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new SCADAPack and existing XTend-PKG radio network have intermittent MODBUS communication

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I am working on a SCADA system that utilizes XTend-PKG radios; one base radio communicates with several remote sites and the base radio is connected to a PC workstation running National Instrument Lookout. The radio is connected through an RS232 to USB converter. In the existing system, the PC workstation and remote RTUs communicate with Modbus Serial. The comms statistics show very strong connection; ten sites are polled every five seconds and there have been no reported communications failures from the client (and there are comms statistics and each site shows >98% comms success).

My project is to replace the existing PC workstation with a SCADAPack PLC. I've copied all the Modbus serial settings (baud rate, parity, etc) from the existing workstation to the new PLC. What is happening is that when I plug the existing base radio into the new PLC, some of the remote sites will communicate but others simply will not; two or three remote sites will communicate with around 90% success rate, but others have literally never had a successful communication but as soon as I plug the radio back into the existing system, every site successfully polls within five seconds. The PLC indicates that checksum errors are occurring every time we have a communications failure. I've had tech support for the PLC check my code, and they say everything is configured correctly and that I should look at how the radios are configured.

On my laptop, I have a Modbus simulator program and when I connect to the existing base radio with an RS232-to-USB converter, I can poll all ten remote sites with no issue.

It seems like there is some sort of issue with the new hardware not going through an RS232-to-USB converter, as in the converter may be providing some sort of latency and now that I'm straight RS232 to RS232, there may be some sort of issue with timing (as in turnaround time, maybe)?

I've been able to pull all of the configuration settings off the existing radio through the XCTU terminal, and I'm working through them but was wondering if there was something I should specifically be looking for and if there is anything anyone can recommend for what to be checking.
asked Jan 20, 2017 in RF Solutions and XBee by defcon.klaxon New to the Community (4 points)
edited Jan 19, 2017 by defcon.klaxon

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1 Answer

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What is the full part number for the radio and what firmware version are you working with?
answered Jan 20, 2017 by mvut Veteran of the Digi Community (15,112 points)
Hi, mvut (you seem to be the guy who is helping me out quite a bit, haha).  Thanks for taking the time to respond to this question.

Long story short, I was able to figure out how to solve the problem.  The solution was to speed up the serial port baud rate; the existing system was 9.6k but I found if the serial baud rate was increased to 19.2k or above, all of the sudden all of the checksum errors vanished.  Not exactly sure why the existing system was happy at 9.6k and the new system wanted faster, but that was the solution and now all sites are reporting in flawlessly.

To answer your specific questions, I don't have the p/n for the radios in front of me (I can look on Monday and respond if you'd like to know) but the firmware version was actually a couple different versions; there was some 2065, and some 2067.  If there is any add'l info I can provide just let me know.  Thanks!
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