Page A-1
Manual 00650-519-1
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. Second, many
RS232 errors are the result of noise induced on the cables. The RS422 standard permits cable
lengths up to 5000 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
9-pin
Connector
Signal
9-pin
Connector
Gnd
5
Gnd
5
TX
+
2
RX
+
9
TX
-
3
RX
-
1
RX
+
9
TX
+
2
RX
-
1
TX
-
3
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.
