The next thing we noticed about mothers was that some of them let their children run hog
wild. This phenomena is called "ringing" (See Oscilloscope Photo 2). Accordingto Murphy's '
Law, no sooner do you think you finally have your kids straightened out than something else
comes along to gum up the works. To corral the little buggers and make them walk the
straight and narrow, anew feature was added to motherboards, called terminators.
There are two types of terminators: active and passive. Apassive terminator is nothing
more than two resistors, one going to avoltage supply and the other going to aground, with
the signal line tied between, and sometimes abypass capacitor which is supposed to short the
high-frequency stuff no one is interested in to ground.
There is aproblem with this approach. All those resistors gobble up power, and they've no
place to dine but the signal; q.e.d. the signal's power gets eaten. Also, there is aspecific time-
constant involved anytime you connect acapacitor to aresistor, and unless you do it exactly
right (which most board designers don't), you foul up the rise and fall times of the signals.
Active terminators are a little bit better. They're about the same as a passive terminator,
the exception being that in aproperly designed active terminator the power drain doesn't
come from the signals that are being terminated, but from the central power supply through
an active component like an op amp.
This cured the ringing--somewhat. But engineers are sometimes like economic
theoreticians--they attack the symptoms and neglect the root causes. And despite all these
approaches, the basic problem of the bad mothers was still there: 95 or so little antennae, and
lots of crosstalk.
Enter JADE.
JADE engineers--being halfway intelligent (or maybe just lucky), and of that peculiar
breed that aren't snowblind--finally hit upon the solution; the JADE ISO—BUS '". The
theory being: if you've got something noisy, put it in anice quiet box--if you surround awire
with ashield that is grounded, it can't radiate.
The ISO—BUS '" is the result and the practical application of that theory--and it works as
slick as a greased pig at acounty fair.
Net only is each and every signal line surrounded on both sides by aground line (thereby
preventing crosstalk), these ground lines are interwoven, top and bottom, into amesh. This
isn't the place to go into network mesh theory, but if you're interested there are several good
books on the subject although they're alittle bit on the esoteric side.
In areal-world sense, the signals think they're running between two infinite ground planes,
sq they stay very quiet and well behaved. But at the same time, the ground planes know
they're amesh network, so they have aminimal capacitance effect.
Asecondary benefit--and this might be one of the most important side effects--is the
inductive reflectance. As asignal reverses in aline, it carries along with it amagnetic field.
This magnetic field sweeping along the line induces acurrent flow in the next signalline. But
the next signal line--on either side and top and bottom--on the ISO-BUS"" is aground line.
This line mirrors the inductive effects of the signal line, and eliminates it.
What all of this technical-sounding gibberish boils down to is that the JADE ISO—BUS '"
is agood mother board. It takes tender care of its children, holding them gently in its claws,
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