Schweitzer Engineering SEL-351P, Quick Start Manual

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7-14 Inputs, Outputs, Timers, and Other Control Logic Date Code 990215
SEL-351P Manual Técnico
Feedback Control
Note in Figure SECTION 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 SECTION 7: .13 and Figure SECTION 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).
For example in Figure SECTION 7: .13, if:
LT1 = logical 0
input IN104 is routed to setting SET1 (as discussed previously):
SET1 = /IN104 = rising edge of input IN104
If input IN104 is then asserted for a few cycles by the SCADA contact (see Pulse 1 in
Figure SECTION 7: .14), SET1 is asserted to logical 1 for one processing interval. This causes
latch bit LT1 to change state to:
LT1 = logical 1
the next processing interval.
With latch bit LT1 now at logical 1 for the next processing interval, input IN104 is routed to
setting RST1 (as discussed previously):
RST1 = /IN104 = rising edge of input IN104