We are using Modbus RTU from/to the control center (master) and 4 pumping stations (slaves). The master radio in a MDS 9790.

Currently, only the master, repeater and one remote are up. We were first unable to get data either way but fixed that by breaking/splitting up the messages from the master to the remote.

We still cannot get any data back from the remote. We have tried breaking up the data requests to the remote but still nothing comes back.

Any help with this issue or with configuration of the MDS radios is greatly appreciated.

Hassan

 

Unless the radio modems can guarantee packets are sent and received with no modem induced inter-character gaps, it's not likely that you'll get it to work with Modbus RTU, since RTU uses inter-character gaps to detect end of packet.

Try Modbus ASCII if possible.

 

Increase your buffers in the remote 9710 radios and the 9790. We set ours to 10. This will take care of the intercharacter timeout. We have a 9790 radio repeater network working with the modbus RTU protocol and a mixture of PLCs. Note:If you use modbus ASCII in a large system you throughput will be terrible.

If that doesn't work there is a possibility that the reply message is confusing the remote PLCs. The remotes will "see" the modbus replies as well as the requests. It doesn't happen often but if in doubt use two of the remote radios and set up a test system without a repeater. Set up one 9710 radio as the master and one as the remote. Get communications to work in this system using one PLC as the polling master and one PLC as the remote. When this is working properly insert the repeater into the system and reconfigure the radio at the master PLC. If it stops working with the repeater then more investigation is needed.