# Overview

The RS-485 communication protocol is a differential, multi-drop, half-duplex, two-wire communication protocol. It was formally specified in 1983 by the Electronics Industries Association (EIA).

It is a very common protocol used in industry for between-room control system to sensor communication.

 Name RS-485 Drive Mode Differential Multi-drop Yes Half-duplex Maximum Data Rate 10Mbps Maximum Bus Length 1.22km (at a data rate of 100kbps)

It is not commonly used for intra-board communication (i.e. between two devices on the same PCB) because in these scenarios there is normally no need for the noise immunity and transmission distances RS-485 provides, and communication protocols such as UART, SPI and I2C are more prevalent.

# Standards

The TIA/EIA 485 standard governs the RS-485 protocol specifications.

The RS-485 standard only defines the electrical specifications of the interface, and does not specify the type of connector to be used, the cable, the pinout, or the messaging protocol/data structure.

Thus, the RS-485 can be incorporated into higher-level standards which specify things such as the messaging protocol, and use RS-485 to define the physical layer.

# Transmission Distances

Because RS-485 is a differential communication protocol, it can achieve far greater communication distances than say, UART, I2C or SPI.

A distance of 1.22km (4000 feet) is achievable at a data rate of 100kbps.

# Baud Rate

RS-485 transceivers usually top out at about 50-100Mbps.

# Node Count

Higher baud rate transceiver IC’s usually support a lower number of total nodes on the RS-485 bus.

RS-485 introduces the term unit load. A unit load is a specified load impedance on the RS-485 bus. Transceivers are rated by the equivalent unit loads of impedance they introduce when connected to the bus.

 Unit Load No. of Nodes Min. Receiver Input Impedance 1 32 $$12k\Omega$$ $$\frac{1}{2}$$ 64 $$24k\Omega$$ $$\frac{1}{4}$$ 128 $$48k\Omega$$ $$\frac{1}{8}$$ 256 $$96k\Omega$$

# Termination Resistors

The TIA/EIA 485 standard does not specifically state what the characteristic impedance of the twisted-pair cable should be, nor the value of the termination resistors. However, it does provide recommendations, and states that the twisted-pair cable should have a characteristic impedance of $$120\Omega$$ whenever possible.

This implies that $$120\Omega$$ termination resistors should be used with this twisted-pair cable.

Receiver hysteresis is normally around 80mV.

# Standard Pinout

There is somewhat of a standard pinout for RS-485 transceivers in 8-pin component packages as follows:

 Pin Number Pin Name Pin Description 1 R, RO Receiver data output. 2 nRE, RE* Active-low receiver outout enable. 3 DE Active-high RS-485 line driver enable. When high, the IC will be driving the RS-485 A and B wires, when low the A and B pins are put into high-impedance and the IC acts as a RS-485 receiver. 4 D, DI Driver data input. If the driver outputs are enabled (DE high), then a low on DI drives A low and B high, while a high on DI drives A high and B low. 5 GND Ground. 6 A RS-485 differential line A. 7 B RS-485 differential line B. 8 $$V_{CC}$$ Supply voltage.

8-pin packages that RS-485 transceivers come include DIP-8, SOIC-8, TSSOP-8 and MSOP-8. Example transceivers that follow this “standard” include the Texas Instruments DS485, Linear Technology LTC1480

# Differential Voltage Specs

The RS-485 specification states that the transmitter must produce a differential voltage of at least ±1.5V when loaded, and the receiver must receive a differential voltage of at least ±200mV.

The waveform below shows the voltage on the A and B nets of a RS-485 bus when operating normally at 115200 baud.

The standard also states that the driver is not allowed to produce a differential voltage of more than ±6V.

# Common-Mode Voltage

Most transceivers can withstand a constant single-ended voltage of around ±15V on each of the A and B wires (the allowed transient voltage can be much higher).

# Specialised TVS Diodes

Dedicated TVS diode components exist for voltage-spike suppression on RS-485 data lines.

They usually are a bidirectional TVS diode (or diode array) with asymmetric breakdown voltages, that match the maximum voltage specifications of +12V and -7V for RS-485 data lines.

The +12V and -7V limits arise from the section of the RS-485 spec which states that up to a 7V ground difference is allowed between any two devices on a RS-485 bus. This, coupled with the spec that allows a single-ended voltage range of 0-5V to be applied to either of the bus nets, gives a possible voltage range of +12V (7V + 5V) to -7V (-7V + 0V).

One such example is the Semtech SM712 diode array. Below is an image of the components internal schematic.