AM7968 AMD [Advanced Micro Devices], AM7968 Datasheet - Page 77

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

Figure 6-4
8. When terminating serial lines to or from the TAXls ensure that the Vcc rail or ground
Decoupling Terminations
6.2 Layout using Fiber Optic Data Links
Because of their small signals levels, fiber optic data links require some care in layout.
Fiber optic data Link receivers consist of a photo sensitive diode and an amplifier. The
photo-diode converts light pulses into currents of around a few hundred nano-amps.
This signal current is then amplified and translated into an ECL signal.
TAXI Receivers and most digital chips switch hundreds of milliamps. If switching noise
from the digital section of the board gets coupled into the optical data link, the signal
from the light pulse data can be corrupted. To prevent the coupling of the optical data
link output with other digital signals, the user must ensure that small signal and digital
switching currents do not flow in the same path. This is done by separating both the
optical V
other digital circuitry. See Figure 6-5.
tap is not at a noisy location. Resistors can couple noise from a power supply rail into
the Serial lines. Vcc to Ground decoupling adjacent to the resistors is recommended
when using pullup/pulldown terminating resistor setups as shown in Figure 6-4.
When using only a pulldown, do not use decoupling as this could add more Vcc noise
into the serial signals.
TAXI
TX
CC
+5 V
plane and the optical ground plane from the V
+
–
RE
TAXIchip Integrated Circuits Technical Manual
RE
No Decoupling
Capacitor Required
R1
R2
R1
R2
CC
and Ground planes used by
+5 V
+
–
TAXI
RX
Decoupling
Capacitor
GND
12330E-21
AMD
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