Allen-Bradley Logix5000 Reference Manual page 151

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