PCI Quad 422/485 Card
Chapter 10 Optional RS422/485 Operation
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Chapter 10 Optional RS422/485
Operation
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 Data Rate
40 Feet = 12m
10 Mbits/sec
400 Feet = 122m
1 Mbits/sec
4000 Feet = 1219m
100 Kbits/sec
D
R
TTL
TTL
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 Data Rate
40 Feet = 12m
10 Mbits/sec
400 Feet = 122m
1 Mbits/sec
4000 Feet = 1219m
100 Kbits/sec
D
D
R
R
R
Terminating Impedance’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.
