RS-485

RS-485#

Differential multidrop serial. A twisted pair carries every byte as the difference between two lines, so the wire survives long runs through noisy industrial environments. Up to 32 standard nodes share a bus (more with low-load receivers), at speeds up to 10 Mbps on short runs or 100 kbps over 1200 m. The backbone under Modbus RTU, BACnet MS/TP, DMX-512, and a fair share of building automation.

Wires#

Signal

Direction

Purpose

A (D+, TXD+)

Bidirectional

Non-inverting line of the differential pair.

B (D-, TXD-)

Bidirectional

Inverting line.

GND

Reference

Signal ground; runs alongside the pair on long buses to limit common-mode swing.

Both ends of the bus terminate with a 120 Ω resistor between A and B. Some installations bias the lines with pull-up / pull-down resistors so the bus holds an idle 1 when no transmitter is active.

Wire format#

The differential voltage between A and B encodes the bit.

  • Logical 1 (mark): A is positive relative to B (typically +2 V or more).

  • Logical 0 (space): A is negative relative to B.

Framing on top of RS-485 is whatever the application layer chooses (usually UART-style, 8N1, with an application protocol like Modbus RTU layered on top). Half-duplex is the common topology; full-duplex needs four wires (sometimes called RS-422).

Pads#

Label

Meaning

A, D+, TX+, TXD+, Y

Non-inverting line.

B, D-, TX-, TXD-, Z

Inverting line.

GND

Ground; sometimes called COM.

Labels are not consistent across vendors. The operator confirms polarity with a multimeter (idle voltage with a known transmitter holding the line) before assuming.

Tools#

Tool

Effect

USB-to-RS-485 adapters (FT232 + MAX485, CH340 + MAX485)

Provide /dev/ttyUSBN with RS-485 transceiver and auto direction control.

MAX485, SN65HVD75

Transceiver ICs; convert a CMOS UART to differential RS-485.

modpoll, mbpoll

Modbus master clients that drive an RS-485 adapter.

pymodbus

Scripted Modbus RTU master / slave in Python.

Logic analyzer with differential probe

Decode the line.

References#

  • TIA/EIA-485-A Electrical Characteristics of Generators and Receivers for Use in Balanced Digital Multipoint Systems.

  • Modbus Organization, Modbus over Serial Line Specification and Implementation Guide.

  • RS-232 for the bipolar point-to-point predecessor.

  • CAN for the differential automotive peer.

  • libmodbus.

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