"Tom Gardner" wrote in message
...

Lloyd had caused me to remember a Trilogi PLC that I had bought many years
ago for some project that got shelved and had forgotten I had. So, with all
the wonderful advice in my last thread, I found the PLC and started
reading.

I found a nice, free beginner's book in ladder.
http://www.tri-plc.com/reqplcbook.htm

I've passed my first stumbling block and I can "see" it in my head once I
understood some misconception I had. PLCs are WAY cool! I imagine all
kinds of possibilities. The only PLC machine we have now was farmed out to
a "guy" that one of the mechanics knew from a previous job. This guy did a
great job and the machine works just fine but I haven't even looked at the
program yet.

Thanks for all the ideas and support guys!

They sure beat the heck out of wiring and re-wiring a bunch of relays!
Sometimes I get something running quick and dirty, then clean it up and
refine later. I have found hind sight is generally better than fore sight
so after the machine has been running a while I see if there are ways to
make it better.

Flow charts are more like a series of steps while the PLC is more like
everything is running in parallel. Once you get your flow chart steps
mapped out to PLC conditions you can make things work like your flow chart.
Starting out I had some problems with things executing when I didn't want
them to, like performing a calculation, one shots or edge detection type
instructions (depends what they are called on your PLC) solve that problem,
they can be used to cause something to happen once per cycle.

Example
if step3 then increment some number.

this will cause the "some number" to increment every scan while the program
is on step 3. To solve this you would do something like:
If step3 and one-shot then increment some number.

The one-shot will be true only one scan cycle when "step3" becomes true
causing "some number" to increment one time per "step3".

When I was doing PLC programming for customers I had to use the PLC they
wanted. I learned to program using mostly a few instructions that were
common to most PLC's, it kept me and the customers technicians from having
to learn their PLC's unique advanced instructions.

RogerN