Rockwell Automation T80016 Application Note Maintenance Override Programming - Manual (Page 9)

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AN-T80016 Maintenance Override Programming
Issue 1 June 08 AN-T80016 9
------------------------------PARAMETERS------------------------------
CALL :
DI
Boolean, Input signal from field device (Active False)
ORQ Boolean, DCS Override Request
MC Boolean, DCS common Override Confirm
GOA Boolean, Group Override Available from safety logic
MOE Boolean, Maintenance Override Enable from keyswitch
RST Boolean, Trip Reset
RETURN :
TRIP
Boolean, Trip output to Cause and Effect logic (Active False)
OS Boolean, Override Status to DCS
OA Boolean, Override Acknowledge to DCS
*)
(*
Maintenance overide
This is a non-timed override that prevents the alarm trip from going false.
To set an override from the DCS:
- a command is sent from DCS to system on tag ORQ,
- and a specific acknowledgement is sent back to the DCS by tag OA.
- The operator must confirm that the acknowledgement matches the command, within
a time window, counted by OR_Timer (here set to 30 seconds). This sends a Confirm
signal on tag MC which is common to all instances of this function block.
MC is reset after all override logic has run, after the last instance of this
function block.
- The confirmation will set the override (effectively Int_OS) and clear the
acknowledgement OA, as long as there are not too many overrides already set
in the safety group (GOA) and maintenance overrideas are enabled (MOE).
- The override is reported to the DCS on tag OS.
The above transaction and points are reset if a confirmation is not received
within the time window.
Override removal is effected by turning off the Maintenance Override Enable
keyswitch or by clearing the command.
*)
(* If an override request has arrived and a group override is available and
maintenance overrides are enabled, acknowledge the request and start the timeout
*)
IF (ORQ AND NOT Last_ORQ) AND GOA AND MOE THEN
Int_OA := True ;
OR_Timer := t#0s ;
TSTART(OR_Timer);
END_IF ;
(* If an override confirm has arrived and an override request is in progress and