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8. When terminating serial lines to or from the TAXls ensure that the Vcc rail or ground
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.
Fig u re 6 -4
De c o u p lin g Te rm in a t io n s
Decoupling
Capacitor
+5 V
+5 V
No Decoupling
Capacitor Required
TAXI
TX
TAXI
RX
R1
R2
R1
R2
+
–
+
–
GND
RE
RE
12330E-21
6 .2 La yo u t u s in g Fib e r Op t ic Da t a Lin k s
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 VCC plane and the optical ground plane from the VCC and Ground planes used by
other digital circuitry. See Figure 6-5.
73
TAXIchip Integrated Circuits Technical Manual
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