Rockwell Automation Publication 1756-RM003N-EN-P - October 2011 151
Timer and Counter Instructions (TON, TOF, RTO, TONR, TOFR, RTOR, CTU, CTD, CTUD, RES) Chapter 3
Figure 2 - Counting Down
When disabled, the CTUD instruction retains its accumulated value. Set the
Reset input parameter of the FBD_COUNTER structure to reset the
instruction.
Arithmetic Status Flags: Not affected
Fault Conditions: None
Execution:
CDEnable
Count-down Enable Bit (CD)
Count-down Done Bit (DN)
Counter Accumulated Value (ACC)
Preset
16637
Condition Function Block Action Structured Text Action
Prescan No initialization required. No initialization required.
Instruction first scan CUEnable
n-1
and CDEnable
n-1
are set. CUEnable
n-1
and CDEnable
n-1
are set.
Instruction first run CUEnable
n-1
and CDEnable
n-1
are set. CUEnable
n-1
and CDEnable
n-1
are set.
EnableIn is cleared EnableOut is cleared, the instruction does nothing, and the
outputs are not updated.
N/A
EnableIn is set The instruction sets CUEnable
n-1
and CDEnable
n-1
.
On a cleared to set transition of EnableIn:
• the instruction executes.
• EnableOut is set.
The instruction sets CUEnable
n-1
and CDEnable
n-1
.
EnableIn is always set.
The instruction executes.
Reset When set, the instruction clears CUEnable
n-1
, CDEnable
n-
1
, CU, CD, DN, OV, and UN and sets
ACC = zero.
When set, the instruction clears CUEnable
n-1
, CDEnable
n-
1
, CU, CD, DN, OV, and UN and sets
ACC = zero.
Postscan No action taken. No action taken.
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