Page 205 - Programmable Logic Controllers, Fifth Edition - Mobile version

L1 Inputs Ladder logic program Outputs L2
ON/OFF
MCR
ON/OFF M
Stop Start M When MCR
is de-energized,
all nonretentive
Stop outputs
de-energize.
PL1
M
Start
Level switch PL1

Level SOL When MCR
switch LS1 SOL is de-energized,
all retentive
L outputs remain
LS1 in last state.
LS2 SOL
U
LS2

MCR

Figure 9-3 Master Control Reset (MCR) instruction.

The programmed MCR instruction is not a substitute for running state but will have to be restarted via the
a hardwired Master Control Relay. It is highly recom- start pushbutton.
mended that all PLC systems include a hardwired MCR • Assume the level switch is closed and the MCR in-
and Emergency Stop switches to provide safe, effective struction becomes disabled. Pilot light PL1 will im-
shutdown of I/O power. mediately become de-energized even though the level
A Master Control Reset (MCR) instruction is an out- switch instruction is true and the rung appears to have
put coil instruction that functions like a master control logic continuity. When the MCR instruction then be-
relay. MCR coil instructions are used in pairs and can comes enabled, PL1 will automatically be energized,
be programmed to control an entire circuit or to con- provided the level switch has remained closed.
trol only selected rungs of a circuit. In the program of • Assume solenoid SOL has been latched energized,
Figure 9-3, the MCR is programmed to control an entire both limit switches LS1 and LS2 are open, and the
circuit. The operation of the program can be summa- MCR instruction becomes disabled. Solenoid SOL
rized as follows:
will remain energized. When the MCR instruction
• The section or zone being controlled begins with the then becomes enabled, the SOL will remain ener-
first MCR instruction and ends with the second MCR. gized, provided both LS1 and LS2 remained open.
• When the first MCR instruction is false, or disabled, • Assume solenoid SOL has been latched de-
all nonretentive rungs below it, in this case, outputs energized, both limit switches LS1 and LS2 are
M and PL1, will be de-energized even if the pro- open, and the MCR instruction becomes disabled.
grammed logic for each rung is true. Solenoid SOL will remain de-energized. When the
• All retentive rungs, in this case SOL, will remain in MCR instruction then becomes enabled, the SOL
their last state. will remain de-energized, provided both LS1 and
• Assume the motor M is running and the MCR LS2 remained open.
instruction becomes disabled. The motor will im- • Retentive instructions should not normally be
mediately become de-energized and stop operating. placed within an MCR zone because the MCR zone
When the MCR instruction then becomes enabled, maintains retentive instructions in the state last ac-
the motor will not revert back to its previous tive when the instruction disabled.

186 Chapter 9 Program Control Instructions

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