RS485 wiring and Modbus RTU installation: the complete guide

RS485 is the physical layer that Modbus RTU runs on. Most "Modbus isn't working" problems are actually RS485 wiring problems: swapped A/B polarity, missing termination, wrong cable type, or shield grounded at both ends. This article covers the complete technical foundation plus the practical installation procedure for a reliable Modbus RTU setup in HVAC, solar PV, and building automation.

The TIA/EIA-485-A standard in brief

RS485 is defined in ANSI/TIA/EIA-485-A-1998 (originally published April 1983, reaffirmed 2012). The standard specifies only the electrical characteristics of drivers and receivers — it does not define the protocol, connectors, or cable. Those come from Modbus, Profibus, and BACnet specifications.

Parameter	Value
Signal type	Differential (A and B)
Driver output (loaded)	Min ±1.5V across 54Ω
Receiver sensitivity	±200mV differential
Common-mode range	−7V to +12V
Max devices (standard UL)	32
Max devices (1/8 UL)	256
Max cable length	1200m at 9600 baud
Rule of thumb	baud rate × length ≤ 10^8

Data is encoded as the difference between V_A and V_B. Noise couples equally into both wires and is rejected by the differential receiver — which is why RS485 works reliably over 1200m while RS232 fails after 15m.

Cable selection

The correct cable is shielded twisted pair with 120Ω characteristic impedance. This is why Cat5/6 often works but isn't ideal — its impedance is 100Ω.

Cable	Impedance	AWG	Capacitance	Application
Belden 9841	120Ω	24	42 pF/m	Standard RS485, indoor
Belden 9842	120Ω	24 (2 pair)	42 pF/m	Full-duplex or extra GND
Belden 3105A	120Ω	22	36 pF/m	Industrial, outdoor
Lapp Unitronic BUS LD	120Ω	24	~42 pF/m	Profibus/Modbus, EU market
Cat5e / Cat6	~100Ω	24	50-55 pF/m	Acceptable <600m, low baud

Maximum length by baud rate:

Baud rate	Max length	Notes
9,600 bps	1200m	Well within rule of thumb
19,200 bps	1200m	Timing margins tighter
38,400 bps	~1200m	Marginal at full length
115,200 bps	~500m	Per Chipkin
10 Mbps	~12m	Per Analog Devices

Topology: why only daisy-chain works

RS485 must be wired as a daisy-chain (linear bus). The cable runs from one end of the bus to the other, tapping briefly at each device's terminals.

Star topology fails because each branch introduces an impedance discontinuity where signals reflect. Texas Instruments (SNLA042A): "Star configurations are discouraged—the device is effectively at the end of a very long stub."

Keep stubs <1m at any baud rate above 9600. For Profibus: 6.6m at 93.75 kbit/s, but 0m at >1.5 Mbit/s. Ring topology is not supported.

If the building layout doesn't allow a daisy-chain, use RS485 repeaters (active hubs). Each creates a new segment with its own termination.

Termination: 120Ω at both ends

When cable length exceeds ~1/10 of the signal wavelength, an unterminated end reflects the signal back — producing CRC errors. The fix: a 120Ω ¼W resistor between A and B at both physical ends of the bus.

Symptom	Cause
CRC errors at distance, works nearby	Missing termination
Weak signals, reduced swing	Double/triple termination
Random CRC at high baud	Termination on stubs/wrong location

Many devices (Eastron SDM630, Siemens Sentron, some gateways) have a DIP switch or jumper for built-in termination. Always check the manual before adding external resistors.

A/B polarity: the number one problem

The TIA-485-A standard states: A = inverting (low during idle), B = non-inverting (high during idle). During idle, B is positive relative to A.

IC manufacturers (TI, Maxim, Analog Devices, FTDI) all follow this convention. But equipment manufacturers don't:

Source	Positive (high in idle)	Negative (low in idle)
TIA-485-A standard	B	A
TI / Maxim / ADI ICs	B	A
Modbus specification	D1	D0
BACnet (ASHRAE 135)	+	−
Profibus	B (red wire)	A (green wire)
Many PLCs and meters	varies	varies

Siemens explicitly warns that A/B labels are not standardized across manufacturers.

Practical solution: ignore the letters. Connect + to + and − to −. Measure idle DC voltage with a multimeter — the wire with the higher voltage is the positive signal (B/D+). No communication? Swap the two data wires. This won't damage any hardware.

Wiring step by step

1. 1

   Plan the route

   Map all device locations. Design a strict daisy-chain. Place the master/gateway at one end. Keep stub lengths <0.3m. Route cables at least 100mm from power cables (per EN 50174-2). Cross power cables only at 90°.

2. 2

   Select and measure cable

   Belden 9841 for standard indoor, Cat5e for short runs, Belden 3105A for industrial/outdoor. Add 10-15% extra for routing. Never use non-twisted cable or standard installation wire — it provides no noise rejection.

3. 3

   Prepare the cable ends

   Strip 50mm outer jacket. Strip 8mm per conductor. Wire ferrules are mandatory — bare stranded wire produces intermittent contact faults (extensively documented in the Teltonika community). Keep the twisted pair twisted as close to the terminal as possible.

4. 4

   Wire the master/gateway first

   Cable the gateway (e.g. MCG-1 or Teltonika TRB246) and configure serial parameters via the web interface. For the TRB246: bridge R+ to D+ and R− to D− for half-duplex (standard Modbus RTU). Power on and verify boot.

5. 5

   Daisy-chain to each subsequent device

   At each device, two cables land: one incoming, one outgoing. Both on the same terminals (A to A, B to B). Maximum 2 wires per terminal — this forces daisy-chain discipline. Set a unique Modbus address (1-247) plus identical baud/parity on every device.

6. 6

   Install 120Ω termination

   At the first device (gateway) and the last device. Never in the middle. Check for built-in DIP switches first. External: 120Ω ¼W resistor between A and B.

7. 7

   Ground shield at one end only

   Connect the drain wire/braid to earth only at the master side. At the far end, leave the shield floating and insulated. Grounding both ends creates a ground loop that injects noise.

8. 8

   Test per device

   Test each device individually with an FTDI USB-RS485 adapter and QModMaster or Modbus Poll before connecting the next one. This localizes faults early.

Grounding and shielding

The signal ground wire (the third wire, alongside A and B) is mandatory. Control Solutions Minnesota calls missing ground "the #1 cause of RS485 failures". Without a common ground reference, the common-mode difference between devices can exceed the −7V/+12V range — resulting in CRC errors and, in severe cases, hardware damage.

The shield (braid/foil) is grounded at one end only — by convention, the master side. Daisy-chain the shield electrically through between devices (all drain wires connected) but earth it only once.

Fail-safe biasing

When no driver is active (bus in idle), the termination resistors pull the differential voltage to 0V — an undefined state that can produce "false start bits". Bias resistors prevent this:

Resistor	Connection	Typical value
Pull-up	B/D+ to Vcc (5V/3.3V)	560Ω or 680Ω
Pull-down	A/D− to GND	560Ω or 680Ω

Install biasing at one location only (typically the master). Modern "True Fail-Safe" transceivers (MAX3080, ISL83082) have a −50mV threshold and don't need external biasing — but older equipment often does.

Top 10 wiring mistakes

1. Star topology — Causes reflections and random CRC errors. Use daisy-chain, or repeaters.
2. A/B swapped — Problem #1 at commissioning. Swap the wires. Won't damage anything.
3. Missing or wrong termination — 120Ω only at both physical ends, never in the middle.
4. No GND wire — #1 root cause per Control Solutions.
5. Shield grounded at both ends — Creates ground loop, injects noise into the signal.
6. RS485 parallel to power cables — EMC problem. Keep 100mm+ away, cross at 90°.
7. Wrong cable type — Non-twisted, standard installation wire, or wrong impedance.
8. Too many devices — Above 32 unit loads without a repeater.
9. Stubs too long — Keep <1m. Long branches cause reflections.
10. Settings not identical — Baud rate, parity, stop bits, data bits must match exactly on every device.

Troubleshooting with a multimeter

Test	Reading	Conclusion
DC voltage A vs B (idle)	B is 200mV-5V higher than A	Normal
DC voltage A vs B (idle)	~0V	Bus undefined — add biasing
DC voltage A vs B (idle)	A higher than B	Polarity reversed
Resistance A-B (power off)	~60Ω	Two terminators — correct
Resistance A-B (power off)	~120Ω	One terminator — one end missing
Resistance A-B (power off)	OL (open)	No termination
Resistance A-B (power off)	<10Ω	Short circuit — cable fault
Common-mode GND-GND	<1V	Acceptable
Common-mode GND-GND	1-7V	Marginal
Common-mode GND-GND	>7V	Isolated repeater required

Software tools: QModMaster (free, open-source) or Modbus Poll (commercial, 30-day trial). Use an FTDI-based USB-RS485 adapter — avoid cheap CH340 chips, which cause timing issues and echo problems.

Regulations and standards

TIA-485-A defines the physical layer (April 1983, revised 1998, reaffirmed 2012). TSB-89A provides application guidance including the baud × length ≤ 10^8 rule.

EN 50174-2 governs data cabling installation inside buildings in Europe. Key requirements:

• Separation distances between data and power cables
• EMC-compatible grounding and equipotential bonding
• Cable management system requirements

EN 50575 (CPR) — since July 2017, all permanently installed cables in EU buildings must meet CPR requirements with CE marking and a Declaration of Performance. RS485 cables typically fall in classes B2ca to Eca depending on building type.

Modbus Serial Line Specification V1.02 (modbus.org, December 2006) standardizes Modbus RTU over RS485: TIA/EIA-485 two-wire, master-slave (1 master + up to 247 slaves), default 19200 bps, 8 data bits, even parity, 1 stop bit (though 9600-8N1 is far more common in practice).

Frequently asked questions

Conclusion

A reliable RS485 installation is mostly a matter of discipline: daisy-chain topology, correct 120Ω termination at both ends, common ground, shield grounded at one end only. Follow these four rules and Modbus RTU works on the first attempt.

With the right cable (Belden 9841 or equivalent), wire ferrules, and a decent FTDI-based test adapter, you can install a 32-device bus over 1200m without a single CRC error. Most installation problems don't come from hardware — they come from shortcuts that ignore the physics.