AM7968-125JC datasheet(73/127 Pages) AMD

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TAXIchip Integrated Circuits Technical Manual

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.

Figure 5-10

Coaxial Cable Interface

12330E-17

TAXI

L, Z0

+5 V

TAXI

–

+5 V

R1R2

R1+ R2

= Z0

5R2

R1+ R2

= Midpoint of Pseudo-ECL Signal Swing = 3.7

186

≤ RE ≤ 7.2 (Z0)

C = Largest RF Capacitor Available

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