AM7968-125JC datasheet(67/127 Pages) AMD

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TAXIchip Integrated Circuits Technical Manual

level of the optical components must match those of the TAXIchip set in order for the DC

connection to work. If the supply voltage or the logic levels are incompatible, an AC

connection must be used.

5.2 Terminated, DC Coupled

The parallel termination shown in Figure 5-2 may be used for a DC connection of a TAXI

Transmitter to a TAXI Receiver. The parallel termination provides both the line termina-

tion (R1llR2 = Z0 = line characteristic impedance) and a pull-down voltage. The Thevenin

′

equivalent of this termination is Z0 pulled down to VCC–2 V, assuring both matched

termination and adequate VOL. Using VCC and a voltage divider provides pull-down

voltage without the need for a separate power supply.

Figure 5-2

Parallel Termination

12330E-9

VCC

R2 (R1 + R2) VCC = VCC – 2 V

R1 | / R2 = Z0

Thevenin

Equivalent

VCC

SERIN

TAXI RX

SEROUT

TAXI TX

Pseudo-ECL

Driver

Copper Media

′

5.3 Terminated, AC Coupled

AC coupling is the connection of choice for many TAXIchip set applications. The typical

arrangement for an AC coupled link is shown in Figure 5-3. RE is returned to ground to

provide the PECL (pseudo-ECL) pull-down for the driver. The capacitor C blocks the DC

voltage, and R1 and R2 terminate the transmission line and provide a DC bias level for

the Receiver. Since only AC variations are passed through the coupling capacitor, the

bias level at the termination should be set to the midpoint of the signal swing expected

by the Receiver input stage. Note that this bias level is not the same as that which is

recommended for the DC coupled case.

The minimum value of RE was previously established as 186

Ω, to avoid exceeding

IOH max. The maximum value of RE must be small enough to supply the transmission

line with enough current to avoid cutting off the output driver. When switching from a

HIGH to the LOW state, the transmission line may cause the emitter voltage of the driver

to fall more slowly than the base voltage, causing the output transistor to turn off. When

the output transistor turns off, its output impedance becomes very high, causing the

falling edge rate to be controlled by the external load (RE and the transmission line).

This variation in edge rate cannot be tolerated until the falling edge crosses the

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