I found an good article on structured PLC programming in
Control Engineering magazine. Go to their web site (
http://www.manufacturing.net/ctl/) and search for "Essentials of Structured PLC". The search should take you here:
Search Results. There will be a couple of links to the archieved article:
"The Essentials of Structured PLC", by Dadla Ponizil from September 1, 2001.
If you click on the "tutorial" link you'll be taken to a summary of the article,
The Essentials of Structured PLC: the tutorial. The article refers to the methodology as Station Mapped Programming. The author uses a conveyor as an example, breaking it down into a state machine representation and then creating ladder logic. There are state diagrams and ladder logic based on the example. The author's e-mail address is at the end of the article. I received a more detailed version of the article via e-mail. You could try the same.
I have found that the code implementation of the logic depended upon my needs (sequential logic or simultaneously running state machines) and the capabilities of the different PLC brands.
There are quite a few threads here about state logic. Perhaps you could search this forum and read some of those for examples. Try this one:
http://www.plctalk.net/qanda/showthread.php?threadid=188
and this one:
http://www.plctalk.net/qanda/showthread.php?threadid=7271
If you already have a code method, great. Otherwise, maybe you could start with the article's or one of the threads' examples.
I know this doesn't provide any immediate help, but maybe it's worth a start.
