An instance of this Function Block type provides an implementation of
the Block Alarm Mode state machine defined in Annex F.3.3 of the
NORSOK I-005 Standard for System Control Diagrams.
This mode is used to determine blocking of actions following alarms.
Annunciation of the alarm is still made. The mode is controlled both
from the logic (FB) and from the operator station (OS). In order to
provide an event-driven implementation, the following changes are made
with respect to Figure F.6 of the NORSOK Standard:
The "OS Blocking Off" input is replaced with a REQ (Request)
event input, and the OSB (Operator Station Blocking) input is used
for the OS blocking signal.
A single BB (Blocked) 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 Blocked" and "OS Status Coincidence" outputs
shown in Figure F.6 are 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
Blocking On" and "OS Blocking 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 "Block Alarm HH" and "Block Alarm LL" functions described
in subclauses F.3.3.2 and F.3.3.3 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.