TC90x modbus problem when 2 buses are connected to Delta PLC

[afecomt]

Dear all

we are facing an issue with PLC Delta AS320 which has 3 RS-485 isolated ports.

We have 16 TC90x thermostats in "bus 1" and 17 TC90x in "bus 2". All modbus addresses are diferent.

TC900 has a no-isolated RS-485 interface and a 1.4 charge

( https://community.se.com/t5/Building-Automation-Knowledge/TC900-Modbus-bias-termination-and-number...)

TC90x are powered by a 230 to 24 VAC transformer, one per bus (two in total). No earth grounded, no secondary neutral to earth, done and designed by contractors (not related to us).

If any of the buses is connected alone, the devices communicate. If both buses are connected, the nightmare appears and the connection is totally caotic.

Can anyone help us or guide us in order to detect where is the error?

Thanks in advance.

[Bastian_Schmitz]

Did you connect 3wires for Modbus or 2wires? If the Gnd of the delta plc is internally connected I would suggest to remove the hand connection on line 1 and 2. 

[afecomt]

Thansk for the answer!

In this case, TC-90x thermostat does not have a GND terminal, only RS-485 A & B

any idea?