BWM ReferenceWiring
RS-485: a half-duplex multidrop bus
Connect differential transceivers on a bus with controlled transmit direction.
Applies to: Two-wire half-duplex topology with a transceiver at every node. RS-485 itself does not define Modbus addresses or a universal connector.
When to use it
Linking compatible industrial sensors or controllers along a cable.
Why it works this way
Differential signaling supports useful noise rejection; an application protocol decides who talks and what bytes mean.
Connection reference
| Signal / feature | Purpose | Check |
|---|---|---|
| Differential pair | Shared data bus | A/B and +/- names can differ |
| DE | Enable the driver | Only one transmitter active at a time |
| /RE | Enable the receiver when low | Automatic-direction modules vary |
| Termination | Match cable impedance at both ends | Do not add one per node |
| Bias / failsafe | Defined idle state | Calculate for the complete loaded bus |
Make the connection
- Use a trunk with short stubs and the cable/transceiver guidance for the chosen rate.
- Plan the signal reference, common-mode limits and isolation before connecting separate power systems.
- Check transmit-enable timing and the protocol settings.
Check before powering
- GPIO cannot directly drive the differential pair.
- Termination and bias are different circuits; modules may already contain either.
Sources & review
Source documents inspected; independent electrical and bench review pending. Reviewed 2026-09-27.
- Texas Instruments: RS-485 Design Guide, SLLA272D
Sections on topology, termination, failsafe and grounding · checked 2026-09-27 - Analog Devices: AN-960 RS-485/RS-422 implementation
Driver enable and half-duplex bus connections · checked 2026-09-27
Original writing and diagram by Kal / Valleytech Solutions, CC BY 4.0. Manufacturer artwork and documentation retain their own rights. CC BY 4.0
Diagram fingerprint
38ead3cb8203d27fc2ea439d17ee104f1ca22470e53df421fe46f9393403aa7f