RS-485 tutorial

- an overview of the RS-485 standard used for the transmission of high speed serial data

RS485 is a standard for the transmission of serial data long a hard wired cable. The system is defined under EIA/TIA-485 and RS-485 provides the ability for multi-drop cabling and for speeds of up to 10 Mbps over short runs of up to 50 feet and slower communications at speeds of 100 kbps at distances of 4000 feet. Although not widely used in domestic environments, it is widely used for data acquisition applications.

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

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)
1.5 V to 5 V (B greater than A)
Space (data = 0)
1.5 V to 5 V (A greater than B>
Driver output current capability 250 mA

By Ian Poole

Read more popular data communications tutorials . . . . .

Ethernet RS-232 RS-422 RS-449
RS-485 Serial data comms USB Current loop

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