19 Dec
2005
19 Dec
'05
2:49 p.m.
I recall reading that the Cray-1 used balanced logic
circuits, in which
a complementary pair of a signal and its complement were always
generated, and each fed a terminated transmission line. The idea was
that the machine drew essentially constant current regardless of its
logic state. Perhaps this placed constraints on the chips that could be
used, as most MSI functions are not implemented in such a perfectly
symmetric fashion.
Complementary outputs are basically a bonus with ECL, due to the circuit
design. In the standard families, often only one output is brought out on
complex functions, but on most of the simple functions (gates, muxes,
etc.) both a brought out. Damn handy, actually. If one is not needed, it
is simply terminated right there on the spot.
Older ECL "backplanes" used differential twisted pair, like the early
Crays. For connections on the same board, however, differential
transmission lines are a waste of resources - standard stripline (a well
defined trace over a power plane or two) works just fine. For short runs,
generally less than three inches for 100K speeds, you could even get away
without using a transmission line, as any reflections would get drowned
out by the relatively long rise and fall times.
William Donzelli
aw288 at osfn.org