RS-485 (often running the Modbus protocol) is the most common wired bus for cost-effective hotel RCU systems. Its "hand-in-hand" (daisy-chain) topology is simple to wire and reliable in practice. This guide covers the topology, cable choices and addressing — aimed at installers and project engineers.
What "hand-in-hand" means
Devices are wired in a daisy chain: the bus runs from the RCU host to the first panel, then on to the next, and the next — like holding hands. Two twisted-pair wires (A and B) carry the differential signal; a third pair or separate line supplies low-voltage power where needed. Termination resistors at the ends keep the signal clean.
Typical RS-485 room wiring
| Element | Notes |
|---|---|
| Bus cable | Shielded twisted pair (e.g. RVSP 2×1.0 mm²); keep away from 220V mains to avoid interference. |
| Topology | Daisy chain / hand-in-hand; avoid long stubs. |
| Termination | 120Ω resistor at the two ends of the bus segment. |
| Addressing | Each device gets a unique Modbus/RS-485 address (set by DIP switch or software). |
| Grounding | Single-point ground to prevent ground loops; use shielded cable. |
RS-485 vs the other wired option (KNX)
RS-485 is simpler and cheaper than KNX, and perfectly adequate for single-supplier wired projects. KNX adds multi-vendor interoperability and a formal standard, at higher device cost. For many budget and mid-scale hotels, RS-485 is the pragmatic wired choice. Compare all three in KNX vs RS-485 vs BLE Mesh.
When wireless is simpler
If running a bus is impractical — renovation, heritage buildings, leased space — BLE Mesh delivers the same room functions with no new cables. The wiring effort drops to near zero; the trade-off is gateway placement and radio planning.
Practical tips
- Keep the bus away from power cables; cross them at 90° if you must.
- Label every device address during install — it saves hours at commissioning.
- Test termination and signal with the RCU's built-in communication diagnostics before closing walls.
- Document the address map; future maintenance depends on it.
Frequently asked questions
- How many devices on one RS-485 segment?
- Up to 32 standard unit loads per segment without a repeater; repeaters extend this for large floors.
- RS-485 or Modbus — what's the difference?
- RS-485 is the physical layer (the wires); Modbus is usually the protocol running on top of it. HOTEKEY RCU supports Modbus over RS-485.
- Can RS-485 RCU reach the GRMS?
- Yes — the RCU host bridges the bus to TCP/IP for GRMS/PMS integration.