Node-RED + Modbus RTU over wireless RS-485 bridge – timeout and LoRa duty cycle experience

General
1

Hi everyone,

I've been using Node-RED to poll Modbus RTU devices over a wireless RS-485 transparent bridge setup. Sharing some notes in case it helps others doing similar industrial IoT work.

Setup:

  • Node-RED running on a Raspberry Pi
  • node-red-contrib-modbus library (Modbus RTU mode)
  • RS-485 wireless transparent bridge between Node-RED server and field devices
  • RF link: LoRa 920MHz for outdoor runs (~1.5km), WiFi 2.4GHz for indoor
  • Field side: 8 Modbus RTU slaves (energy meters, temperature controllers)

Modbus RTU configuration in Node-RED:
Used the Modbus-Flex-Getter node. Key settings:

  • Serial port: /dev/ttyUSB0 (connected to wireless bridge master unit)
  • Baud rate: 9600, 8-N-1
  • Modbus timeout: bumped from default 1000ms to 2000ms for LoRa link
  • Poll interval: 5 seconds per device

LoRa-specific notes:
LoRa 920MHz adds 100-180ms of round-trip latency at default spreading factor (SF7). The wireless bridge handles retransmission internally, so Node-RED just sees a slightly slower serial port.

The key gotcha: LoRa duty cycle. In dense deployments, if you're polling multiple slaves at high rates, you can saturate the RF channel. I found that 5-second poll intervals with 8 devices kept RF utilization comfortable. With faster polling (1s), I saw occasional Modbus CRC errors which were actually from packet collisions on the RF link.

WiFi mode notes:
For the indoor portions, the bridge supports 2.4GHz WiFi transparent mode. Much lower latency (<20ms), so Modbus timeouts of 500ms work fine. Main issue: WiFi channel congestion in offices -- had to set a fixed channel (ch11 in my case) rather than auto.

MQTT bridge mode:
The bridge also supports a Modbus TCP gateway mode, which lets Node-RED use the Modbus TCP node instead of serial RTU. This worked well for the WiFi link -- set up the bridge as a Modbus TCP slave at 192.168.x.x:502, and Node-RED connects over LAN.

Has anyone else done Modbus RTU over LoRa with Node-RED? Curious what timeout values others use and whether there are other node-red-contrib-modbus settings I should tune.

1 Like