73
A note on standards
8
Wiring
The higher data rates and longer cable lengths supported by the RS-422 and RS-423
standards allow the component wavelengths of the digital signals to be shorter than
the electrical length of the cable. Consequently, the connection should be treated as
a transmission line.
The characteristic impedance of the cable connection should be around 100 ohms
for frequencies greater than 100kHz. Additionally, the DC series loop resistance of
the cable should be less than 240 ohms. The cable can be twisted pair or untwisted
pair (flat cable) with these characteristics.
The conductors should be 24 AWG or larger with a resistance not exceeding
30 Ohms per 1000 feet for each conductor.
Mutual pair capacitance between 2 conductors in a pair should not exceed 20 pF per
foot.
Stray capacitance between 1 conductor of a pair with all other conductors connected
to ground should not exceed 40 pF per foot.
A single driver and a single receiver are utilised when a terminal or printer is
directly connected to a system, via an RS-422 or RS-423 interface. As a
two-conductor pair is needed to connect a driver to a receiver, four conductors are
necessary to make a direct connection.
RS-423 connections using more than one signal driver can benefit from its
single-ended nature by using a common ground return line for all signals. This is
common for connections using hardware flow control, where each device has a data
signal, a flow control signal and a shared ground return.
RS-422 and RS-423 are intended for long-distance connections in
datacommunications applications. These long distances make the use of
prefabricated cables difficult and uneconomical. Consequently, there are presently
no RS-422 or RS-423 datacommunication cables available from Hewlett-Packard.
Instead, cables must be made using the advice provided in this document.
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