RS-485 for long distance wired transmission



The RS-485 (or TIA-485(-A), EIA-485) standard is widely used in wired communications, for two-wire connection with a data transmission rate up to 10Mbps. The main reason why RS-485 can be used to communicate over long distances is the use of differential or balanced lines.

The following figure is the schematic diagram for an RS-485 circuit. We named the two differential lines A and B.
In the case that a logic high is received at the Transmitter Input (DI = 1), the line A voltage level at the output of the driver will be higher than line B (VOA> VOB).
Conversely, if logic low is received (DI = 0), the driver will charge line B, so its voltage level will be higher than line A (VOB> VOA).

On the receiver, using a differential opamp for two voltage comparisons to ground, the output will to be logic high RO = 1 (when VIA − VIB > 200 mV) and logic low RO = 0 (when VIB − VIA > 200 mV).

The RS-485 standard conforming drivers provide a differential output of a minimum 1.5 V across a 54-Ω load,
whereas standard conforming receivers detect a differential input down to 200 mV. The two values provide
sufficient margin for a reliable data transmission even under severe signal degradation across the cable
and connectors. This robustness is the main reason why RS-485 is well suited for long-distance
networking in noisy environment.

Since the noise coupled into the system is transmitted equally on both differential lines, one line radiates the waves in the transmission and just offsets the waves on the other line, which reduces the electromagnetic interference (EMI) in the system.

Check out ADI’s ADM3065/ADM3066 which is an IEC ±12 kV ESD protected transceiver with RS-485 protocol. These transceivers offer half duplex with high speed bidirectional data communication up to 50 Mbps and are fully compliant with Profibus® standard at VCC greater than or equal to 4.5 V.

RS-485 cables are generally twisted-pair, such as Alpha Wire’s 6453 series, which can increase noise resistance and reduce noise emissions, both useful for long-range transmission.

More articles and information about RS-485 can be found here:

ADI:RS-485/RS-422 Circuit Implementation Guide

Prodcuts of RS485 interface including drivers, receivers and transceivers