RS-485 Wiring and Termination Resistor Guide
Neva Otomasyon · 03.08.2026 · 6 min read
RS-485 is the most common industrial communication layer over which field energy meters, analyzers and PLCs speak Modbus RTU. A correctly built RS-485 line carries data reliably for years; a poorly built one leaves you fighting random dropouts, corrupted readings and lost data. This guide explains how to wire the bus so that a healthy stream reaches an energy management platform like Argus EMS.
Multidrop Bus and Daisy-Chain Topology
RS-485 is a multidrop bus architecture: every device is wired in series on a single pair. The correct topology is a daisy-chain, where the cable enters a device, leaves it and continues to the next one. A star layout or long stubs ruin the line impedance, causing signal reflections and data errors. The two ends of the cable must sit on the two most distant devices, and these two ends carry the termination resistors.
Twisted Pair, Shield and Common GND
Always use twisted pair cable for the data pair. The twist couples noise equally onto the A and B lines, so the receiver's differential reading stays clean. In industrial environments choose a shielded cable and ground the shield at a single point only; grounding both ends creates a loop current through the shield. A common GND reference between devices is also required, because RS-485 receivers operate within a limited common-mode voltage range.
- Twisted pair for data, mind A (D+) and B (D-) polarity
- Shield grounded at one point, the other end left floating
- Common GND via a third conductor or separate wire
- Do not route alongside power or high-current cabling
Termination Resistor and Fail-Safe Bias
At high baud rates and over long runs, the signal reflects back from the cable end and corrupts the data. To prevent this, a 120 ohm termination resistor is fitted at each end of the line; this value matches the characteristic impedance of typical twisted pair. It goes only on the ends, never on intermediate devices. In addition, when no device is driving the bus the voltage between A and B is undefined and noise creates false bits. A fail-safe bias (pull-up at one end, pull-down at the other) holds the line at a known idle level. In Argus EMS installations, these two settings are the direct cure for the most common intermittent dropouts.
Distance, Baud and Load Relationship
RS-485 can theoretically span hundreds of meters, but distance is inversely related to baud rate: the faster the speed, the shorter the safe length. On long runs, operating Modbus RTU at a lower baud value usually produces fewer errors. Each receiver on the bus consumes a certain unit load; standard drivers typically feed 32 unit loads, and beyond that you need 1/8 unit-load receivers. The table below summarizes the symptoms field teams meet most often.
| Symptom | Likely Cause | Fix |
|---|---|---|
| Intermittent timeouts, random device drops | No termination or on the wrong end | Fit 120 ohm at the two most distant ends |
| All devices unresponsive | A/B polarity wired in reverse | Swap the D+ and D- terminals |
| Only distant devices fail | Baud too high for the distance | Lower the baud, check cable length |
| False idle traffic, boot-time errors | Missing fail-safe bias | Add pull-up/pull-down at one end |
| Corruption after lightning or noise | Shield grounded both ends or no GND reference | Single-point shield ground, pull a common GND |
Why Solid Wiring Is Critical for Argus EMS
When field devices are read over Modbus RTU, every communication error means a missed reading and therefore a gap in the cumulative consumption chart. In Neva Otomasyon field deployments, the most frequent data losses come not from device faults but from cabling left without termination and bias. Once the line is built correctly, with the right topology, proper termination and a clean GND reference, the data flowing into Argus EMS stays continuous and reliable.
Related Content