LD for PLC

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Should we still use these PLCs with
RLL (Relay Ladder Logic) language?
Yes, and for very good reasons!!

By: Hillel Hachlili

PLC (Programmable Logic controller) is considered by many
professional control people to be a very old machine (which is true, they
are in use for about 35 years) with a primitive operating system and
a programing language (the Relay Ladder Logic). Base on these immature
conclusions they look for "better control platforms" like the very
popular personal computers with add on hardware and newer programing
The reality proves the opposite. PLC by almost any aspect
fit the latest concepts in the modern computerized world.

The first major misunderstanding of the concept behind PLC is
thinking of it as a "Relay Ladder Logic" machine rather than
a "Scan based" machine. The scan is the heart of the PLC operating
system more than any thing else. The most modern operating systems
are the multi-tasking ones (UNIX ,WINDOWS etc.). Since most of the
current computers are still based on one CPU (the PII or PIII for example),
the "time sharing" is the only way these machines can perform
more than one task.
The SCAN in PLC is exactly time sharing operating system,
and should be considered as such. The PLC scans all the tasks under
its control every 15 - 25 millisecond, which is consider as one "Scan"
The order of the tasks in the scan and the scan length are easily
controlled in most of today PLCs. All the I/Os and other resources (like
data registers, timers, counters, etc.) are easily shared among all the
tasks, and each task can use others tasks' results.
Since every line of code in the PLC program can deal with an independent task,
the number of tasks in this multi-task platform is almost unlimited.
Everything in the PLC platform is monitored, calculated and controlled every
scan. In other platforms it takes a complicated interrupt system with
many interrupt priorities and interrupt masking to do the same.
If you compare it with other multi-tasking operating systems you'll be
convinced that PLCs are still current.

Graphical Operator Interface:
Few companies try to convince the Control Engineers' Community
that it's time to shift to "Plain English Control Language"
and make history of those "strange relay symbols".

Yet when the computer media shift more and more to
graphics using icons, graphic switches, windows etc., isn't PLC's
relay symbols just another graphic way to present logic combinations?
The RLL symbols...
Should we still use these PLCs with
RLL (Relay Ladder Logic) language?
Yes, and for very good reasons!!
By: Hillel Hachlili
LD for PLC - Trang 2
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