishes a zone in
the user program which controls the same outputs, through separate rungs, at
different times.
Chapter Objectives
Introduction
Output Override Instructions