Just noticed that you’re using Modbus to integrate, why not try this in Bacnet? It will be simpler than digging through the modbus register list. You can set the protocol to Bacnet using the keypad, hold the left and rightmost buttons down at the same time for 5+ seconds, the display will jump to menu mode after which you can scroll around to find the protocol setting. Check the baud rate setting while you’re at it. Then use T3000 to scan, if the hardware is OK it will respond to the Bacnet who-is command and from there you can manage it using T3000 and/or Niagara.
If you cannot connect with the thermostat in a simple test setup with known good converter, etc, we will have to get the unit here for a look. Please coordinate with Candy (at) temcocontrols (dot) com to coordinate this and you can cc to me as well.
One final note: Since the system was working before and now suddenly its not, that could well point to a cabling issue or hardware issue. You can use one of these little pocket scopes to have a look at the network communications.
