from the Forum department...
Multi-Slave Modbus Network Problems
Communications systems and equipment. topic
Posted by Chris on 5 May, 2012 - 12:54 am
I've got a situation where I have 3 modbus slave devices connected together as 2 wire 485 which I have coming into a 485 to 232 converter with a short cable which is connected to a serial Freewave Radio (talking 232). The other radio is connected to a PLC which is the modbus master. I had these devices setup correctly originally and was able to both send and receive data from all 3 slave devices. However, after about 12 hrs to a day of communicating one or two of the devices stop communicating, although one device always stays connected. The only way I've found so far to cure the problem is to reset the devices that are no longer communicating. I just read on several websites that modbus 232 only supports 1 master and 1 slave - so my question is could this be part of my problem since I am using a 232 to 485 converter in my setup? Any other suggestions to the problem would be helpful as well.

Posted by Andrzej on 7 May, 2012 - 6:11 am

Try check the communication with the 3 slaves on RS485 without the radio. You can use Modbus Tester from to test this.


Posted by Lynn August Linse on 7 May, 2012 - 11:43 am
This is a classic example of '2-wire RS-485' when 3 wires are required. 2-wire RS-485 without ground wire is only predictable when ALL of the devices involved come from a single vender.

Likely your 232/485 converter has a floating signal ground, and since you are only using 2 wires, you don't have a shared signal reference with the RS-485 of the 3 slave devices. Eventually, the RS-485 voltages shift enough due to ground potential that the 485 chip over-voltage prevents detection.

You need to either link the signal ground of all 4 devices together (the converter and the 3 slaves), or at least try to reference the converter's signal ground to the power supply ground for your 3 slaves.

Posted by Grant Douglas on 6 September, 2012 - 7:44 am
Hi Chris, we have a similar problem onsite. Did you solve the problem and if you did what was the cause and solutions to resolve the problem.

Grant Douglas

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