Commissioning RS485 Subnets

1

I am trying to connect a Temco thermostat to a T3-BB controller over RS485 so that T3000 can detect it as a sub-device.

Hardware / network:

  • Main controller: T3-BB, reachable over Ethernet at 192.168.1.2:502
  • T3-BB Modbus ID / panel ID appears to be 42
  • T3-BB serial/object instance read successfully: 256318
  • Thermostat: configured as Modbus
  • Thermostat Modbus ID: 1
  • Baud rate: 19200
  • Serial format tested: 8N1

What works:

  • The T3-BB responds correctly over Modbus TCP.
  • The thermostat works when connected directly to the PC with a USB-RS485 adapter.
  • Direct USB test on COM4, 19200, 8N1, Modbus ID 1, FC03 holding registers 0..22 returned valid data.
  • So the thermostat itself is alive and its Modbus settings appear correct.

What does not work:

  • When the same thermostat is connected to the T3-BB RS485 port, it is not detected.
  • I tested both RS485 MAIN and RS485 SUB on the T3-BB.
  • I tested both polarities on MAIN.
  • I also tested SUB after moving the wiring.
  • In all cases, T3000 / Modbus through the T3-BB cannot read thermostat ID 1.
  • MODBUS_TOTAL_NO on the T3-BB remains 0.
  • The T3-BB internal thermostat table stays empty.

T3-BB settings tested:

  • RS485 MAIN configured as Modbus Master: register 61 = 7
  • RS485 SUB configured as Modbus Master: register 59 = 7
  • Baud set to 19200
  • RS485 fixed mode enabled
  • T3000 tunnel tested with:
    • register 96 = 2 for RS485 MAIN
    • register 96 = 0 for RS485 SUB
    • register 97 = 6 for 19200

Observed behavior:

  • Over the T3-BB, Modbus reads to thermostat ID 1 always timeout.
  • Direct USB-RS485 reads to the thermostat succeed immediately.
  • On the T3-BB RS485 port, the RX LED sometimes stays red or flashes, but no valid Modbus response is received.

Question:
What is the correct way to wire and configure a thermostat as a Modbus RS485 sub-device under a T3-BB in T3000?

More specifically:

  1. Should the thermostat be connected to RS485 MAIN or RS485 SUB on the T3-BB?
  2. Should the T3-BB port be set to Modbus Master, and which register/setting is the correct one?
  3. Does the T3-BB require a specific RS485 GND / 485G reference connection?
  4. Is there a termination or bias jumper required when only one thermostat is connected?
  5. Are the T3-BB + / - / G labels equivalent to common USB-RS485 A / B / GND, or are they reversed?
  6. Is there a specific T3000 scan procedure required after setting the thermostat to Modbus ID 1, baud 19200?
  7. Why would the thermostat answer correctly through a USB-RS485 adapter but not through the T3-BB RS485 port?

Any wiring diagram or exact T3000 configuration steps for this setup would help a lot.

2

Hi Sebastien,

Thanks for the details. Here are a few pointers to help get your network communicating smoothly:

  • Main vs. Sub: Both ports are actually functionally equivalent, so you can use either one.

  • Software/Firmware: Since you mentioned the “Master” setting, it sounds like you might be running an older version of T3000 and device firmware. You can head to T3000 → help → check for updates to get the latest version of both.

  • RS485 Ground (G): This is optional and your small setup will work fine without it. It is mainly used in larger networks with multiple power supplies to prevent ground loops. Tying it to earth at each cabinet shunts differences in potential away from the network cables.

  • Termination: Not required for a small network like yours.

  • Wiring Consistency (Crucial): Ensure your RS485 A and B lines are consistent across the entire network. More importantly, double-check that your 24VAC power polarity is perfectly consistent for all devices. A single device with 24vac swapped can drop communications for the whole network.

  • Troubleshooting the Subnet: If it is still not communicating, verify that your subnet settings (like baud rate and protocol) match exactly. Also, ensure that every device on the subnet—including the T3-BB itself—has its own unique ID. You mentioned they are both at ID=1 so thats very likely the culprit. T3000 is ‘supposed’ to pop up a message that there’s an ID conflict so I will be passing this on to our team why you didnt get that message.

  • Use your RS485 adapter and try communicating to the T3-BB by itself over RS485. If you have trouble try with the other subnet port. Once you have that going on its own, and the Tstat going on its own, try with all devices: BB, Tstat and PC.

I believe there’s a long and detailed post on this forum for configuing RS485 subnets. Standing by to help more.