An instance of this Function Block type provides an implementation of
the Alarm Suppression Mode state machine defined in Annex F.3.8 of the
NORSOK I-005 Standard for System Control Diagrams.
This mode is used to control suppression of alarm annunciation and
alarm actions. The mode is controlled both from the logic (FU) and from
the operator station (OS). In order to provide an event-driven
implementation, the following changes are made with respect to Figure
F.13 of the NORSOK Standard:
The "OS Suppression Off" input is replaced with a REQ
(Request) event input, and the OSS (Operator Station Suppress) input
is used for the OS suppression signal.
A single BU (Suppress) output is used, and a CNF (Confirm)
event output is used to provide an indication that the REQ event has
been processed.
The "OS Status Suppressed" output is not shown on the mode
control block. As described in NORSOK I-005-F.2.2.4, reporting of
mode states to the OS is to be handled separately.
The state machine is implemented with a simple Ladder Diagram
(LD) OR operation.
This implementation has the following advantages:
The NORSOK Standard does not explicitly specify that the "OS
Suppression On" and "OS Suppression Off" inputs shall be
complementary. If it could happen that both of these inputs were
simultaneously TRUE, this could cause an unanticipated race condition
or oscillation between the two states. This condition will not occur
in the event-driven implementation.
The LD (Ladder Diagram) Boolean OR implementation exhibits
the required behaviors in a simpler and more robust manner.
The "Suppress Alarm HH", "Suppress Alarm LL", "Suppress Alarm
WH", and "Suppress Alarm WL" functions described in subclauses
F.3.8.2, F.3.8.3, F.3.8.4 and F.3.8.5 of NORSOK I-0005, respectively,
can be implemented by reuse of instances of this single type with
appropriate routing of input and output signals, without having to
define separate state machine types. This provides improved
encapsulation, reusability and maintainability.
Enter data for the input variables.
Exercise the FB by clicking on the REQ event input.