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.