Schweitzer Engineering Laboratories SEL-2020 - Page 254

7-8 SELOGIC Control Equations Date Code 20010518
SEL-2020 Instruction Manual
Auto-Message
Operation
Completes
Completion of
\DAC2/ Processing
Clears 2:UMB
Completion of
\DAC1/ Processing
Clears 1:UMB
Auto-Message
is Triggered
1:UMB Set by
New Unsolicited
Messages
1:UMB
2:UMB
ISSUE2
D2
Figure 7.6: Trigger Lock-Out Problem
You can prevent this problem by writing a SEL
OGIC Control Equation that forces the trigger
condition to reset itself immediately.
The following SEL
OGIC Control Equation adds !D2 to the previous equation:
ISSUE2 = 1:UMB*!D2+2:UMB*!D2
Now, as shown by Figure 7.7, the trigger condition will only be true momentarily; then it will
retrigger upon completion of the message processing.
Auto-Message
Operation
Completes, Causing
D2 to Retrigger
Completion of
\DAC2/ Processing
Clears 2:UMB
Completion of
\DAC1/ Processing
Clears 1:UMB
Auto-Message
is Triggered
1:UMB Set
by New
Unsolicited
Messages
1:UMB
2:UMB
ISSUE2
D2
Figure 7.7: Forcing Re-triggering Avoids Trigger Lock-Out