Connecting legacy RS-485 PLCs to Ignition via wireless transparent bridge – Modbus RTU over LoRa/WiFi experience

Ignition
1

Hi everyone,

I've been working on several projects that require connecting older RS-485 equipment (Modbus RTU slave devices, legacy PLCs, flow meters) to Ignition without running new Ethernet cables. We've been trialing a wireless transparent bridge approach -- basically a device that presents as a virtual RS-485 serial port on each end of a LoRa or WiFi link.

Setup overview:

  • Master side: Ignition gateway running Modbus RTU driver
  • Field side: RS-485 devices (energy meters, PLCs, temperature controllers) connected via wireless bridge
  • RF link: LoRa 920MHz for long-range outdoor (up to ~2-3km), 2.4GHz WiFi for indoor/short-range

Ignition config:

  • Modbus RTU driver pointing to a TCP gateway address (bridge device runs a Modbus TCP server on port 502, bridging to RTU)
  • Standard RTU settings: baud 9600, parity none, 1 stop bit
  • Initial poll rate 1000ms

Challenges we encountered:

Modbus timeout tuning: The RF link adds latency -- typically 80-150ms for LoRa. Default Ignition Modbus timeout of 500ms generally works, but on congested LoRa channels we'd occasionally see timeouts. Bumped timeout to 1000ms and reduced poll rate to 2000ms for non-critical tags. Problem mostly resolved.

LoRa duty cycle: For 920MHz (Americas ISM band), a single Modbus request-response at 9600 baud is roughly 30-40ms of RF airtime per device. Polling 20 devices required careful scheduling to avoid collisions. The bridge device handles internal collision avoidance.

WiFi channel conflicts: For 2.4GHz mode in a busy warehouse, had to pick channel 11 to avoid congestion on 1 and 6. Interference caused intermittent packet loss until we isolated the channel.

Transparent bridging benefit: Ignition treats the wireless link as a normal RS-485 serial port -- zero protocol conversion needed. Modbus RTU frames pass through intact, so the Ignition driver doesn't need any modifications.

Multi-slave addressing: Had 12 devices per RS-485 segment using addresses 1-12 without issues.

Has anyone else connected RS-485 Modbus RTU devices to Ignition via wireless? Curious whether others prefer transparent bridging vs. Modbus TCP with a hardwired converter, and what timeout values people use with wireless links.

Happy to share more details on antenna placement, baud rate tuning, and Ignition tag configuration.

2

My experience with this kind of setup is that if the data doesn’t arrive within 200ms of the request it probably will never arrive, so it was better to just set the timeout low and discard the request. Try setting the timeout low, and the tag scan rate high.

I would also convert the RTU to TCP at the device.