RS-485

showstack / protocols / rs-485

Also called TIA-485, EIA-485, TIA/EIA-485-A, RS485.

The balanced, multi-drop electrical layer that DMX512, many Modbus installations and most machinery buses actually run on. It defines voltages and drive capability, not data — what the bits mean is always somebody else's standard sitting on top.

Device managementPaid specTIAReported

Stewarded by TIA (Telecommunications Industry Association).

Not a traffic flow

The electrical layer DMX and many machinery buses sit on. It has no payload of its own.

What goes wrong

RS-485 says nothing about connectors, pinout, baud rate or protocol. Two devices can both be "RS-485" and share nothing else. That is a feature of the standard and a recurring surprise on site.

The load limit is stated in unit loads, not devices. One unit load is a 12 kΩ impedance across the pair and a driver must handle 32 of them. A quarter-unit-load receiver puts four devices in one unit load, which is why a modern DMX line can exceed 32 fixtures and an older one cannot.

Termination is 120 Ω at each physical end of the bus and nowhere else. Two terminators give 60 Ω across the pair, which is what the driver expects. A terminator part-way along a run, or a missing one, both show up as intermittent errors rather than a clean failure.

The common-mode range is −7 V to +12 V. A ground potential difference outside that window between two ends of a long run will corrupt data on a pair that measures fine, which is the case for a proper ground reference rather than just two conductors.

Typical use

Under DMX512 and RDM
Under most Modbus RTU field wiring
Motion and machinery control buses

The words for it

Glossary entries that name RS-485, with the false friends that catch people out.

Skew時間偏差The gap in arrival time between signals that are supposed to land together. On a parallel bus, skew is why wi…Unit load單位負載The standard reference load an RS-485 bus segment is rated against — one unit load is roughly a 12 kΩ receive…

Sources

TIA-485-A — Electrical Characteristics of Generators and Receivers for Use in Balanced Digital Multipoint Systems — Telecommunications Industry Association primary
The RS-485 Design Guide (SLLA272) — Texas Instruments
Guidelines for Proper Wiring of an RS-485 (TIA/EIA-485-A) Network — Analog Devices

Understand the mechanism

DMX on the wire Show networks

Something wrong, or missing?

Known gaps on this entry: default_ports, first_published, implementations. If you can source one of them, that is a ten minute pull request.

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