Modbus over RS232 - odd connection problem

[AdrianEL]

OK so here is the setup;

RedLion G3 HMI Panel is communicating over an RS232 port using Modbus RTU protocol to an AB Micrologix 1400 PLC. The communication is not a direct wire but via a radio link at 9600,8,N,1. All timeouts are set very high (like 5000ms) to give propagation time.

I found that;

1. Using a PC with a Modbus Slave simulator in place of the Micrologix PLC I can send data up and down the radio link with absolute reliability and the RedLion G3 requests and receives responses and puts data in the right tags etc.

2. Using a PC with a Modbus Master simulator, plugged directly into the MicroLogix PLC and simulating exactly the same Modbus sentence as the RedLion G3 the PLC responds and sends back the data requested.

3. All the cables used are those in the final installation to prove they are working.

BUT

Plug the radio directly into the Micrologix PLC and there is no communication Radio shows receive transmissions from the G3 but the PLC does not respond indicating that it thinks it is not receiving (anything?) a valid Modbus sentence.

I have tried swapping Tx and Rx on the cables just in case.

It is like the RS232 port on the Micrologix and the port on the Radio are incompatible. Is that possible? Anyone got any other ideas I could try.

Thanks

Adrian

 

[OkiePC]

What is the make and model of your radio? And, what is the frequency?

 

[Peter Nachtwey]

Modbus RTU is very timing dependent. I would try the A-B protocol that uses start and stop characters.
Also, how are your serial port cables wired up? There are wires like DTR, RTS, and CTS that must be connected correctly.

 

[AdrianEL]

Thanks for the replies -

The radios are 169MHz on a licenced frequency. I am certain that the radios are working.

I have wired only the Tx Rx and GND connections on the RedLion G3 and the AB PLC. There does not seem to be a need for any other signals because using only these three connections I can communicate with both devices in both directions perfectly using a PC.

 

[OkiePC]

Thanks for the replies -

The radios are 169MHz on a licenced frequency. I am certain that the radios are working.

At that low frequency, you may need a slower baud rate. When we use UHF (~450 Mhz), we have to limit the baud to 4800. I have not dealt with any VHF radios, so I cannot say what the baud rate restrictions might be. I would also follow Peter's suggestion about trying DF1 protocol, just to prove all the connections. Some data radio modems will handle the Modbus timing just fine, and they usually advertise that fact. Those that don't advertise it...who knows?

I have wired only the Tx Rx and GND connections on the RedLion G3 and the AB PLC. There does not seem to be a need for any other signals because using only these three connections I can communicate with both devices in both directions perfectly using a PC.

I normally only need these signals, so I think your are okay there.

 

[AdrianEL]

Again I appreciate your reply -

The radios are X8200 models from Warwick Wireless who have been very helpful. They programmed the radios for us and selected the baud rate. They also carrie out a site survey to select the frequency etc and we have a wider than normal bandwidth in our licence from OFCOM. Basically the radios were plug and play - once we found a blown fuse they just worked fine. I can reprogram the AB PLC and the G3 panel to a slower baud rate but what puzzles me is that when I replace the AB PLC with a simulator everything works OK.

Oh and this - we are polling the Modbus slave only once every 5 seconds. The radios have a data buffer so have plenty of time to transmit the Modbus sentence and they know to wait a few ms to check that they have all bytes to send.