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SoftPLC Partners Solve Timing/Integration Problem
Engineers frequently encounter difficulty in control applications when multiple
tasks need to occur in a coordinated manner. A food processing OEM found this
to be the case in one of their machines. In the past, the OEM used an SLC-500
PLC for machine control, in conjunction with a Basic module which communicated
to a high speed Servo via an RS-232 channel. Timing problems resulted because:
(a) the data coming into the Basic module was limited to 19.2Kbaud, (b) the
Basic program needed to run a large number of calculations for the Servo before
it passed the information to the PLC, and then (c) based on where the PLC
was in its cycle, the control did or did not happen in the proper sequence
or fast enough.
The solution chosen by the OEM was to use SoftPLC, a PC based open architecture
control system that runs "PLC-5 like" ladder. The servo calculations
required were extremely lengthy and needed to be done within 1/100
second, so rather than burden the control PC with this, they were still done
in a separate PC in Basic. The hardware selected was a Ziatech STD-32 multi-processor
system--1 CPU running SoftPLC for control, 1 CPU performing the calculations,
and a third CPU running an operator interface program and the SoftPLC ladder
programming software, TOPDOC. High-speed APIX I/O was selected, which fits
right in the Ziatech backplane. In a global memory area, information is passed
between the SoftPLC and the Basic program.
The OEM was able to convert the usable SLC-500 logic to run in SoftPLC, and
wrote a C function to read/write to the global memory area - all in less than
one day! All objectives for the machine re-design were reached, including
intercommunications between the various control elements, cost of machine,
processing speed and reliability of operation.
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