A physical interface, not a protocol
RS-485 (ANSI/TIA-485) describes electrical characteristics for differential drivers and receivers on a shared cable. It does not specify device addresses, message formats, checksums, baud rates or a master–slave protocol. Those rules come from the equipment or a higher-level protocol such as Modbus RTU.
Balanced signalling helps reject noise common to both conductors. Reliable operation still depends on the transceiver, cable, grounding, topology and correct driver control.
Key properties
- Standard
- ANSI/TIA-485
- Signalling
- Balanced differential pair
- Half-duplex
- Shared pair, alternate transmit
- Full-duplex
- Separate transmit/receive pairs
- Bus loading
- 32 unit loads (classic rating)
Keep the trunk continuous
A typical 2-wire, half-duplex network uses one twisted differential pair with multiple transceivers connected along a main cable. Keep branches short relative to signal rise time and cable propagation delay; avoid arbitrary star wiring. Separate transmit and receive pairs are possible in 4-wire full-duplex designs.
The green line represents the differential pair, not a single conductor. Termination is typically across the pair at each physical end when required by signal timing.
| Item | Practical engineering note |
|---|---|
| End termination | On transmission lines needing termination, match the cable’s differential characteristic impedance at the two physical ends. A common value is 120 Ω; do not add a terminator at every node. |
| Resistance check | With power safely removed and suitable isolation, two passive 120 Ω end resistors appear approximately as 60 Ω across the pair. Bias networks and connected equipment can alter the reading; consult the device manual. |
| Idle-state bias | Older receivers may require external fail-safe biasing to hold a defined idle state; modern transceivers may have built-in fail-safe features. Avoid stacking independently designed bias networks. |
| Polarity labels | A/B, D+/D− and other labels are not used consistently across every product family. Check the transceiver datasheet or device pinout rather than assuming a universal A-to-A connection. |
Loading, speed and distance
Classic RS-485 drivers are rated for 32 unit loads (UL), not necessarily exactly 32 physical devices. Receivers rated at fractional unit loads can allow more nodes, subject to the total loading budget and the transceiver specification.
Maximum useful cable length depends on bit rate, cable characteristics, rise time, noise, termination and topology. Treat widely quoted distance-versus-speed figures as examples, not guaranteed universal limits.
Common trouble spots
- Reversed conductors or mismatched pin labels
- Too many or missing end terminators
- Long stubs, stars and cable impedance mismatch
- Incorrect transmitter enable / turnaround timing
- Ground potential difference, transients or insufficient isolation
- Wrong serial settings or higher-layer protocol parameters
Measure the right signals
Identify the higher-layer protocol and serial settings first. Use a compatible differential probe or isolated analyser when measuring on powered machinery. Compare differential waveforms, idle bias and receiver error reports; confirm any connection to signal common or protective earth against the equipment documentation.
Do not connect an earthed oscilloscope ground lead to an unknown live conductor. Industrial networks may bridge different ground potentials; follow site isolation, electrical safety and machine-service procedures.