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170
Limitations Each TC number can be used as the definer in only one TIMER or COUNTER
instruction.
Description The CNTR(12) is a reversible, up/down circular counter, i.e., it is used to count
between zero and SV according to changes in two execution conditions, those in
the increment input (II) and those in the decrement input (DI).
The present value (PV) will be incremented by one whenever CNTR(12) is
executed with an ON execution condition for II and the last execution condition
for II was OFF. The present value (PV) will be decremented by one whenever
CNTR(12) is executed with an ON execution condition for DI and the last execu-
tion condition for DI was OFF. If OFF to ON changes have occurred in both II and
DI since the last execution, the PV will not be changed.
If the execution conditions have not changed or have changed from ON to OFF
for both II and DI, the PV of CNT will not be changed.
When decremented from 0000, the present value is set to SV and the Comple-
tion Flag is turned ON until the PV is decremented again. When incremented
past the SV, the PV is set to 0000 and the Completion Flag is turned ON until the
PV is incremented again.
CNTR(12) is reset with a reset input, R. When R goes from OFF to ON, the PV is
reset to zero. The PV will not be incremented or decremented while R is ON.
Counting will begin again when R goes OFF. The PV for CNTR(12) will not be
reset in interlocked program sections or by the effects of power interruptions.
Changes in II and DI execution conditions, the Completion Flag, and the PV are
illustrated below starting from part way through CNTR(12) operation (i.e., when
reset, counting begins from zero). PV line height is meant to indicate changes in
the PV only.
Execution condition
on increment (II)
Execution condition
on decrement (DI)
ON
OFF
ON
OFF
Completion Flag
ON
OFF
PV
SV
SV – 1
SV – 2
0001
0000 0000
SV
SV – 1
SV – 2
Precautions Program execution will continue even if a non-BCD SV is used, but the SV will
not be correct.
Flags ER: SV is not in BCD.
Indirectly addressed DM word is non-existent. (Content of DM word is
not BCD, or the DM area boundary has been exceeded.)
Timer and Counter Instructions Section 5-14