Chapter
7
71
Output Override and I/O Update
Instructions
The user may need programming instructions for certain applications
requiring output overrides or I/O updates. They are:
Master Control Reset instruction –(MCR)–
Zone Control Last State instruction –(ZCL)–
Immediate Input instruction –|I|–
Immediate Output instruction –(IOT)–
The two output instructions that can be used to override a group of outputs
are:
Master Control Reset –(MCR)–
Zone Control Last State –(ZCL)–
These instructions are similar to a hard-wired master control relay in that
they can affect a group of outputs in the user program. The MCR and ZCL
instructions, however, are not a substitute for a hard-wired relay, which
provides emergency stop capabilities for all I/O devices.
WARNING: A programmable controller system should not
be operated without a hard-wired master control relay and
emergency stop switches to provide emergency I/O power shut
down. Emergency stop switches can be monitored but should
not be controlled by the user program. These devices should be
wired as described in the PLC-2/30 Assembly and Installation
Manual, publication no. 1772-805.
To override a group of output devices, two MCR or ZCL instructions are
required: one to begin the zone and one to end the zone (Figure 7.1). The
start fence is always programmed with a set of input conditions. The end
fence must be programmed unconditionally.
When the MCR or ZCL start fence is true, all outputs within the zone are
controlled by their respective rung conditions. When the MCR or ZCL
start fence is false, the outputs within the zone are controlled by the MCR
or ZCL instruction.
7.0
General
7.1
Output
Overrides
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