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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 Te rm in a t e d , DC Co u p le d
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-
′
Thevenin
tion (R1llR2 = Z0 = line characteristic impedance) and a pull-down voltage. The
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.
Fig u re 5 -2
P a ra lle l Te rm in a t io n
VCC
TAXI TX
TAXI RX
Pseudo-ECL
Driver
R1
Copper Media
Z0
SEROUT
SERIN
R2
VCC
R1
12330E-9
′
Thevenin
Equivalent
R1 | / R2 = Z0
R2 (R1 + R2) VCC = VCC – 2 V
R2
5 .3 Te rm in a t e d , AC Co u p le d
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
I
OH 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
63
TAXIchip Integrated Circuits Technical Manual
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