Relay Ladder; Structured Text - Allen-Bradley Logix5000 Reference Manual

Program Control Instructions (JMP, LBL, JSR, RET, SBR, JXR, TND, MCR, UID, UIE, AFI, NOP, EOT, SFP, SFR, EVENT)
Arithmetic Status Flags:
Fault Conditions:
Execution:
Condition
Prescan
Rung-condition-in is false
Rung-condition-in is true
EnableIn is set
Instruction execution
Postscan
error_bit
error_check.FD
Not affected
None
Relay Ladder Action
The rung-condition-out is set to false.
The rung-condition-out is set to false.
The instruction executes.
The rung-condition-out is set to true.
N/A
The UID instruction prevents interruption by higher-priority tasks.
The UIE instruction enables interruption by higher-priority tasks.
The rung-condition-out is set to false.
Example:
When an error occurs (error_bit is on), the FSC instruction checks the error code
against a list of critical errors. If the FSC instruction finds that the error is critical
(error_check.FD is on), an alarm is annunciated. The UID and UIE instructions
prevent any other tasks from interrupting the error checking and alarming.

Relay Ladder

Structured Text

UID();
<statements>
UIE();
Rockwell Automation Publication 1756-RM003N-EN-P - October 2011
Structured Text Action
No action taken.
N/A
N/A
EnableIn is always set.
The instruction executes.
No action taken.
FSC
File Search/Compare
Control
Length
Position
Mode
Expression error_code=error_list[error_check.POS]
Chapter 11
UID
EN
error_check DN
10 ER
8
ALL
alarm
UIE
471