[sebhc] H8 bus termination

Lee Hart leeahart at earthlink.net
Sun Sep 3 13:15:56 CDT 2006

Carroll Waddell wrote:
> I've been looking at the H8 bus on a scope. They sure are noisy.
> Lots of glitches, etc. I was just looking on the internet at bus
> termination, and I read something by Lee Hart. Is this the Lee
> in the group?

Could be. What did I say? :-)

> Is there some way that we could come up with a bus termination
> for the H8 bus?

Yes, you could. But you may not need to, except as an aethetics issue. 
The original design was noisy; but it was designed to work in spite of
the noise. Tri-state lines are allowed to "float" when all drivers are
off. The bus line is high impedance during these times, so it is easily
bounced around by noise. But the cards aren't paying attention to the
noise during these times. Also, the design favors level-sensitive logic
rather than edge-sensitive so noise pulses are less of an issue.

Trionix made an improved motherboard for the H8 that helps a lot. If you
can find one, it would be a big improvement.

Or, you can replace the old TTL chips with modern CMOS 74HC- or 74HCT
logic. It generates far less noise, and is a lot less sensitive to it.
"Never doubt that the work of a small group of thoughtful, committed
citizens can change the world. Indeed, it's the only thing that ever
has!" -- Margaret Mead
Lee A. Hart, 814 8th Ave N, Sartell MN 56377, leeahart_at_earthlink.net

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