PLC textbook?
Greetings Dan,
sorry to let you down ... but I’ve never been able to find a textbook that would work with the unusual (weird?) methods that I use to teach PLCs ... my classes are usually five days of hard-core nuts-and-bolts training intended for plant maintenance technicians ... the idea is to take a guy who might start out knowing absolutely nothing about a PLC and send him back to the plant the next Monday morning ready to go to work with all of the basic skills that he needs to handle the PLC end of his job ... and that includes everything from discrete (on/off) signals - all the way through analog signals – and interpreting ladder logic programs ...
all of the PLC textbooks that I’ve ever come across are targeted more toward classes which last for a full sixteen-week college semester ... so the truth is, even if I COULD find the “right” textbook, the students in my one-week “high-impact” courses certainly wouldn’t have enough time available to read it anyway ...
so to answer your question, the “textbook” that I use for my classes is a three-ring binder containing about 350 pages ... most of this is made up of material that I’ve developed myself over the years ... and a collection of “important pages” from various online manuals for processors, modules, software, etc. ... just the type of material that the students need for the hands-on lab exercises during my classes – and that they’ll require in order to survive once they get back on the job ...
but even though I personally can’t help you out with a PLC textbook recommendation, my distinguished colleague, Mr. Randy Ludington, might be able to provide some assistance along those lines ... I was able to take a quick tour of Randy’s lab last week while picking up the
PLC hardware that he so graciously bestowed upon me ... he mentioned that the textbook he uses for his PLC classes was written by Frank Petruzella and is published by McGraw-Hill ... since I haven’t had a chance to look over a copy of it myself, maybe Randy will step in and continue the discussion from here ...
as for PLC textbooks in general, I’ve found (as you mentioned) that they all seem to “lose steam” as soon as they hit the transition from discrete to analog signals ... to be fair to the authors, one perfectly valid reason for this is that each PLC manufacturer has different ways of handling these more-advanced processes ... and even considering just one manufacturer, the analog procedures can vary widely from one platform to another (for example: the procedures for handling analog signals for the Allen-Bradley PLC-5 platform are quite different from the procedures required for the Allen-Bradley SLC-500 platform) ... and so writing a single “one-size-fits-all” textbook would be an extremely challenging (if not impossible) task ...
as a matter of fact, I actually have TWO slightly different “textbooks” ... one which I use for the five-day “PLC-5/RSLogix5” class ... and another which covers exactly the same type of material – but for the five-day class for the “SLC-500/RSLogix500” platform ...
I’m sorry that I couldn’t nail this “textbook” issue down for you ...
as for the 1746-NT4 module that you mentioned, as you might already know, you can find the user manual for it online
here ...
the biggest problem that most people seem to have with one of these SLC “configurable” type modules is in getting the “big picture” ideas sorted out ... the thing to remember is that the configurable module is able to “help out” the SLC processor by doing some of the basic math operations “on-board” before the input signals are sent over to the input image tables ... this allows the module to handle different types of thermocouples (J, K, T, etc.), different temperature scales (Celsius or Fahrenheit, etc.), and other things along those lines ... the question is: “how do we (the programmers) tell the module how we want it to work?” ... the answer is: by putting the correct “code numbers” into the module’s configuration words ... and these “configuration words” will be located on the SLC’s OUTPUT data table ...
let’s take an example ... suppose that your 1746-NT4 input module is located in slot #3 of the SLC’s local chassis ... in that case the signal data (the thermocouple readings) from the four available input channels would be located in these four words: I:3.0, and I:3.1, and I:3.2, and I:3.3 ... there will also be four extra input words (I:3.4 through I:3.7) assigned for this module ... these will eventually contain “status information” about the individual channels ... we’ll ignore these for the rest of this discussion ... the number “3” which shows up in each address is used to denote the module’s slot number ...
so far this should be making pretty good sense ... specifically, we have an INPUT module ... and we have INPUT (“I”-type) words to handle the signal data coming in from the module ...
now let’s talk about the configuration words for the input module ... when you first add the 1746-NT4 module to your program’s IO Configuration list, in addition to the 8 words set up on the INPUT table, there will also be 8 words automatically set up on the OUTPUT table ... these will be O:3.0 through O:3.7 ... the last four words (O:3.4 through O:3.7) will be unused ... the first word (O:3.0) will be used for the configuration of the module’s first input channel ... the second word (O:3.1) will be used for the configuration of the module’s second input channel ... and so on for all four of the module’s channels ... the fact that we have OUTPUT words associated with an INPUT module sometimes confuses people the first time they run into this type of system ...
now all that we have to do is figure out exactly what “code values” to use for those four configuration words and we’re just about finished ... the best way to tackle that particular problem is to print out a copy of page B-4 from the module user manual (linked above) ... then mark up the chart by penciling in the desired set up information ... then finally go to the output table and manually enter the proper 1 or 0 pattern of bits for each configuration word ... then turn the system on and test it out ... if you’re lucky you should be home free ...