Rs-485 Standard; Rs-485 Operation - Brainboxes ES-357 Product Manual

This mode is generally used between one transmitter / receiver to only one other transmitter /
receiver, but it is possible for each output to drive up to 10 receivers.

RS-485 Standard

The RS-485 standard is similar to the RS-422 standard upon which it is based. The main difference is
that up to 32 transmitter / receiver pairs may be present on the line at one time.
Generally not all the lines available on the 9 pin D connector or terminal block are used for RS-485
systems. The RTS+/- and CTS+/- lines, though driven by the device, are usually not connected. In
two wire Half-Duplex configuration, the TXD+ line is connected to RXD+, whilst the TXD- line is
connected to RXD-. Only one pair of twisted wire cable is used in RS-485 Half Duplex
communication.
RS-485 Standard - Up to 32 Driver/Receiver Pairs
Line Length
40 Feet = 12m
400 Feet = 122m
4000 Feet = 1219m

RS-485 Operation

The RS-485 standard is intended for up to 32 driver receiver pairs on the bus. The line drivers used in
the RS-422/485 card are designed to work correctly in both RS-422 and RS-485 systems. The main
difference therefore is in how the system is implemented. Though the card uses a 9 pin D connector,
in general, not all the lines are used for RS-485 systems. The RTS+/- and CTS+/- lines, though driven
by the card, are usually not connected. In two wire, Half-Duplex configurations the TXD+ line is
connected to the RXD+ whilst the TXD- line is connected to the RXD-, only one pair of twisted wire
cable is used in RS-485 Half Duplex communications. The hardware handshaking performed by the
CTS+/- and RTS+/- lines in RS-422 systems are handled by a software protocol in RS-485 systems.
In situations where more than one device may transmit data on the shared data line, each cards RTS
line is used as a gating signal to enable the TXD driver only when that card needs to transmit data.
This mechanism prevents bus contention caused by multiple transmitters holding the line in opposing
states. Our cards have a facility which automatically "gates" the RTS line, thus enabling the
transmitter independently of any software. The three wiring schemes given described below are: -
1. RS-485 One Talker Many Listeners (HALF DUPLEX)
2. RS-485 Many Talkers Many Listeners (HALF DUPLEX.)
3. RS-485 Many Talkers Many Listeners (FULL DUPLEX.)
ES Range Product Manual
Max Data Rate
10 Mbits/sec
1 Mbits/sec
100 Kbits/sec
Version 2.9
© Copyright Brainboxes Limited 2013
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