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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:
- 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 3.1 - Data Signaling Rate Versus Cable Length for Balanced
Interface Using 24 AWG Twisted Pair Cable
Losses in a transmission line are a combination
of AC losses (skin effect), DC conductor loss, leakage, and
AC losses in the dielectric. In high quality cable, the conductor
losses and the dielectric losses are on the same order of
magnitude. Figure 3.2 is included in this application note
to point out the significant difference in performance of
different cables. This chart shows Attenuation versus Frequency
for three different Belden cables. Note that the polyethylene
cables offer much lower attenuation than PVC cables.
Note 1: Above attenuation data is supplied courtesy Belden
Wire and Cable Company
Note 2: Datalene is a registered trademark of Belden Wire
and Cable Company. Datalene is a type of foamed cellular polyethylene
insulation.
Figure 3.2 - Attenuation Versus Frequency for Several Data
Cables
Another approach to choosing transmission
line is the "E-GRADE Program," which has been established
by Anixter Bros. Inc. Anixter is a worldwide distributor of
wiring system products. Under this program, Anixter divides
data interface cables into four categories as follows:
E-GRADE 1
LIMITED DISTANCE
E-GRADE 2 STANDARD DISTANCE
E-GRADE 3 EXTENDED DISTANCE
E-GRADE 4 MAXIMUM DISTANCE
Simple charts are used to help the user
select the proper cable without any technical understanding
of the cable parameters. This program divides the usage categories
into EIA-232-D, EIA-422-A, and EIA-423-A. When using this
literature, use the EIA-422-A charts for choosing RS-485 cable.
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