RS-485 – The Wired Communication Standard

TIA/EIA-485 is a popular wired communication standard published by the TIA/EIA or the Telecommunications Industry Association/Electronics Industries Association. This standard is also known as the RS-485 and uses differential signaling enables the standard to transmit data over long distances for factory automation and in noisy industrial environments.

This is because differential signaling allows rejection of common mode noise, while the twisted pair cable ensures the most received interference comes as common mode. When used over long distances, the standards improve the chances for ground potential differences, while the wide CMR or common mode range of the standard ensures that the network operates satisfactorily, even when there are large common mode voltages present.

In practice, both the transmitter and the receiver have non-inverting and inverting pins. Bidirectional communication over a single cable can use half-duplex devices, where the corresponding Receiver and Transmitter terminals connect to the same IC pins. Networks can also use two cables for bidirectional communication, and employing full-duplex devices, only, the Receiver and Transmitter terminals now must connect to separate pins.

The number of transceiver models available in the market is huge, and that makes it a challenge picking out the best and most cost-effective device for a specific application. That requires considering the common design considerations, examining the electrostatic discharge (ESD) protection and comparing the Human Body Model. Other important points to be considered are the over voltage protection (OVP) and data skew in case of high-speed transmissions.

Requirements of RS-485

Although the published standard for the RS-485 is over 14 pages long, the most important requirements are:

The differential output voltage generated must be over ±1.5V, while the receiver must be capable of detecting signals with a minimum of ±200mV. This combination makes sure the devices can tolerate attenuation from long cables and there is a robust noise margin available.

As the standard allows multiple drivers on the bus, each transmitter must have an enable pin giving it tri-state output capability. This ensures true bidirectional transmission over a single cable.

Transmitters must have high output current capability to drive long cables and cables with double termination, especially for high-speed bidirectional transmission.

The CMR should be at least -7V to +12V. This allows using RS-485 over networks of 1220 m or 4000 feet. Long distances can involve ground potential differences and a high CMR helps to tolerate them in noisy environments. Additionally, devices with different supply voltages can also communicate on the same bus because of a large CMR.

The receiver input resistance must be over 12KΩ. According to the standard, there can be 32 devices on a bus.

The Basic RS-485 Transceiver

People often use less expensive RS-485 transceivers for simple, short, low-node-count networks. This works because short networks do not involve much CMV or common mode voltage and OVP, and they can work with the CMR specified by the standard.

When there are less than 32 modes in the network, fractional unit load devices are not necessary. Moreover, when cables are not frequently connected and disconnected, ESD protection is also not necessary. However, most basic devices now include the ±8 to ±15 KV Human Body Model for ESD protection.