Hello all,

We have been using modbus RTU over utilinet radios for years. We have one master (FIX32) polling many Modicon PLC's (very reliable). We want to add another base radio to a new Quantum w/ Xmit loadable to be a second master.

When I use my PC Modbus software as a master to the radio, I am able to communicate with three Compact 141's (nodes 5, 6, and 8). I am also able to use the Quantum as a master to a software slave using a direct cable connection to my PC. The problem is when I connect the Quantum to the radio, I get timeouts. I know that normally only one master is allowed, but with the Utilinet radios this is allowed.

It has been suggested to use ASCII over radios instead of RTU because of the 3.5 character times issue. Because RTU mode has been working for so long I hestitate to convert the entire system to ASCII.

If the 3.5 character times is an issue, why does FIX32 work in RTU mode?

Might there be a difference in the drivers that allow FIX32 to work, but not the Quantum?

Any suggestions?

 

The problem is likely the "packet" nature of your Radio modems, so perhaps there are some new settings in the modem or your application you can
select. For example, if the radio can handle 64 or 128 byte chunks, you may be able to solve this problem by limiting the size of each Modbus/RTU
message to fit within a single packet. How you do this is application specific - should be easy on a XMIT block.

For example, I had one customer using satellite and they found if they limited each request to 54 registers they had a flawless error-free system.
55 or more registers gain them a error rate up to 50%.

Best Regards

Lynn August Linse, alist (at) linse.org
Foothill Ranch CA 92610

 

MODBUS protocol is a Master/Slave protocol. One Master many Slaves. You can not just through in another Master and hope that is will work. Try connecting one Master at a time and see if they work. Also try increasing the timeout on both Masters. This will give both of them more time to get a response if the radios allow for more than one Master. You said that with the Utilinet radios this is allowed. Please explain?

I have used a system with two Masters before. It required that I use a Black Box to switch between Masters. The first master would grab the connection and hold it. It used the RTS and CTS lines for this. The RTS was held until the Master received a response or timed out. Then it would allow the other Master to get control. If the other Master exerted it’s RTS while the first Master was communication, it would not get a CTS until the first Master had finished. Since you are using radios, I don’t think it is possible to coordinate the communications between the two Masters. Other alternatives would be to use a separate channel for the second Master and add a second slave port to the slave devices. Or some sort of buffered mux at the slaves.

The question about ASCII vs RTU. Changing to ASCII may not help. It would just make the overall communications time slower. Some implementations of MODBUS ASCII do not adhere to the 3.5 inter character timing and just rely on the overall timeout.