Re: [foxboro] Ladder Logic
- From: Corey R Clingo <corey.clingo@xxxxxxxx>
- To: foxboro@xxxxxxxxxxxxx
- Date: Thu, 3 May 2012 15:00:52 -0500
>> "We have found over the years that electricians have a much better time
>> troubleshooting ladder logic. Block logic becomes a problem when
multiple
>> calc blocks are tied together all with many connections and 50 steps.
At
>> 3:00 in the morning no one ever has the correct documentation either."
True. I like CALC* blocks, don't get me wrong. Maybe that's because I'm a
long-time HP calculator user. But I like to think it's because CALC blocks
are an interesting hybrid approach to logic -- basic combinatoric &
sequential boolean logic AND some higher-level programming facilities,
that run with bounded cpu/memory in one block scan. (I never bothered with
PLB; we always had PLCs available for fast processing needs, and CALC code
could do most everything else I needed.)
However, documenting CALC code is difficult compared to ladder and
IEC61131-ish function block, and if you don't come from a good programming
background, they are not particularly easy to implement cleanly. As with
most general-purpose programming languages, deciphering someone else's
code takes perseverance at times. The fact that the entire comment line
that is stored in the workfile is not available to the detail displays
doesn't help, either. All these cause lots of 3 am problems.
I'm not sure ladder is the answer, as I believe it was a transition
language to make old-school electricians comfortable with those newfangled
PLC-thingys. But most contemporary PLCs (and increasingly DCS as well) use
some variant of IEC61131 function block. It seems in my experience to be
more easily understood by I/E personnel of all stripes, and is generally
straightforward enough for non-I/E personnel to grok. Like ladder, it's
mostly inherently self-documenting; you don't need the Integrated Control
Block Descriptions Vol.1 handy to decipher the instructions and remember
how they manipulate the stack. And speaking of stack, there are no
"hidden" variables or values, either.
Hey, I like Forth and HP calculators as much as the next geek. But maybe
it's time to think about moving forward. Using an industry standard to
boot is a nice bonus.
Corey Clingo
My Own Opinion :)
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