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20 RS-422/485 Application Note

 Copyright B&B Electronics -- Revised October 1997

B&B Electronics -- PO Box 1040 -- Ottawa, IL 61350

PH (815) 433-5100 -- FAX (815) 434-7094

Chapter 3: 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.

Number of Conductors

The signal ground conductor is often overlooked when ordering cable. An

extra twisted pair must be specified to have enough conductors to run a signal

ground. A two wire system then requires two twisted pair, and a four wire

system requires three twisted pair.

Shielding

It is often hard to quantify if shielded cable is required in an application or

not. Since the added cost of shielded cable is usually minimal it is worth

installing the first time.

Cable Characteristics

When choosing a transmission line for RS-422 or RS-485, it is necessary

to examine the required distance of the cable and the data rate of the system.

The Appendix to 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 (see

Figure 3.1). This curve is based on signal quality requirements of:

a). Signal rise and fall time equal to, or less than, one-half unit interval at

the applicable modulation rate.

b). The maximum voltage loss between driver and load of 6 dB.

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