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. 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 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
25 pin Signal
9 pin
25 pin
Gnd
5
7
Gnd
5
7
TX
+
2
24
RX
+
9
12
TX
-
3
25
RX
-
1
13
RX
+
9
12
TX
+
2
24
RX
-
1
1
TX
-
3
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 always 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.
A balanced differential line receiver senses the voltage state of the transmission line across the two
signal input lines. If the differential input voltage is greater than +200 mV, the receiver will provide a
specific logic state on its output. If the differential voltage input is less than -200 mV, the receiver will
provide the opposite logic state on its output. The maximum operating voltage range is from +6V to -6V
allowing for voltage attenuation that can occur on long transmission cables.
Manual PCI-COM-1S
18
