I might have screwed RS-232 ports

3 posts in this topic


I just wanna share something that's happened to me this week and want to know your thought about it. Here the topo :

We have a CNC machines in USA that we want to link it to our headquarter DNC's server in Canada in passive mode to get programs from. Passive mode is that we output a program out of the machine that tells the server which program to get ready to send and when we hit input on the machine, the server send the program.

To do so, the output/input method only work in RS-232 communication on this machine. With the distance, RS-232 is not possible. So I bought an Ethernet serial server device that would do the bridge. That device works on 10vdc-50vdc. So I plugged in the source out of one of the 24vdc power supply that the machine has. Now that when things get ugly. I have to say that I have some basics in PLC and electrical stuff, enough to get me going in troubleshooting. But one thing slipped out of my knowledge.

I checked the electrical diagram to check which power supply to get source from. 2 was big DC power supply that feeds coils, lights and stuff. The second feeds switches and proxs. There were also 2 small ones. The first is to supply the Devicenet devices, so I though I would not want to mess with this one. The other one feeds the probes on our machine so again, wouldn't want to touch this one.

The electrical diagram shown that the first 2 big power supply has there Positive grounded. GROUNDED. I question myself why for a little bit but thought that most of the switched and prox would work on PNP. I didn't tought it would bring that supply to be ZERO+ and 24NEG. Tag was written POS24 and NEG24. Big mistake and lack of knowledge there.

So I plugged the network device on one of them, after his breaker of course. After getting everything wired, I turned the power up. Click, the breaker tripped. Hum... but the device was getting power as the ready light has lit a little. So I give it another shot. Click. Now that's no good...

I double checked everything, POS was on POS terminal and NEG was on NEG. Checke with a mutlimeter, getting 24vdc. The rest was the 2 RJ-45 ethernet cable and the RS-232 cable. So I unplugged everything except the supply. Put back the breaker on. Things was getting supply, all was going good. So I thought that the only thing that would make a short would be the RS-232 cable, there's no way ethernet cables would make the supply tripped so I did bring the cable near the housing of the RS-232 port. Bzzzz. A little spark. So that when I suspected that the problem would probably come from the supply because the device has nothing to get grounded. So that's where I reach the multimeter and checked both of the RS-232 housings. 24vdc. Oh my...That's when I suspected the power supply itself. It was giving NEG 24 so my device was getting 0+dc and 24-. The ethernet serial device was still working cause the polarity was good and there were 24v delta between supply terminal BUT the ground of the device (housing and signal reference) was NEG24v.

I decided to get a clean supply so I hooked it to the devicenet powersupply which gives a real POS24vdc and NEG0vdc.

Now, I've been a week to try to get things working without success. I think I've fried both of the RS-232 port. I've checked physically to see if something would be fried but nothing on both RS-232 devices. I tryed the loopback test which mean to jump port 2 and 3 (RX TX) but it didn't give me any response on both devices.

Am I right that I've screw things up? Is there any other test that I can run to see if there some hope getting things work? I don't have access to an oscilloscope or a good multimeter to check the signals.

Learn the hard way or my mistakes they say? Well I sure learn something out of it.

Please give my your opinion on that, good or bad.



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If you have double checked your settings and tried the loopback test with no luck, the port is probably gone, as you suspect.  I am not aware of a better test to try.  The loopback test is the base test I always start with.  If that doesn't work, there is usually a physical port issue.

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And you did not try to transfer signals and data using software and not a hardware solution? As it seemed to me programs that cope with the transfer of data that support som ports, can easily do this without additional hardware.
This reduces the time and amount of work.

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