# Who's rewired their house for this hobby?

Jon Elson elson at pico-systems.com
Thu Nov 27 11:10:14 CST 2014

On 11/26/2014 09:47 PM, Billy Pettit wrote:
> Will wrote:
>
> "On the other hand, ECL does not suffer from the spikes, burps and
> farts TTL tends to make.
>
> Late model Cybers tend to have very little in the way of filtering,
> simply because the ECL load is "nice"."
>
> ECL, if done properly, uses a terminator on every signal line.  On MECL it is usually it's 560 ohms to -5.2v or 220 ohms to -2.2 volts.
There are two ways to do it.  You can have a terminator at
the far end of the line and
a pull-down at the driver end.  The pull-down will be as you
mention above, but the
terminator will be a much lower resistance, somewhere
between 51 and 91 Ohms,
typically.  The other way is to have the terminator resistor
combine both functions,
so you only need one resistor at the far end of the line.
Very short lines can be
done with only the pull-down resistor.  Generally, you can
only get away with that
for a couple inches.  The terminator power is so large in
ECL systems it can
often exceed the power dissipation in the chips themselves.

The terminator needs to match the characteristic impedance
of the transmission
line.  It is probably around 80-90 Ohms for wire wrap wire
over a backplane.
Differential ribbon cable is 110 Ohms, and coax can be
anywhere from 51 - 91
Ohms.  So, if you have a 91 Ohm terminating resistor to -2.2
V, and the logic
high is -0.8 V, then the terminating resistor draws 15 mA!
Now, assume 1000
single-ended terminated lines and you've got 15 Amps.  Since
damn near EVERY
line needs to be terminated, it is real easy to get to 1000
terminators.  So, that's
why short lines only use pull-down resistors, to save
power.  A logic low on the
same scheme draws about 6.5 mA, but the 220 Ohm pull-down
only draws 6 mA
on the worst case (logic high).

Jon