485电路介绍

Jan Axelson

RS-485 is often the interface of choice.The network nodes can be PCs,microcontrollers, or any devices capableof asynchronous serial communications.Compared to Ethernet and other net-work interfaces, RS-485’s hardwareand protocol requirements are simplerand cheaper.

The RS-485 standard is flexibleenough to provide a choice of drivers,receivers, and other components de-pending on the cable length, data rate,number of nodes, and the need toconserve power.

Several vendors offer RS-485 trans-ceivers with various combinations offeatures. Also, there are options formethods of terminating and biasingthe line and controlling the driver-enable inputs.

In this article, I show you severalcircuits for RS-485 networks. Even if

20 Issue 107 June 1999

CIRCUIT CELLAR®

you use prebuilt cards or converters,understanding the options will helpyou choose the right product and

configure it to get the best results foryour application.

RS-485 IN BRIEF

But first, a quick look at RS-485.The interface popularly known as RS-485 is an electrical specification formultipoint systems that use balancedlines. RS-485 is similar to RS-422, butRS-422 allows just one driver withmultiple receivers whereas RS-485supports multiple drivers and receivers.The specification document (TIA/EIA-485-A) defines the electrical char-acteristics of the line and its driversand receivers. There are brief sugges-tions relating to terminations andwiring, but there’s no discussion ofconnector pinouts or software proto-cols (as there is for RS-232).

An RS-485 network can have up to32 unit loads, with one unit load equiva-lent to an input impedance of 12k. Byusing high-impedance receivers, youcan have as many as 256 nodes.

An RS-485 link can extend as far as4000′ and can transfer data at up to10 Mbps, but not both at the sametime. At 90 kbps, the maximum cablelength is 4000′, at 1 Mbps it drops to400′, and at 10 Mbps it drops to 50′.For more nodes or long distances, youcan use repeaters that regenerate thesignals and begin a new RS-485 line.Although the RS-485 standard saysnothing about protocols, most RS-485links use the familiar asynchronousprotocols supported by the UARTs inPCs and other computers. A transmittedword consists of a start bit followed bydata bits, an optional parity bit, and astop bit.

Two ways to add RS-485 to a PCare on an expansion card and by at-taching an RS-485 converter to anexisting port. Converters for RS-232are widely available and Inside OutNetworks has developed a USB–to–RS-485 converter, also available fromB&B Electronics. On microcontrollers,you can connect an RS-485 transceiver

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Designing

RS-485 Circuits