Rosemount 848L
Reference Manual
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Alarm Variations
Those applications that use a local switch with automatic control have a three position switch arranged as
•The Boolean expression is:
•The 848L expression is:
•EQ1 contains OR(AND(NOT(IN(4)),DO(1)),IN(3));
If there is just one close confirm on input 2, then the Boolean expression is: ALARM = TON ( ( !DO1 & !IN2 ) ( DO1 & IN2 ), TravelTime)
If there is just one open confirm on input 1, then the Boolean expression is: ALARM = TON ( ( DO1 & !IN1 ) ( !DO1 & IN1 ), TravelTime)
It both confirms are used, then the Boolean expression is: ALARM = TON ( ( !DO1 & !IN2 ) ( DO1 & !IN1 ) ( IN1 & IN2 ), TravelTime)
The equivalent 848L expressions are: TON(OR(AND(NOT(DO(1)),NOT(IN(2))),AND(DO(1),IN(2))),100);
TON(OR(AND(DO(1),NOT(IN(1)),AND(NOT(DO(1)),IN(1))),110);
TON(OR(AND(NOT(DO(1)),NOT(IN(2))),AND(DO(1),NOT(IN(1)),AND(IN(
1),IN(2))),120);
Output Variations
Output with Interlock
The chosen expression goes in the last expression used, which must be linked to a DI to generate an alarm.
A valve actuator may be spring return, requiring one output, or bistable, requiring two outputs. Output 1 is used for Open and output 2 for Close. Bistable valves often require a short pulse instead of maintained power. The
848L expressions for spring return are: OUT1 contains DO(1);
or bistable has:
OUT1 contains TP(DO(1),30); OUT2 contains TP(NOT(DO(1)),30);
The interlock may be wired, or internal, or from the bus. The example uses a DO from the bus:
OUT1 contains TP(AND(DO(1),DO(3)),30); spring return is AND(DO(1),DO(3));
OUT2 contains TP(OR(NOT(DO(1)),NOT(DO(3))),30);
