AMD
common mode range. The average DC value of the input signal is therefore relatively
unimportant.
There are three broad classes of TAXl-to-media interface:
1. Very short (<3″ link length), usually DC coupled.
2. Terminated, DC coupled.
3. Terminated, AC coupled.
The short link is typical of a TAXIchip set to optical components connection. The
terminated cases are used for driving cables, also optical or other components with
incompatible power supply and/or logic level requirements may sometimes need circuits
and layout that exceed 3″.
5 .1 Ve ry S h o rt Lin k , DC Co u p le d
For DC coupled inter-connections in which the distance between the serial pins and the
next device is less than 3″, transmission line terminations are not necessary. All that is
required is an appropriate PECL pull-down resistor, RE. Elimination of reflections is not
required for these short line lengths because the round-trip propagation is significantly
less than the 2 ns TAXIchip set rise and fall time. The effect of media mismatch in this
case is distortion and slowing of the transition due to the addition of the reflection to the
still changing edge.
Fig u re 5 -1 a S t a n d a rd Lo a d Circ u it
VOH = 4.1 V
IOH
50 Ω
VCC –2 V = 3 V
12330E-7
Fig u re 5 -1 b P u ll-Do w n w it h IOH Ma t c h e d t o S t a n d a rd Lo a d
C
VOH = 4.1 V
IOH
RE
12330E-8
The lower limit for RE is that value which produces the maximum value of lOH. In a
standard PECL load circuit (Figure 5-1a) lOH max is given by:
(VOH – (VCC –2))/50 = (4.1–3)/50 = 22 mA
If we return RE to ground instead of 3 V (Figure 5-1b), the minimum value of RE
becomes 4.1 V/22 mA, or 186 Ω.
Reflections due to mismatch can be minimized by locating the pull-down resistor at the
end of the line, rather than the source. A mismatched line termination will give a
reflection coefficient less than one while leaving the end of the line open will give a
reflection coefficient of one (maximum reflection). Note that the supply voltage and logic
62
TAXIchip Integrated Circuits Technical Manual
AMD [ AMD ]
