Specifications

Chapter 3 - Selecting RS-422 and RS-485 Cabling RS-422 And RS-485 Applications Ebook
Page 45 of 137 Manual Documentation Number: <DocNumber>
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Cable Characteristics
When choosing a transmission line for RS-422 or RS-485, it is
important to know the total length of the communication cable and the
data rate of the system.
Cable Length vs Data Rate
The Appendix to the EIA RS-422-A Standard presents an empirical
curve that relates cable length to data rate for 24 AWG twisted-pair
telephone cable that has a shunt capacitance of 16 pF/ft, and is
terminated in 100 ohms. This curve is based on the following signal
quality requirements:
Signal rise and fall time equal to, or less than, one-half unit interval
at the applicable modulation rate.
The maximum voltage loss between driver and load of 6 dB.
Figure 32: Cable Length vs Data Rate
In practical terms this graph indicates that RS-422/485 systems can
have cable lengths up to 4000 feet when operating at less than