I have configured a Unistream PLC as Modbus Master (periodic working mode) and connected with 11 Modbus Slave devices using RS485. Communication works five or six hours as it must, but suddenly Modbus RS485 communication hangs up. It stops to polling just one (random) slave device and does not poll other slaves at all. After hard reset of whole PLC (power off and on), it starts polling all slaves normally. I make tests with "Modbus Poll" software and PC, result was that the communication with all slaves working correctly, but Unistream PLC as master does not.
By the way, there is no possibility to adjust "retries count" and "time out" values for Modbus communication. This is very serious disadvantage. If a one of the slaves does not answer to the master requests, the communication with other slaves must still work!