Internet PLC Forum
General => Technical support => Topic started by: Joel Moore on December 07, 2006, 07:18:45 AM
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(I put "failure" in quotes because the PLCs are still working as far as I can tell.)
We fried the resistor located next to the COMM1 port on two T100MD888+ PLCs recently (both part of the same machine). I had done this once before on a 1616+ PLC and when I emailed Triangle back then about this resistor I got the following response:
The large resistor next to COMM1 reader is a current limiting
resistor that connects the ground voltage of the PLC and that of the
PC (or other devices) being hooked up to COMM1. For some reason your
connection caused a ground fault on the system, meaning that the
PLC's ground could have been connected to a power source non-
isolated from the power supply of the PC. When the devices are
connected the ground potential difference between the PC and the PLC
cause a large current to flow through the resistor and cause it to
burn out. Usually you just need to replace the resistor and check
the power source to ensure some isolation.
In the most recent situation it appears the problem (as far as we can tell) was that someone put a lock washer under the mounting screws and one of them penetrated the solder mask and made contact with a trace. I believe it is a ground trace -- it's the one nearest to the mounting hole next to U12, on the component side of the board.
So if I'm analyzing this correctly the PLC's ground was tied to the machine's earth through the screw (since our chassis is tied to earth). However, our machine uses power from a 208 outlet while the PC is plugged into a seperate 110 outlet. So it seems there was a difference in ground potentials between the PC and the machine (a seperate matter we need to address).
Does this make sense?
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That is a definite possibility. By right PC's DC power supply should be isolated from the AC system. But grounding of the casing can cause the system to assume a certain ground potential and if that is at a fixed potential with respect to the machine's ground then it can burn up the resistor.
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What's the value of the resistor? If it's fried, probably can't measure or read the colors. :)
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It is a 100 ohm 1/2W resistor.
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Thanks, I'll write it in the manual, probably never need it, but if in 20 years or so, will be easy fix. :) :)