AM7968-125DKC Advanced Micro Devices, AM7968-125DKC Datasheet - Page 67

• Manufacturer Part Number: AM7968-125DKC
• Description: TAXIchip Integrated Circuits(Transparent Asynchronous Xmitter-Receiver Interface)
• Manufacturer: Advanced Micro Devices

Figure 5-2
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 =
equivalent of this termination is Z0 pulled down to V
termination and adequate V
voltage without the need for a separate power supply.
Parallel Termination
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
I
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
OH
max. The maximum value of RE must be small enough to supply the transmission
Pseudo-ECL
TAXI TX
Driver
SEROUT
Z0
TAXIchip Integrated Circuits Technical Manual
= line characteristic impedance) and a pull-down voltage. The Thevenin
V
R1
R2
Equivalent
Thevenin
CC
OL
Copper Media
. Using V
Z0
R
1
| / R
CC
2
and a voltage divider provides pull-down
= Z
0
R2 (R1 + R2) V
CC
–2 V, assuring both matched
V
R1
R2
CC
CC
, to avoid exceeding
= V
CC
SERIN
TAXI RX
– 2 V
12330E-9
AMD
63