RS-485 and Modbus RTU Termination: Resistors, Bias, and Pinouts
An RS-485 network fails in two different ways. Miss the termination, and reflections corrupt frames at random. Miss the bias network, and the bus floats when every driver goes quiet. The floating bus picks up noise, and receivers read phantom start bits. Both faults look like software. Both are resistors.
This guide covers Modbus RTU and RS-485 termination and the fail-safe bias math. It also covers the vendor pinout trap on RJ45 ports. The article includes a bias calculator.
Terminate both ends, and only the ends
RS-485 (TIA/EIA-485-A) runs on a balanced pair with a characteristic impedance near 120 Ω. The Modbus over Serial Line specification calls for a 120 Ω resistor at each extreme end of the trunk. The same physics as CAN applies. An unterminated end reflects the edge energy back into the line, and the reflection collides with live data.
A pattern that appears again and again in field reports: a plant network runs for years, then dies overnight. The cause is a terminator that nobody installed. Or a drive DIP switch that vibrated off. Or a replaced end device with its internal termination left disabled. An oscilloscope shows the ringing in seconds. A multimeter test works too: power down and measure across the pair. A healthy dual-terminated trunk reads near 60 Ω.
The bias network: why idle buses need three resistors
When no driver transmits, every RS-485 transceiver goes high-impedance. Nothing drives the pair. The differential voltage collapses toward 0 V, inside the ±200 mV band where a receiver output is undefined. Any coupled noise then toggles the receiver, and UARTs log framing errors on an idle line.
The fix is a fail-safe bias network. A pull-up resistor ties line A toward +5 V. A pull-down resistor ties line B to ground. Together they hold the idle bus above +200 mV. The Modbus specification recommends 450–650 Ω per leg for a 5 V supply. Install the bias at one location only, usually the master. The green dots below trace the bias current that holds the pair apart:
Bias calculator: will your idle bus stay defined?
The two terminators and every connected receiver load the bias divider. More nodes pull the idle voltage down. Check your values before the network ships:
The divider works against the parallel combination of both 120 Ω terminators and the node input resistances. With 32 full-load nodes the bus load falls near 52 Ω. A 5 V supply then needs roughly 620 Ω per leg to hold 200 mV. Smaller networks give you margin. Use it.
The RJ45 pinout trap
Serial RS-485 has no universal RJ45 pinout. Each vendor chose its own map. A standard Ethernet patch cable between two brands can short a data line into a power pin. The same trap catches terminators. A plug with the resistor on the wrong pins terminates nothing.
| Equipment | Data + / − | Watch out for |
|---|---|---|
| Modbus.org standard, Schneider Altivar / Zelio / Modicon | Pins 4 + 5 | Pin 7 carries supply voltage on some drives |
| Allen-Bradley PowerFlex, Control Techniques | Pins 1 + 2 | Pin 7 carries +24 V on Commander SK |
| Siemens compact drives | Pins 2 + 3 | Reversed polarity relative to most maps |
| SolarEdge inverters | Pins 4 + 5 | Ground on pin 7, not pin 8 |
Always confirm the map in the device manual before you plug anything in. Our terminators state their terminated pins on the listing, so the resistor lands where your equipment expects it:
- RJ45, pins 4+5 (Modbus / Schneider), 120 Ω, 1%, soft silicone — pair, $20 for indoor panels.
- RJ45, pins 4+5, 120 Ω, 1%, shielded, epoxy — pair, $30 for cabinets with drives and inverters.
- RJ45, pins 4+5, RC 120 Ω + 1 nF, 1%, soft silicone — pair, $26 for battery and solar sites where idle power matters.
Every order ships as a pair, one for each end of the trunk. We also build custom fail-safe bias plugs with pull-up and pull-down networks to your values. For wiring faults on vehicle and machine networks, our CAN Bus Wire Tester finds bad wires, connectors, and shorts.
Quick answers
Does Modbus RTU need termination resistors?
Yes, on any trunk of real length. The Modbus serial specification calls for 120 Ω at both extreme ends. Very short bench links sometimes survive without them. Plants should not gamble on that.
One terminator or two?
Two, one per end. A middle node never gets one. Extra terminators load the drivers and shrink the signal.
What pull-up and pull-down values do I need?
For a 5 V supply the Modbus specification recommends 450–650 Ω per leg at one location. Run your node count through the calculator above and confirm the idle voltage clears +200 mV.
Why does my RS-485 network log errors only when idle?
That is the signature of missing bias. Terminated but unbiased buses float at 0 V between frames, and noise writes phantom characters. Add the bias network.