Problem with RS232/RS422 for connecting PLC with a display

[Philip Kilpatrick]

I have a CLX PLC and using the serial port to operate message displays. This system has ran for about 8 years with no problems other than replacing a display. Recently something happened and none of the displays work. I connect a display to the PLC directly and the display works correctly. I connect my computer to the PLC serial port and I can read and write to and from the port. When I connect the RS422 convertors it all stops, I have replaced all the convertors, still does not work. The power supplies seem fine at around +-12vdc. I have set up test wire wire new convertors and still will not work. I even changed to RS485 to test but still does not work.

You you have any ideas? I have been working on this for almost two weeks, getting a bit flustrated.

Thanks

 

[wp100]

rs422 problems

Hi,

I assume the converter and display are on a pcb - are there any other parts that may have failed ? - where and what voltage does the display get is power from ?

Have you examined / resistance checked the pcb for any broken tracks / dry joints / bad serial cable ?


Not familiar with that controller - but is there any way a parameter to the PLCs serial output to the display could have changed so making comms down that line fail ?

 

[bohumilfulin]

how far can rs422 work

try to use galvanic isolating rs422/rs422 converter. I thing the problem is noncorrect ground loop.

 

[barry]

rs422 telephone wire

I have run into problems with 232-422 convertors before. Depending on what you're using, some of these converters (B&B for example) actually don't meet the RS-232 requirement (they require more than the minimum guaranteed input voltage). Can you monitor the 422 output of the converter with an oscilloscope and verify that signals are coming through?

 

[Philip Kilpatrick]

rs232 to rs422 serial converter does not work

Thanks for your answers and questions.

All the converters are from B&B, they are the same model that was used before.
I got a software program from B&B to test the converters with a loop back connector, it never worked for any of the conveters, I thought it was something I did wrong, maybe not.

I will look into another manufacture.

We have verified the PLC port and everything is good and the same as the last 8 years. We go from serial port to a conveter, the rs422 side then goes to 4 other ProLite Message boards and at each Display we then have a RS422/RS232 conveter . None of the displays work, the common link is the first converter, this was changed two times. The then changed all of the converters. The power supplies are 12 vdc, with voltage varation from 11.5 to 16.5 on some of them.
The RS232 serial cable is fine, I use this same cable for testing, when I connect a display directly to the RS232, it works fine, the only reason I use the converters is the distance.


Thanks

 

[banjo]

hyperterminal rs-422

This sounds like a cable fault. RS422 is a five wire interface. Two differential pairs and a ground. Often the ground is forgotten. If the two pieces of equipment have a connection to a common earth ground, then it will work at least temporarily. However, if this earth ground is lost an the cable contains no ground wire, it will stop working.

My advice is to get the longest spare piece of cable you have and use it to connect one display to the controller. Leave the rest of the signs and the original cable unattached. Fire up the system and verify that this display works. If it does not work, swap displays and converters until it does. Now you have a known working pair. Swap in the rest of the displays and converters until you have proven the condition of all the hardware. If you have not found any faults, the only thing that remains is the cable.

How long is the cable? Any chance that it has been cut? Perhaps it was cut by another party and then incorrectly spliced? We used this same kind of scheme several years ago at another company where I worked. The cable lengths were very long. Our baud rates were low, so we did not always use twisted pair wire. Sometimes they would use "silver satin" which resembles telephone wire. Our signs would quit working and we would often find that a contractor found our wire, assumed it was a telephone signal, and tapped into it!!! Let me tell you that RS422 does not work if a POTs phone is connected to it!

 

[Philip Kilpatrick]

b&b loop back test not working +rs485

The longest run of cable would be about 400 feet. I have done simular testing but with a very short wire in the shop. I do have a new roll of wire that I will try. I will also check the grounds in all the existing cables.

Do you know how reliable a loop back test is, I tried this with several converters but never see anything return in the receive window. If I connect directly from the PLC it works fine with no converters. I have been connecting a wire from the Tx to the Rx of the connector.

Thank you

 

[banjo]

connect my rs422 to my computer

Yes, loopback should work. B&B makes good equipment and has for many years. If you converters are powered from the RS232 line, then you cannot just unplug them and jumper transmit to receive as the unplugged converter will not have power. You can contact B&B support for details on looping back your particular converters.

Here are some other ideas of how to troubleshoot. Get two computers some cable and two of the RS232-422 converters. ( One or both computers can be laptops, and old ones work fine for this.) Connect the converters to the computers and then attach the cable as normal. On both computers, open a serial port terminal program. Windows ships with one called Hyperterminal. There are many others, some are free. TerraTerm also works well. Configure the serial port terminal program for your baud rate and whatever serial port you attached to. Turn off any hardware handshaking. Also turn off any local echo of characters. Tell both terminal programs to "connect", in Hyperterminal, I think you have to click a button that looks like a telephone. Now start typing on one computer, if the cable and converters are good, then the output should appear on the other computer. If you go to the second computer, you should be able to type messages back to the first.

Lets call the computers "A" and B". Take them to the customer site and connect "A" at the PLC end and connect "B" at the first display. Again open the serial port terminal software, configure as before and type on both keyboards. If "A" can type characters to "B"'s screen, but "B" cannot type to "A", then the return path is broken. If "B" can type characters to "A"'s screen, but "A" cannot type to "B", then the transmit path is broken. If neither can type to either, then you got a shorted or cut cable.

If either computer shows characters but they are wrong or jumbled, the cable is got a broken wire or an incorrect splice. These are twisted pairs, if the cable is connected wrong and one of the transmit wires is twisted with a receive wire rather than its mate, then really wierd stuff happens.

Once you have the two computers talking over the customer's cable. Reattach the PLC in place of computer "A". And monitor the screen of computer "B" when running the PLC. You should see the traffic on the cable. If it is binary data, it will appear as a bunch of nonsense characters. This is OK. The big thing is something came through.

 

[Philip Kilpatrick]

rs422 baud rate on oscilloscope

I have sort of done part of this but will do it to the extent you have suggested. It will be next week before I get back to the site.

You have given me lots of information to try.

Thank you