FUNCTION_BLOCK SCDModeDT

An instance of this Function Block type provides an implementation of the Disable Transition Mode state machine defined in Annex F.3.4 of the NORSOK I-005 Standard for System Control Diagrams. This mode is used to prevent the output of a Function Block from going to a high or low state. If the output is in the Low state when BCH is asserted, it will remain in the Low state, but the next time it goes High it will remain so until BCH is de-asserted. Similar semantics apply to the BCL variable.

In order to provide an event-driven implementation, the following changes are made with respect to Figure F.9 of the NORSOK Standard:

The "OS Status Disable" output is replaced with BDH (Status Disable Transition High) and BDL (Status Disable Transition Low) outputs. 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) latching operation.
A REQ (Request) event input is used to trigger the latching operation, and a CNF (Confirm) event output is used to provide an indication that the REQ event has been processed.

This implementation has the advantage that the LD (Ladder Diagram) latching operation exhibits the required behaviors in a simple, efficient and easily understood manner.

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