Product manual

Serial Solutions Range Product
Manual
© Copyright Brainboxes Limited 2010 Page 34 of 47
7. Optional RS422/485 Operation
7.1.1. The RS422 Standard
The RS422 standard defines a serial communications standard. RS422 is a high speed and/or long distance
data transmission. Each signal is carried by a pair of wires and is thus a differential data transmission
system. Over distances up to 40 feet the maximum data rate is 10 Megabits per second, and for distances
up to 4000 feet the maximum data rate is 100 Kilobytes per second. A 120-Ohm resistor should be used to
terminate the receiving end of the line. It is generally used between one transmitter receiver pair to ONLY
one other transmitter receiver pair, but each output can drive up to 10 receivers.
RS422 Standard
1 Driver up to 10 receivers
Line Length Max Baud Rate
40 feet = 12m 10 Mbits/sec
400 feet = 122m 1 Mbits/sec
4000 feet = 1219m 100 Kbits/sec
7.1.2. The RS485 Standard
The RS485 standard is similar to the RS422 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. A 120-Ohm resistor should be used
to terminate either end of the main line. If more than one device may transmit data, the RTS line is used as
transmit enable signal, so preventing contention between talkers.
RS485 Standard
Up to 32 Driver/Receiver Pairs
Line Length Max Baud Rate
40 feet = 12m 10 Mbits/sec
400 feet = 122m 1 Mbits/sec
4000 feet = 1219m 100 Kbits/sec
7.1.3. Terminating Impendence’s
RS422 and RS485 lines should be terminated at the end of the main branch of the RECEIVER, in the cables
characteristic impedance. These terminating impedance’s stop echoes caused by the serial data being
reflected back at the cable ends. It is not necessary to terminate the transmitter end of the twisted pair.
PCI Quad 422/485 Card Chapter 10 Optional RS422/485 Operation • 49 The AT Dual Port RS422/485, Opto
Isolated AT Dual Port RS422/485 and AT Velocity RS422/485 cards have the correct 120 Ohm (nominal)