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Old April 5th, 2009, 05:21 PM   #1
Archie
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Secret/Undocumented PLC Features

After making my latest video and discussing some features about Beckhoff and a few features that are undocumented or buried very deep, it came to mind that it would be good to start a thread and find out if others know of PLC features that are not well known. I'm NOT looking for hacks, cracks, or software pirating, but useful legit features that are typically only found by accident.

Does anyone know of some useful features that are not very well known? Looking for features to any brand of PLC.

This is my contribution on Beckhoff TwinCAT:

http://www.youtube.com/watch?v=2xUttaGaKhE
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Old April 5th, 2009, 07:33 PM   #2
rwy34ils
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I would be happy if all the "known" features worked the way they should!!!!!!!
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Old April 6th, 2009, 08:04 AM   #3
Rheinhardt
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LOL Agreed...
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Old April 6th, 2009, 10:37 AM   #4
Ron Beaufort
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Greetings Archie ...

not sure if this is what you’re looking for, but here’s a trick that my more experienced students like a lot ... suppose that you’re out working on a system in the field and would dearly love to have something like a “simulator switch” to use for temporary troubleshooting purposes ...

the pictures show how to use the keyswitch on a PLC-5 or on an SLC-5/03 (or higher) to control something in your ladder program ... just turn the key between the Run and the REMote positions to switch between ON and OFF ...

disclaimer: obviously this is NOT something to arm Bubba the operator with – because you KNOW he’s going to turn that switch too far and shut everything down ... but used carefully, this can be a VERY handy TEMPORARY troubleshooting tool ...
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Old April 6th, 2009, 06:07 PM   #5
TConnolly
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Using that trick right now in a CLX. The first rung of the main routine in the continuous task uses GSV to get the key swtich position. Status is an INT tag.
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Old April 7th, 2009, 01:47 PM   #6
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Siemens S5 had loads of "un documented" instructions, well some I came across were not in the manuals.
some of their standard function blocks used system instructions not shown on any manuals I found.
In fact some i think were actual instructions but by using the way the ram structure was it was possible to create large DB's on the fly to fill the memory with a DB of several kilowords other RS instructions were used in standard blocks i.e. load a hex value into a flagword that is a MC5 machine code instruction then DO RSxx (process the instruction contained in the flagword).
It was possible to make self modifying code or transfer a datablock containing the MC5 instructions & create a function block nasty but have seen it done.
Once came across a system that checked how much memory was left in the plc & in the startup OB it created a DB just large enough to fill the memory, created the header for the block etc.
this was not to stop anybody from copying as if you uploaded it & tried to download it would say not enough memory.
but as a printer buffer on a critical process but then again unless you new how to re-load the plc you had great problems doing a mod.
Although not strictly an undocumented instruction I came across a function block that passed a timer (or the decimal value of a timer address) this foxed me for a while as the value was greater than 255 (the max number for a standard S5 timer, however I realised what the engineer had done.
Timers are addressed as an 8 bit word for the timer number, so 256 is timer 1 as it only processes the lower byte it sees the pointer as a "1". don't know what was in his mind apart from confusing other engineers but passing 0-255 would have done.
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