Newsgroups: comp.robotics
Path: brunix!sgiblab!swrinde!ihnp4.ucsd.edu!ames!sgi!wdl1!mail!cps233!dombrows
From: dombrows@lds.loral.com (Brian Dombrowski, 5424)
Subject: Re: What are PLCs?
Message-ID: <1994Apr12.130024.10381@lds.loral.com>
Sender: news@lds.loral.com
Reply-To: dombrows@lds.loral.com
Organization: LORAL Data Systems
References: <2oc8sp$fte@cae.cad.gatech.edu>
Date: Tue, 12 Apr 1994 13:00:24 GMT
Lines: 44


In controls engineering PLC does stand for Programmable Logic Controller.
These are microprocessor based controllers that, in the simplest sense,
take a set of discrete digital inputs and perform some logical function
on them to produce a digital ouput that can represent a condition or
drive a device.  The logic functions are evaluated 'scanned' in quick
iterations by the microprocessor to give a near real-time response.

PLCs are typically programmed in a language called 'ladder logic' which
is a graphical programming language.  Each line in a ladder logic program
is known as a 'rung'.  An example of output that is based on the function
( A OR B ) AND C would look like this:

	       A                      C		                 Output

	------| |--------+-----------| |--------------------------( )--
			 |
			 |
	       B	 |
	------| |---------


Programs that are only a dozen rungs in length are easy to understand.
Programs that are hundreds of rungs long are the biggest convoluted
mess you've ever seen, especially if your used to structured programming
in high-level languages.  A fews years programming in ladder logic will
most likely toast your brain if you've got a degree in CS or CE and
are used to modular design.

These things are extensively used in automated machine tools.  The
auto industry is probably the biggest consumer of PLCs.

Ladder logic is an old concept the originated from relay based control
systems.  And you can blame the UAW for it still being used despite the
advance in computer technology.  This is because that even though educated
engineers design the systems, union electricians usually trouble shoot and
maintain them.  The unions want to hang on to the 'picture book' concept
of ladder logic because there used to it from the relay control days.

Most popular PLCs are expensive and are probably out of the price range
of most hobbyist.

Bd

