RS-485 Wiring, Termination and Biasing: A Practical Guide for BMS Networks
RS-485 networks need a daisy-chain topology, 120 Ω termination at both ends, correct biasing and a shared reference. Here is how to wire them right first time.
Most "Modbus problems" and "BACnet MS/TP problems" on site are not protocol problems at all. They are RS-485 wiring problems: a star instead of a daisy chain, a missing terminator, too many terminators, no bias, or a floating reference. This guide explains how RS-485 works, how to wire it, where to put termination and bias resistors, which cable to use, and how to find faults quickly.
What is RS-485?
RS-485 (formally TIA/EIA-485) is an electrical standard for serial communication over twisted-pair cable. It defines voltages and drivers, not a protocol. Building automation protocols that run on top of it include:
- Modbus RTU (see What is Modbus?)
- BACnet MS/TP (see What is BACnet?)
- many proprietary field buses, including I/O module buses such as eBMS/IO
RS-485 uses differential signalling: data is sent as the voltage difference between two wires, usually labelled A and B (or D+ and D−, or + and −). Noise picked up along the cable affects both wires equally and cancels out at the receiver. That is why RS-485 works reliably over long distances in electrically noisy plant rooms.
Key specifications:
| Parameter | Typical value |
|---|---|
| Maximum cable length | About 1,200 m (4,000 ft) at lower baud rates |
| Devices per segment | 32 unit loads (more with fractional-load transceivers) |
| Typical BMS baud rates | 9,600 to 76,800 bit/s |
| Cable characteristic impedance | 100–130 Ω (120 Ω nominal) |
| Termination resistor | 120 Ω at each end |
Longer runs are possible at low baud rates; higher baud rates shorten the safe cable length.
Rule 1: daisy chain, never star
RS-485 must be wired as a single line, a bus that runs from the first device to the second, the second to the third, and so on to the last. Each device connects to the trunk with a stub as short as possible, ideally at the terminals themselves.
Do not wire:
- a star, with several cables radiating from one point
- long spurs or tees off the trunk
- rings that join the last device back to the first
Every branch creates a signal reflection. At low baud rates and short distances a small star may appear to work, which is why it is so common, but it fails intermittently as the network grows, the baud rate rises or the weather changes. If the building layout forces branches, use an RS-485 repeater or hub to give each branch its own properly terminated segment.
Rule 2: terminate both ends, and only the ends
Why termination matters
A signal travelling along a cable reflects when it reaches an impedance mismatch, such as an open cable end. On long cables at higher baud rates, those reflections arrive back while the next bit is being read and corrupt it. A termination resistor matching the cable's characteristic impedance absorbs the signal at each end, so nothing reflects.
Where to put the termination resistors
- Fit one 120 Ω resistor across A and B at each physical end of the bus: the first device and the last device.
- Do not terminate devices in the middle of the bus.
- The controller (JACE, IONA or master) is often at one end, but not always. Terminate the physical ends, wherever they are.
Many devices have a built-in termination resistor enabled by a DIP switch or jumper. Check every device: a common fault is a mid-bus device shipped with termination switched on, which loads the bus and reduces signal levels.
How to check termination
With all devices powered off, measure the resistance between A and B at any point on the bus:
| Reading | Meaning |
|---|---|
| About 60 Ω | Correct: two 120 Ω resistors in parallel |
| About 120 Ω | Only one end terminated |
| About 40 Ω or less | Three or more terminators: remove the extras |
| Very high (kΩ) | No termination |
This one-minute check finds a large share of RS-485 faults.
When can you skip termination?
On very short, slow networks (a few metres at 9,600 bit/s), reflections settle before the receiver samples the signal, and termination makes little difference. But it never hurts on a correctly biased network, and networks grow, so terminate anyway.
Rule 3: bias the bus
Why biasing matters
When no device is transmitting, the bus is idle and the A–B voltage drifts near zero. Receivers can then read noise as data, producing framing errors and phantom characters. Bias resistors (also called fail-safe resistors) hold the idle bus in a defined state: B pulled towards the supply and A towards 0 V (or the reverse, depending on the manufacturer's labelling), giving at least about 200 mV of difference.
Where biasing comes from
- Many masters and controllers provide bias, often switchable. On BACnet MS/TP, bias is commonly provided at one point on the trunk, typically the controller or router.
- Some I/O modules and meters have switchable bias.
- Modern "fail-safe" transceivers handle idle-bus detection internally and need less external bias.
Bias at one point only, normally the master. Biasing at many devices lowers the effective resistance and can overload the drivers, just like over-termination.
Termination and bias work together
Termination resistors lower the bus impedance, which makes biasing harder to achieve. That is why bias resistor values are chosen with termination in mind (values in the range of 500 Ω to 1 kΩ are typical with 120 Ω terminators). Follow the controller manufacturer's recommendation and avoid improvising resistor values.
Rule 4: connect the reference (0 V / common)
RS-485 is described as "two-wire", but in practice it needs a third conductor: a reference or signal ground connecting the 0 V of every transceiver. Without it, the common-mode voltage between devices on different power supplies can exceed the transceivers' range (typically −7 V to +12 V), and communication fails or devices are damaged.
Use the spare conductor of a twisted pair, or a dedicated conductor, to connect each device's RS-485 reference terminal (often labelled COM, GND, REF, 0 V or SC). Do not use the cable screen as the signal reference.
Rule 5: cable and screen
Cable choice
Use twisted-pair cable with a characteristic impedance of about 120 Ω, designed for RS-485 or data use. Belden 9841 and 3105A-type cables and their equivalents are common choices; a two-pair version gives a pair for A/B and a pair for the reference. Avoid:
- untwisted multicore or "bell wire"
- mixing cable types on one segment
- running data in the same containment as power cables without separation
Screen (shield)
Connect the screen through continuously from device to device, and earth it at one point only, usually at the controller panel. Earthing at both ends creates ground loops that inject noise. Insulate the screen at the far end.
Rule 6: addresses, baud rate and settings
Once the physical layer is right, configuration problems are next:
- every device on the trunk needs a unique address (Modbus unit ID 1–247, BACnet MS/TP MAC 0–127)
- all devices must share the same baud rate, and for Modbus the same parity and stop bits
- never mix Modbus RTU and BACnet MS/TP on the same trunk
- on BACnet MS/TP, keep MAC addresses contiguous from 0 and set Max Master just above the highest address, so the token does not waste time polling empty addresses
Planning an RS-485 network
| Design rule | Guideline |
|---|---|
| Devices per segment | Plan for 32 unit loads; check each device's load (1, 1/2, 1/4 or 1/8) |
| Practical BMS device count | Often 20–40 per trunk for responsive polling, fewer on slow baud rates |
| Cable length | Under 1,200 m; shorter at 76.8 kbit/s and above |
| Segments | Use repeaters to extend or isolate long or noisy runs |
| Ports | Split large systems across several controller ports rather than one long trunk |
Device count is usually limited by polling performance before it is limited electrically. A single Modbus RTU trunk at 9,600 bit/s can only carry so many reads per second. See how many devices a Modbus network supports and how many Modbus networks a JACE supports.
Fault-finding an RS-485 network
Work from the physical layer upwards.
- Power everything off and measure A–B resistance (about 60 Ω expected).
- Check for shorts: measure A to reference, B to reference, and each to the screen; there should be no low-resistance paths.
- Check polarity. Manufacturers do not agree on what "A" and "B" mean. If no device responds, or only some do, swap A and B at the controller as a test.
- Power up and measure idle voltage. With nobody transmitting, A–B should sit at a steady few hundred millivolts or more. Near 0 V means no bias.
- Halve the network. Disconnect the second half of the trunk (terminating the new end) and see whether the first half works. Repeat until the faulty section or device is found.
- Check addresses and settings on any device that does not answer.
- Look at the traffic. Driver statistics in Niagara (timeouts, CRC errors, retries) or a USB RS-485 adapter with a serial sniffer show whether errors are electrical or configuration-related.
Symptoms and likely causes
| Symptom | Likely cause |
|---|---|
| No devices respond | Wrong port or settings, A/B reversed, no bias, short circuit |
| Some devices never respond | Duplicate or wrong address, mismatched baud/parity, device-level polarity |
| Devices drop in and out | Star wiring, missing or extra termination, noise, poor reference |
| Errors increase over the day or with plant running | Electrical noise from drives or power cables, screen earthed at both ends |
| Works at 9,600, fails at 38,400 | Reflections: termination or topology problems |
| One device takes the whole bus down | Faulty transceiver or a device stuck transmitting; isolate it |
Repeaters, isolators and boosters
- Repeaters regenerate the signal and start a new segment, extending distance and device count.
- Isolators break ground loops between buildings or between areas on different earthing systems.
- Boosters such as the TNW Modbus booster strengthen signals on long or heavily loaded runs.
- Gateways such as Modbus TCP to RTU converters move part of the network onto IP, which is often the cleanest fix for a sprawling serial bus.
Frequently asked questions
Does RS-485 need a termination resistor?
Yes, on anything other than very short, slow networks. Fit one 120 Ω resistor across A and B at each physical end of the bus, and none in the middle.
Where should RS-485 termination resistors be placed?
At the two devices at the physical ends of the cable, wherever they are. Not at every device, and not necessarily at the controller unless it is at an end.
What value is an RS-485 termination resistor?
120 Ω, matching the characteristic impedance of standard RS-485 twisted-pair cable.
What is RS-485 biasing?
Biasing uses pull-up and pull-down resistors, usually at the master, to hold the idle bus in a defined state so receivers do not interpret noise as data.
Can RS-485 be wired in a star?
No. RS-485 must be a daisy chain. Use repeaters or hubs if the layout needs branches.
Is RS-485 obsolete?
No. RS-485 is still the standard physical layer for Modbus RTU and BACnet MS/TP field devices because it is cheap, robust and reliable over long distances.
How many devices can be on an RS-485 bus?
32 unit loads per segment by the standard. Devices with fractional-load transceivers allow more, but polling speed usually limits practical BMS trunks to a few dozen devices.
What cable should I use for RS-485?
Twisted-pair data cable with a characteristic impedance of about 120 Ω, ideally screened and with a second pair for the reference conductor. Do not use untwisted multicore cable.
Do I need to connect the RS-485 ground?
Yes. Connect the reference (0 V/COM) terminal of every device with a dedicated conductor so all transceivers share a common reference. Do not use the screen for this.