FUNCTION_BLOCK SCDModeSG

An instance of this Function Block type provides an implementation of the Safeguarding Mode state machine defined in Annex F.3.7 of the NORSOK I-005 Standard for System Control Diagrams. The Safeguarding mode is controlled by process shut down functionality of the plant.

A close examination of the state machine in Figure F.12 of the NORSOK Standard shows that each state corresponds to exactly one priority-ordered input of the block, as shown in the table below, where an "X" entry in the table corresponds to a "don't care" input for the given state. Furthermore, as shown in this table, the full state of the state machine can be made externally visible by the provision of an additional BLS ("Status Locked Safeguarding") output.

NORSOK State		Inputs				Outputs
No.	Name	LSL	LSH	FSL	FSH	BLS	BSL	BSH
3	Locked Safeguarding Low	1	X	X	X	1	1	0
4	Locked Safeguarding High	0	1	X	X	1	0	1
2	Safeguarding Low	0	0	1	X	0	1	0
5	Safeguarding High	0	0	0	1	0	0	1
1	No Safeguarding	0	0	0	0	0	0	0

This priority-driven state machine is easily implemented by using the IF..ELSIF..END_IF structure of the IEC 61131-3 ST (Structured Text) language, as shown in the REQ algorithm below.

Enter data for the input variables.
Exercise the FB by clicking on the REQ event input.