Wiring Guide

How to wire modules on the RS-485 bus, cable selection, termination, and cabinet layout tips.

RS-485 bus topology

RS-485 is a bus (daisy-chain), not a star. Modules connect in series from the USB adapter through each module in turn:

USB-RS485 adapter
  │
  ├── Module 1 (address 1)
  │       │
  ├── Module 2 (address 2)
  │       │
  ├── Module 3 (address 3)
  │       │
  └── [far end] ── 120Ω terminator

Each module has two RS-485 screw terminals — use both: one for the incoming wire from the previous device, one for the outgoing wire to the next.

⚠ Star wiring causes problems

Connecting multiple modules to a central hub (star topology) creates impedance mismatches and reflections that cause intermittent communication errors, especially at higher baud rates.

Cable selection

Cable type Notes
CAT5e UTP Use one twisted pair for A/B. Cheap and widely available.
CAT6 UTP Slightly better performance at longer runs.
Belden 9841 Industrial RS-485 cable, rated for continuous flex.
LIYCY 2×0.5mm² Screened twisted pair, good in electrically noisy environments.

Use one twisted pair for A and B. If your cable has a screen/shield, connect it to GND at one end only (typically at the USB adapter end) to avoid ground loops.

Wire gauge

Bus length Minimum wire gauge
Up to 100 m 0.2 mm² (AWG 24)
100–500 m 0.5 mm² (AWG 20)
500–1200 m 0.75 mm² (AWG 18)

For a typical home installation (single building, sub-100m runs), CAT5e is entirely adequate.

Termination

A 120Ω resistor at the far end of the bus reduces signal reflections:

  • Place it between the A and B terminals of the last module on the bus
  • Many USB-RS485 adapters have a built-in switchable terminator — enable it if the adapter is at the far end
  • For short runs (under ~10 m, one or two modules), termination is optional but still recommended

💡 When to add termination

If you see intermittent communication errors or the bus status reports 'error' frequently, try adding or removing termination. Incorrect termination is the most common cause of RS-485 problems.

Power wiring

Each module requires 24V DC on its power terminals. Options:

Single power supply, one run

Run 24V and GND from a single DIN-rail PSU alongside the RS-485 cable:

PSU 24V+ ─────┬────── Module 1 terminal 1
               └────── Module 2 terminal 1 (daisy-chained)
PSU GND  ─────┬────── Module 1 terminal 2
               └────── Module 2 terminal 2

Per-module power (larger installations)

For many modules spread across a building, you may prefer a PSU in each cabinet zone and only run the RS-485 signal cable between zones.

In this case, ensure all GND references are common (connect the GND of each PSU together, or to PE).

Cabinet layout example

A typical DIN-rail cabinet section for a 4-room floor:

DIN rail (top section):
  [MCB breakers] [RCDs] [24V PSU]

DIN rail (middle section):
  [Relay Module A: addr 1] [Relay Module B: addr 2]

DIN rail (bottom section):
  [Switch Module: addr 3] [USB-RS485 adapter] [Patch terminals]

The USB-RS485 adapter is mounted near the Raspberry Pi / HA Yellow, which can live adjacent to the cabinet or inside it if space permits.

Grounding and shielding

  • RS-485 A, B, and GND should all share a common 0V reference
  • If running cables through electrically noisy environments (near motor drives, fluorescent lights), use screened cable and ground the screen at one end
  • Do not run RS-485 signal cable in the same conduit as mains wiring for long parallel runs

Maximum bus length

At 9600 baud (Domoriks default), RS-485 is rated to 1200 m. In practice, for a home, the limit will never be reached. Even a large house will have total bus runs well under 200 m.

If you do need longer runs, drop to 4800 baud (set via holding register) or add a bus repeater.

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