Tuesday, 26 October 2010

S88 Phase Programming for DeltaV

With Emerson’s DeltaV and some other systems an S88 Phase is constructed around their version of the S88 “State transition diagram for example states for procedural elements.”
The Emerson structure uses Sequential Function Charts for each of the major state changes, such as that the Running SFC that goes between  Idle and Complete. In addition a Fail Monitor evaluates conditions that cause the phase to go into the Holding state.
The diagram below shows an overview of this.
Now, there are other and in my opinion better ways of constructing the phases. For example a Phase could just have one SFC, but with Running Holding, Aborting, Stopping And Restarting logic all handled within the same SFC.
Actually it is quite possible to do that even using the Emerson structure, if you don’t slavishly follow the Emerson examples.