RS485 - an introduction

The RS485 standard is used when high speed serial data communications of 10 Mbps is needed.


Serial Data Transmission Includes:
Serial data standards    

Common standards:     RS232     RS422     RS449     RS485     20 mA current loop    


RS485 was developed to provide high speed data. The standard is defined by industry telecommunications bodies and may be referred to most commonly as RS485, but references to EIA485 or TIA-485 may also be seen.

RS485 is able to provide a headline data rate of 10 Mbps at distances up to 50 feet, but distances can be extended to 4000 feet with a lower speed of 100 kbps.

Although RS485 was never intended for domestic use, it found many applications where remote data acquisition was required.

RS-485 applications

Often RS-485 links are used for simple networks, and they may be connected in a 2 or 4 wire mode. In a typical applications several address able devices may be linked to a single controlled (PC), and in this way a single line may be used for communication. It is also possible to convert between RS485 and RS232 using simple interface converters that may include optical isolation between the two circuits as well as surge suppression for any electrical 'spikes' that may be picked up.

Using RS-485, it is possible to construct a multi-point data communications network. The standard specifies that up to 32 drivers or transmitters along with 32 receivers can be used on a system. This means that there can be 32 nodes capable to both transmit and receive. This can be extended further by using "automatic" repeaters and high-impedance drivers / receivers. In this way it is possible to have hundreds of nodes on a network. In addition to this, RS485 extends the common mode range for both drivers and receivers in the "tri-state" mode and with power off. Also, RS-485 drivers are able to withstand "data collisions" (bus contention) problems and bus fault conditions.

As RS485 networks become larger, the problem of data collisions becomes greater. This can be solved, at least in part by ensuring the hardware units (converters, repeaters, micro-processor controls) are designed to remain in a receive mode until they are ready to transmit data.

Another approach is to design a 'single master' system. Here the master initiates a communications request to a "slave node" by addressing that unit. The hardware detects the start-bit of the transmission and thereby enables the transmitter. Once a requested data is sent the hardware reverts back into a receive mode.


RS485 specification overview

The table below provides the highlight details behind RS485.


RS485 Highlight Specifications
Attribute Specification
Cabling Multi-drop
Number of devices 32 transmitters
32 receivers
Communications modes half duplex
Maximum distance 4000 feet @ 100 kbps
Maximum data rate 10 Mbps @ 50 feet
Signalling Balanced
Mark (data = 1)
condition
1.5 V to 5 V (B greater than A)
Space (data = 0)
condition
1.5 V to 5 V (A greater than B>
Driver output current capability 250 mA


RS485 was able to provide high speed data communications. Although many other formats have taken over, it was widely used and was able to meet the needs for a number of applications where data needed to be transmitted at high speeds for the time. RS485 proved to be a robust standard and able to provide reliable data communications over extended distances.



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