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Appendix A: Application Considerations
Introduction
Working with RS422 and RS485 devices is not much different from working with standard RS232
serial devices and these two standards overcome deficiencies in the RS232 standard. First, the cable
length between two RS232 devices must be short; less than 50 feet at 9600 baud. Second, many
RS232 errors are the result of noise induced on the cables. The RS422 standard permits cable lengths
up to 4000 feet and, because it operates in the differential mode, it is more immune to induced noise.
Connections between two RS422 devices (with CTS ignored) should be as follows:
Device #1 Device #2
Signal Pin No. Signal Pin No.
Gnd 7 Gnd 7
TX 24 RX 12
+ +
TX 25 RX 13
- -
RX 12 TX 24
+ +
RX 13 TX 25
- -
Table A-1: Connections Between Two RS422 Devices
A third deficiency of RS232 is that more than two devices cannot share the same cable. This is also
true for RS422 but RS485 offers all the benefits of RS422 plus allows up to 32 devices to share the
same twisted pairs. An exception to the foregoing is that multiple RS422 devices can share a single
cable if only one will talk and the others will all receive.
Balanced Differential Signals
The reason that RS422 and RS485 devices can drive longer lines with more noise immunity than
RS232 devices is that a balanced differential drive method is used. In a balanced differential system,
the voltage produced by the driver appears across a pair of wires. A balanced line driver will produce
a differential voltage from ±2 to ±6 volts across its output terminals. A balanced line driver can also
have an input "enable" signal that connects the driver to its output terminals. If the "enable" signal
is OFF, the driver is disconnected from the transmission line. This disconnected or disabled
condition is usually referred to as the "tristate" condition and represents a high impedance. RS485
drivers must have this control capability. RS422 drivers may have this control but it is not always
required.
Page A-1
Manual MRDAG12-8H.Bc2