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.
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Enter data for the input variables.
Exercise the FB by clicking on the REQ event input.
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Last updated: 2013-03-29.
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