An instance of this Function Block type provides an implementation of
the Outside Operation Mode state machine defined in Annex F.3.6 of the
NORSOK I-005 Standard for System Control Diagrams.
When in outside operation state, a binary actuator (e.g., a valve or
engine) is controlled (e.g., opened/closed or started/stopped,
respectively) from a location "outside" the central control system,
e.g., from a local panel. The central control system can only observe
(and if desired display) what happens, but not control the actuator.
The mode is controlled both from the logic (LO) and from the operator
station (OS). In order to provide an event-driven implementation, the
following changes are made with respect to Figure F.11 of the NORSOK
Standard:
A single OSO (Operator Station Outside) input variable, in
combination with a REQ (Request) event input, replaces the "OS Set
Outside" and "OS Reset Outside" inputs.
A single LO (Lock Outside) 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 Auto/Manual/Outside" signal 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 redundant "Outside Operation 2" state in the
NORSOK-defined state machine is eliminated. This state has no effect
on the externally observable behaviors of the state machine block.
The LD (Ladder Diagram) Boolean OR implementation exhibits
the required behaviors in a simpler and more robust manner.
Enter data for the input variables.
Exercise the FB by clicking on the REQ event input.