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146
Limitations Each TC number can be used as the definer in only one TIMER or COUNTER
instruction.
Description CNT is used to count down from SV when the execution condition on the count
pulse, CP, goes from OFF to ON, i.e., the present value (PV) will be decre-
mented by one whenever CNT is executed with an ON execution condition for
CP and the execution condition was OFF for the last execution. If the execution
condition has not changed or has changed from ON to OFF, the PV of CNT will
not be changed. The Completion Flag for a counter is turned ON when the PV
reaches zero and will remain ON until the counter is reset.
CNT is reset with a reset input, R. When R goes from OFF to ON, the PV is reset
to SV. The PV will not be decremented while R is ON. Counting down from SV will
begin again when R goes OFF. The PV for CNT will not be reset in interlocked
program sections or by power interruptions.
Changes in execution conditions, the Completion Flag, and the PV are illus-
trated below. PV line height is meant only to indicate changes in the PV.
Execution condition
on count pulse (CP)
Execution condition
on reset (R)
ON
OFF
ON
OFF
Completion Flag
ON
OFF
PV
SV
SV – 1
SV – 2
0002
0001
0000
SV
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.)
In the following example, the PV will be decremented whenever both 00000 and
00001 are ON provided that 00002 is OFF and either 00000 or 00001 was OFF
the last time CNT 004 was executed. When 150 pulses have been counted down
(i.e., when PV reaches zero), 00205 will be turned ON.
00000
CP
R
CNT 004
#0150
00002
00001
00205
CNT 004
Address Instruction Operands
00000 LD 00000
00001 AND 00001
00002 LD 00002
00003 CNT 0004
# 0150
00004 LD CNT 004
00005 OUT 00205
Here, 00000 can be used to control when CNT is operative and 00001 can be
used as the bit whose OFF to ON changes are being counted.
Example 1:
Basic Application
Timer and Counter Instructions Section 5-14