from the Automation List department...
Communication Problem Between PC-PLC
PLCs and related questions. topic
Posted by Joska on 10 June, 2003 - 5:39 pm
I work with omron cx programmer v3.0, but since I
use laptop pc and USB-Serial converter UC-232A
always have communication problem between PC and

Many thanks for solution!

Posted by Bruce on 12 June, 2003 - 4:59 pm
If you are using a "dumb" RS-232 to RS-485 converter, the problem could be due to unfaithful control of the RTS line of the converter by the USB serial converter. I had this problem with an Edgeport USB to Serial converter as it did not maintain the correct phasing between assertion of the RTS line and transmission of the serial data. I had to switch to a PCMCIA RS-232 port in my laptop to get true serial port equivalent performance.

If you are going straight to the PLC by way of RS-232 then it could be something else. You might still consider switching to a PCMCIA serial port converter instead.

Posted by Anonymous on 12 June, 2003 - 5:10 pm
Good day sir!

i also experienced the same problem but with a SIMATIC 505 PLC. what i did was to lower the communication baud rate for both the laptop and the PLC instead of using a higher (default) baud rate. You could try this method as i heard that usb to serial converters sometimes don't perform well with faster baud rates. It worked for me but i am not sure if it will work for you.

Good luck and god bless.

Posted by David Wooden on 13 June, 2003 - 11:25 am
Things to try:

1) Reduce the baud rate.
2) Increase the response time-out.
3) Reduce the frame length.

You could also try the Omron USB to serial converter. (Part number is

Best of luck,

David Wooden
Senior Software Engineer, Systems Integration
TAS Division of Omron Electronics LLC
Office: (847) 884-7034 Extension 432
Fax: (847) 884-9383

Posted by BobB on 16 June, 2003 - 8:41 pm
I could not use the USB/232 converter that came with my HP laptop (RTS problems). Bought a Wiretek brand and it works OK.

Still have problems sometimes with both Syswin and CX-Programmer. I have a fairly long 232 cable so I can get clear of bigfooted electricians on site when commissioning. I have found that if there is a loop in the cable (crossover) I have trouble. Sometimes up and down loading takes 4 or 5 goes with a large programme. If you can afford it, buy a PCMCIA serial port. They are more reliable. Also. keep your cable short. A good PCMCIA/232 can be purchased from BB Electronics.

Posted by David Adams on 19 June, 2003 - 1:21 pm
We always had good success with long (400-500 ft) when we used RS422 for the cable and put converters on either end.

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