Specifications

Chapter 3 - Selecting RS-422 and RS-485 Cabling RS-422 And RS-485 Applications Ebook
Page 43 of 137 Manual Documentation Number: <DocNumber>
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Selecting RS-422 and RS-485 Cabling
Cable selection for RS-422 and RS-485 systems is often neglected.
Attention to a few details in the selection process can prevent the
costly prospect of re-pulling thousands of feet of cable.
The EIA-422 specification recommends 24 AWG twisted pair cable with
a shunt capacitance of 16 pF per foot and 100 ohm characteristic
impedance. While the EIA-485 standard does not specify cabling, the
EIA-422 specification should be used for RS-485 as well.
3 In This Chapter
Number of Conductors ................................................................................ 43
Shielding .................................................................................................... 44
Cable Characteristics ................................................................................... 45
Using Cat 5 for RS-422/485 ......................................................................... 47
Number of Conductors
Describing RS-422/485 communications systems as "two-wire" or
"four-wire" is somewhat misleading. To ensure that common mode
voltage does not compromise the data, or damage the equipment, an
extra wire should always be used to connect the signal grounds. This
signal ground conductor is often overlooked when ordering cable. This
means that a "two-wire" system actually requires three conductors,
and a "four-wire" system requires five conductors.
Although it is possible to obtain cable with a twisted pair and a third
conductor, or one with two twisted pairs and a fifth conductor, typically
it is simpler just to use a cable with an extra twisted pair and use one
CHAPTER 3