
7-14
Inputs, Outputs, Timers, and Other Control Logic
Date Code 20020215
SEL-351R Instruction Manual
Figure 7.13: Latch Control Switch Controlled by a Single Input to Enable/Disable
Reclosing
Feedback Control
Note in Figure 7.13 that the latch control switch output (latch bit LT1) is effectively used as
feedback for SEL
OGIC
control equation settings SET1 and RST1. The feedback of latch bit LT1
“guides” input IN104 to the correct latch control switch input.
If latch bit LT1 = logical 0, input IN104 is routed to setting SET1 (set latch bit LT1):
SET1 = /IN104 * !LT1 = /IN104 * NOT(LT1) = /IN104 * NOT(logical 0)
= /IN104 = rising edge of input IN104
RST1 = /IN104 * LT1 = /IN104 * (logical 0)
=
logical
0
If latch bit LT1 = logical 1, input IN104 is routed to setting RST1 (reset latch bit LT1):
SET1 = /IN104 * !LT1 = /IN104 * NOT(LT1) = /IN104 * NOT(logical 1) =
/IN104 * (logical 0) = logical 0
RST1 = /IN104 * LT1 = /IN104 * (logical 1)
= /IN104 = rising edge of input IN104
Rising Edge Operators
Refer to Figure 7.13 and Figure 7.14.
The rising edge operator in front of Relay Word bit IN104 (/IN104) sees a logical 0 to logical 1
transition as a “rising edge,” and /IN104 asserts to logical 1 for one processing interval.
The rising edge operator on input IN104 is necessary because any single assertion of optoisolated
input IN104 by the SCADA contact will last for at least a few cycles, and each individual
assertion of input IN104 should only change the state of the latch control switch once (e.g., latch
bit LT1 changes state from logical 0 to logical 1).
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