Kontron PCI-COM-8ULP, Руководство пользователя

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PCI-COM-8ULP User’s Guide page 29
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 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 Pin No. Signal Pin No.
Gnd 5 Gnd 5
TX
+
2 RX
+
9
TX
-
3 RX
-
1
RX
+
9 TX
+
2
RX
-
1 TX
-
3
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.
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.