ISA88 Part 1 Update: more useable, clearer, see new state diagram

Dennis I dont know if you read the rest of my comment on my blog. Summarising, I can see that transient states are present, but what I am trying to say is why do they need to be exposed outside of the controlling object. So for example if a control recipe asks an EPE to run a phase, what does it care whether the phase is starting, running or stopping, pausing or paused ? Once it is pausing or paused that is all the control recipe needs to know until it is running again. What is the control recipe supposed to do with the transient information ? Propagating modes to other objects maybe, but that should be only what is reasonable in Recipe Control – the real time stuff should be down in basic control. And I suspect that the transient states are really about the real time world.

The transient states are there because the recipe is not in control, it is only requesting that the equipment perform the action (Start, Stop, Abort, ...). Because some of these can take time, there should be a state that shows that the action was started but has not yet completed. In many cases the transient states may not be used (because the action is essentially immediate) or goes by so fast that it is not seen (assuming some sort of polled scan of the state).

Who needs the Transient Procedural States? I always work on the principle that the recipe controller (that handles the control recipe) is a transaction based sequencing and data handling system. It can tolerate short delays. It is in effect the process operator; it cares little about the equipment provided it can do what the recipe asks of it. It can request the equipment to do some process action, then wait until it has. Basic control that drives the equipment is real time and cannot tolerate such delays. The original Part 1 provides a lot of suggestions and some basic models that help to specify both the recipe and the basic control. And it shows how they can meet, via Procedural Entities. To interface the control recipe with the equipment, there must be EPE’s (Equipment Procedural Entities), typically phases, that actually execute the process action. But think about this – why should the recipe control have any real time capability other than being fast enough to avoid introducing significant delays Read more here www.s88control.blogspot.com/2009/03/who-needs-transient-procedural-states.html