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A note on standards
8
Guidelines for application
The RS-422 and RS-423 standards do not specify maximum distances or maximum
data rates for connecting cables. These standards do, however, include “Guidelines
for application” which give conservative values for maximum cable length and data
rates based upon the following cable type:
• twisted pair, copper
• 52.5pF/meter shunt capacitance
• 24 AWG
The following information is based upon these guidelines. The performance
achievable in a real configuration will differ from that suggested by the guidelines,
as performance is also determined by the environment and the configuration itself.
Single-ended versus Differential Transmission
Two methods are used for sending digital signals between components (such as a
system and a terminal). The first method, single-ended data transmission, is based
upon the use of one signal line. The second method, differential data transmission,
uses two signal lines.
Single-ended data transmission is used by equipment which conforms to the RS-232
or RS-423 standards. Differential data transmission is used by the RS-422 standard.
RS-232 The RS-232 standard, though widely used, has a maximum data rate of 20kBaud
and cannot be used over long distances (maximum of 50 feet/15 meters). However,
it has the advantage of being commonly used and understood. It can also operate
with just three conductors, if necessary, for direct terminal connection.
Data
In
Data
Out
RS-232C
Interface
RS-232C Application
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