Experimental Physics and
Industrial Control System

"Johnson, Andrew N." <[email protected]> · Sat, 14 Mar 2015 03:06:12 +0000

Hi Eric,

The SNL compiler implements a version of the well-known finite state machine (FSM) known as a Mealy Machine. Each SNL state set in a program corresponds to a single FSM, comprising a set of states connected by transitions, which are made up of input (when) conditions and output actions. An entry block just provides a set of actions (i.e. FSM outputs) to be executed whenever its state is entered. Action sequences cannot contain state changes, those are determined only by the transition conditions which trigger the actions.

If you discover that an action sequence needs to change the transition it's implementing, you will usually have to add one or more states and transitions to properly model and code the result. The advantage of state programming is that (in theory) for any series of inputs you can determine exactly how the machine will respond. In practice state programs with lots of states can be complex beasts to follow; drawing up a corresponding state transition diagram can help in these circumstances, but I wouldn't try to detail all of the actions on the diagram.

HTH,

- Andrew

-- 
Sent from my iPad

> On Mar 13, 2015, at 7:36 PM, Eric Williams <[email protected]> wrote:
> 
> I had put a state change statement in the entry block of one of my states and the 2.2.1 compiler kicked it out.  Evidently it only allowed them in transition blocks.  I re-arrange my code to get around the restriction but I was curious: What is the problem with having a state change statement in an entry block?
> --
> Eric Williams
> ALS Controls, LBL

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System