Page 206 - Programmable Logic Controllers, Fifth Edition - Mobile version

L1 Inputs Ladder logic program Outputs L2
Input A Active
MCR Start fence
Input A Input B Output A Output A ON

Input C TON
Input B Output B ON
TIMER ON DELAY EN
Timer T4:1
Time base 1.0 DN
Preset 10
Input C
Accumulated 0
Input D Latch output B
L
Input D
Input E Unlatch output B
U
Input E
MCR End fence

Figure 9-4 MCR fenced zone with the zone true.

Allen-Bradley SLC 500 controllers use the master con- end fence, which is a rung with an unconditional
trol reset instruction to set up single or multiple zones MCR.
within a program. The MCR instruction is used in pairs • Input A of the start rung is true so all outputs act ac-
to disable or enable a zone within a ladder program, and cording to their rung logic as if the zone did not exist.
it has no address. Figure 9-4 shows the programming of Figure 9-5 shows the programmed MCR fenced zone
an MCR fenced zone with the zone true. The operation of with the zone false. The operation of the program can be
the program can be summarized as follows: summarized as follows:
• The MCR zone is enclosed by a start fence, • When the MCR in the start fence is false, all rungs
which is a rung with a conditional MCR, and an within the zone are treated as false. The scan

Inputs Ladder logic program Outputs
L1 Input A L2
MCR Start fence

Input A Input B Output A
Output A OFF
Input C TON
Input B
TIMER ON DELAY EN
Timer T4:1
Time base 1.0 DN Output B ON
Preset 10
Input C
Accumulated 0
Input D Latch output B
L
Input D
Input E Unlatch output B
U

Input E
MCR End fence

Figure 9-5 MCR fenced zone with the zone false.

Program Control Instructions Chapter 9 187

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