Hi all,
I’m hoping someone here has run into this before. I have a TRB255 (serial 6003720977, firmware currently updated to the latest available) that I’m trying to use as an RS485↔Ethernet Modbus bridge for a Delta DT320 temperature controller (Modbus RTU, 9600 8N1, unit ID 1, holding register 4096).
The problem: the built-in RS485 port never receives a valid response from the slave device, no matter which Modbus feature I use:
- Modbus TCP over Serial Gateway — configured with device=rs485, 9600/8/1/None, Full Duplex off, Server ID “Obtained from TCP”, listening on 192.168.1.1:502. TCP connects fine, but every request times out or the connection closes with zero bytes received.
- Modbus Serial Client — same serial parameters, polling unit ID 1 / register 4096 / function 03. Status shows “Up” and it’s actively polling, but Successful requests stays at 0 while Failed requests increments every cycle.
What I’ve ruled out:
- The DT320 itself is fully functional — connected directly to a PC via a separate USB-RS485 adapter with identical serial settings, it responds correctly within milliseconds.
- Wiring double- and triple-checked against the terminal labels (D+/D-), tried swapping polarity, tried a different physical channel, added a 120Ω termination resistor, bridged R+/D+ and R-/D- on the combo RS422/485 terminal.
- Tried Full Duplex on and off.
- IP filter / firewall ruled out on the TCP path (handshake succeeds fine).
- Rebuilt the config from scratch multiple times, full reboots in between.
- Updated to the latest firmware — no change in behavior.
The interesting bit: I measured the DC voltage between the D+ and D- terminals directly on the router with a multimeter:
- At rest: ~4.79V
- During an active poll/test: only dips to ~4.69V
That seems way too high for RS485 idle bias (I’d expect a few hundred mV with normal fail-safe biasing), which makes me suspect the RS485 transmitter on this unit isn’t releasing to a high-impedance receive state between transmissions — basically jamming the bus so it can never hear a reply.
Has anyone seen this specific symptom before? Trying to figure out if this is a known firmware issue or if I’ve just got a unit with a hardware fault on the RS485 transceiver. Already opened a support case, just curious if this rings a bell for anyone here in the meantime.
Thanks!