Rockwell Automation 1761 MicroLogix 1000 Programmable Controllers - Manual (Page 206)

Preface MicroLogix 1000 Programmable Controllers User Manual
11–18
Selectable Timed Disable (STD) and Enable (STE)
These instructions are generally used in pairs. The purpose is to create zones in
which STI interrupts cannot occur.
Using STD
When true, this instruction resets the STI enable bit and prevents the STI subroutine
from executing. When the rung goes false, the STI enable bit remains reset until a
true STS or STE instruction is executed. The STI timer continues to operate while
the enable bit is reset.
Using STE
This instruction sets the STI enable bit and allows execution of the STI subroutine.
When the rung goes false, the STI enable bit remains set until a true STD instruction
is executed. This instruction has no effect on the operation of the STI timer or
setpoint. When the enable bit is set, the first execution of the STI subroutine can
occur at any point up to the full STI interval.
STD/STE Zone Example
In the program that follows, the STI function is in effect. The STD and STE
instructions in rungs 6 and 12 are included in the ladder program to avoid having
STI subroutine execution at any point in rungs 7 through 11.
The STD instruction (rung 6) resets the STI enable bit and the STE instruction (rung
12) sets the enable bit again. The STI timer increments and may time out in the
STD zone, setting the pending bit S:2/0 and lost bit S:5/10.
The first pass bit S:1/15 and the STE instruction in rung 0 are included to insure that
the STI function is initialized following a power cycle. You should include this
rung any time your program contains an STD/STE zone or an STD instruction.
False
6.69
True
3.16 STD
10.13 3.16 STE
Execution Times
(
µ
sec) when
:
STD
SELECTABLE TIMED DISABLE
STE
SELECTABLE TIMED ENABLE