Modbus RTU Communication Stops

I got some room thermostats from China. Those have standart room control functions and commmunicate with Modbus RTU. I set up a good RS485 line for those. terminating resistor, maximum 32 devices per line etc. The problem is that these devices randomly stops communicating and only way to start communication with them again is reset their power. Even when I try with only two devices , that problem still occurred after some time. I tried communication with many master devices and softwares. Even I myself wrote a master software to read/write with them. But still no luck. So I will appreciate much any of your ideas around this problem.

I think the source of your thermostat is the problem.. maybe try a different manufacturer!

I have run into a couple of different Modbus slave devices that will freeze if polled too fast. Your master should have a method of adding a delay between polls and/or a delay at the end of each poll cycle.

Another cause is common mode voltage. See this forum thread for details:

http://www.control.com/thread/1404403082

poll interval is even 5 seconds and then interval for poll cycle is 30 seconds.

Are all the slave devices the same brand/mode/firmware? If not, how many different types?

How many slave devices are on the network link?

Is the network terminated with a resistor at both ends?

Is there a firmware update for the Modbus option for the serial slave devices?

Have you tried 'sniffing' (monitoring the network) with either software or an oscilloscope to find out if the failure is whether the master fails to continue to poll or if the slaves do not respond?

Have you tried running a single slave for a long periods of time to see if a single slave works, whereas multiple slaves do not?