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Because of the resultant lower system costs, coaxial cable is the recommended serial
medium for short-to-moderate length links. At longer lengths, the advantages of fiber
optic transmission (low attenuation, immunity to EMI and ground loops, etc.) make it the
media of choice.
The maximum length possible for a coaxial cable TAXI link depends on the type of
coaxial cable used and the data rate. Higher data rates will tend to limit link lengths
because attenuation and pulse dispersion on coaxial cable increases with frequency.
Many different types of coaxial cables are available. Some have far less attenuation
than others however, with low loss generally comes increased size and rigidity.
RG-58 is a commonly used, readily available type of coaxial cable. In lab tests using this
type of cable, it was found that the TAXlchip link could operate with a byte error rate of
better than 10–10 with a confidence limit of 95%, at byte rates of up to 12.5 MHz, at
distances of up to 200 feet. The confidence limit accounts for the statistical nature in
which errors occur in a digital system and it implies that we can be 95% sure that, under
the given circumstances, the byte error rate will be 10–10 or better. Note that a byte error
could have been due to a single bit error or more hence, the bit error rate may not be
equal to the byte error rate divided by ten.
Using the TAXlchip set in conjunction with coaxial cable as the serial media is quite
simple. Appropriate line terminations are required and AC coupling is strongly recom-
mended to eliminate ground loops. The recommended configuration, including the
necessary design equations, is shown in Figure 5-10. Each of the components that
make up the interface serve the same purpose as in the AC-coupled TAXl-fiber optic
interface shown in Figure 5-9.
Note that two coaxial cables comprise the link, one for each of the differential pseudo
ECL signals. These two lines should be calibrated for a propagation delay difference of
less than 0.2 ns.
Fig u re 5 -1 0 Co a x ia l Ca b le In t e rfa c e
+5 V
+5 V
R1
TAXI
TX
TAXI
RX
R1
R2
L, Z0
C
C
+
–
+
–
RE
R2
RE
R1R2
= Z0
R1+ R2
5R2
= Midpoint of Pseudo-ECL Signal Swing = 3.7
R1+ R2
12330E-17
186 ≤ RE ≤ 7.2 (Z0)
C = Largest RF Capacitor Available
69
TAXIchip Integrated Circuits Technical Manual
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