It smoked

[Yoseph]

      I am new to PLCs and I am willing to do what ever it takes to learn it.
I have a T100MD. I wrote a one line ladder logic with a NO contact and
a relay, just to see if I can monitor the out put port with a scope; just when
I did a program transfer to the PLC, a resistor behind the DB9 connector
started to smoke. Now I can't talk to the PLC. Did I get a defective PLC
or was the ckt that distructive?
                                                        thanks

[support]

you may want to check your PLC power supply and its wiring. The reason is as follow:

That resistor you mentioned connects the ground between your PLC 0V and the PC's ground. It serves to tie the two side ground potential together and to limit the ground current flow between the two devices when they are connected together. Under normal circumtances, there is very little ground current flow if the two sides power supply are cleanly isolated from each other.

It appears that you have a ground fault, meaning that your power supply to the PLC is not a clean isolation from the PC's power suppy. You need to check the power supply. You can verify this by measuring the voltage between pin 5 of the PC's serial port and the 0V of your PLC's power supply (before the two are connected). Normally these should be just some mV of random readings, but if there is a ground fault then there is a fixed voltage between the two devices and if the voltage difference is large enough it will burn the resistor after prolong exposure. Put your meter to both AC and DC range to make sure that there is no AC leakage to your PLC power supply.

Normally if there is the a ground difference of a few volts the resistor may turn brown but it may not damage it. You can measure it with an ohm
meter. A good resistor is about 100 Ohm. The resistor is supposed to protect either serial port of the PC or the PLC from being damaged. However, if the voltage difference is quite severe then it may still damage the PC serial port.  If the resistor actually grows and flame out, it means that the ground voltage difference is very large - it could be an AC voltage leakage somewhere. The resistor heat up according to the formula:

     P = V^2/R  (voltage square divided by resistance).

YOU MUST CHECK OUT THE GROUNDING DIFFERENCE before connecting your PLC to the PC again. You may want to remove your PLC from your
panel and test it with a small desktop power supply that you have tested previously to work well and try to use a PC to communicate with it.

Try to use a power supply that has a plastic casing (such as the PS-DR4524) instead of metal casing and don't connect any of the DC voltage lead to the casing which may be connected to the AC ground.

[Yoseph]

     I have made the measurements as you said and it is
150mV (milli volts), give or take 5mV AC. On DC setting
it is 0Volts to three decimal places. The measurement was
between pin 5 of my PCs serial port to the ground terminal
of the PLCs power supply. I have also made a similar
measurement to a serial port of another PC and got same
results.

    I had the PLC powered up and connected to the PC for
several days while I was reading up on the manuals to
educate myself. I had reset the RTC many times to clear the
RTC error alarm.. There was no indication of any problem at
any of these  times. I did not see any problem  until I did
a program transfer to the PLC;  that I do not understand.
Shouldn't the grounding problem have affected the PLC even
before I did the program transfer?

    I would be glad to try another power supply if you can tel
me who the supplier is for power supply you mentioned.  I am
willing to try everything and anything to get this project done.

                                   thank you for taking the time to help.
                                                     Yoseph

[support]

We had already received your returned parcel yesterday. We immediately tested it and found out that both the PLC and the LCD display are working fine. We have connected the PLC to our desktop PC and let it communicate via the serial port continuous for nearly 24 hours and there was no problem at all. The conclusion is still that somehow the PLC power supply and the PC power supply are not properly isolated from each other,  resulting in large ground potential difference that burned up the resistor.

Even though the resistor appeared burnt, it is still working properly with the correct resistance value. However, for cosmetic reason we will replace it.

More information about the power supply we sell can be obtained from the following website:

1) DR4524  - 24V, 2A output - $49.00

   http://www.meanwell.com/product/DR-45/default.htm

2) DR7524  - 24V, 3.5A output $79.00

   http://www.meanwell.com/product/DR-75/default.htm