Most Modbus RTU faults that look like software bugs are really reflections, floating lines or missing grounds on the twisted pair. Two small resistor choices at the ends of the cable often turn random CRC errors into a rock solid link.
RS485 is the workhorse of Modbus RTU, BACnet MSTP and many drive networks. Its reliability depends heavily on how the cable ends are terminated and how the idle line is held in a known state.
What Is RS485 Termination?
RS485 termination is the practice of placing a resistor equal to the cable characteristic impedance, usually 120 Ω, across the A and B lines at both physical ends of the bus to absorb signals and prevent reflections. It is part of the physical layer compared in RS232 vs RS485.
Opto 22 explains that the terminator should match the cable impedance, typically 120 Ω, with 100 Ω to 130 Ω seen on common twisted pair cables. A mismatch reflects part of each edge back down the line.
The line must be a daisy chain, not a star, so there are only two ends. Use a shielded twisted pair cable rated around 120 Ω.
Poor RS485 termination usually shows up as CRC errors and timeouts in Modbus diagnostics.
Why Biasing Is Needed
When no driver is transmitting, the bus floats and noise can look like data. Bias resistors pull A high and B low so receivers see a clear idle state.
Texas Instruments recommends at least 200 mV of differential voltage in the idle state for reliable failsafe operation. Many modern transceivers have built in failsafe, which reduces the need for external bias.
Bias is applied at one point only, usually the master. Adding bias at every node lowers the bus impedance and loads the drivers.
When RS485 Termination Matters
| Condition | Terminate? | Reason |
|---|---|---|
| Long cable, high baud rate | Yes | Reflections distort bits |
| Short cable, 9600 baud | Often optional | Reflections settle within each bit |
| Many nodes spread out | Yes | Longer electrical length |
| Star wiring | Fix wiring first | Termination cannot cure stubs |
Opto 22 points out that signals travel at roughly two thirds of light speed in copper, so reflections from long runs arrive while the next edge is forming. At low baud rates they die away before the receiver samples.
Coax networks follow the same idea as RS485 termination, as in 75 Ω ControlNet termination.
6 Easy RS485 Termination Rules
The common reference wire is often missed, and large ground differences then damage transceivers. Shield practice is explained in ground loop prevention.
Opto 22 notes that up to 256 devices can share a bus when they use fractional unit load transceivers.
Bias Resistor Formula
Rbias maximum = (Vcc × Rt ÷ 0.2 minus Rt) ÷ 2
Rt = parallel value of both terminators = 60 Ω
Example, 5 V supply:
Rbias maximum = (5 × 60 ÷ 0.2 minus 60) ÷ 2 = 720 Ω
Use 680 Ω for about 211 mV idle voltage
This is a simple voltage divider with the terminators in the middle. Lower bias values add current, so do not go much below what is needed.
Bias Resistor Calculator
With a 3.3 V supply the maximum bias resistor drops to about 465 Ω. Always check the transceiver datasheet for its own failsafe features.
- Daisy chain topology.
- Terminators only at the two ends.
- One bias point.
- Common reference conductor.
- Terminators on every node.
- Star or tree wiring.
- No bias with old transceivers.
- No reference wire.
For systematic fault finding, see SCADA communication problems. A scope on A and B shows reflections and bias levels at once.
TI RS485 Design Guide PDF
Bias and Termination Video
RS485 Termination FAQ
Related Articles
- RS232 vs RS485
- Differences Between RS232 and RS485
- Modbus RTU vs Modbus TCP
- Modbus Protocol Explained
- SCADA Communication Problems
External References
What We Learn Today
- RS485 termination uses 120 Ω at both ends of a daisy chain.
- Bias once for at least 200 mV idle voltage.
- Use a common reference wire and count unit loads.