Unibus / Qbus bus drivers and receivers

From: Richard Erlacher <edick_at_idcomm.com>
Date: Fri Jun 29 08:01:07 2001

Why not see if you can find a copy of the old (and still occasionally available)
Motorola MECL System Design Handbook. It consists, mostly, of a discussion of
signal integrity issues from the standpoint you're concerned with in this case.
It does focus on MECL, but the principles are the same irrespective of the
technology. The author, Blood, is the authority behind most of the current
generation of signal integrity software in use today, theough he's seldom given
credit.

Dick

----- Original Message -----
From: "Chuck McManis" <cmcmanis_at_mcmanis.com>
To: <classiccmp_at_classiccmp.org>
Sent: Thursday, June 28, 2001 11:06 PM
Subject: Re: Unibus / Qbus bus drivers and receivers


> At 09:30 PM 6/28/01 -0700, you wrote:
> >Well, the question is "how worried about ringing *should* I be in a
> >single backplane configuration if I use bus drivers which have a
> >rise/fall time of 9 ns, when the bus spec says that the rise/fall
> >time of bus drivers must not be *less* than 10 ns?"
> >
> >I'm guessing that it will probably be OK, but I really want this
> >thing to "work first time" and be reliable so I feel a little
> >uncomfortable cutting corners like this right at the outset.
>
> I take it you are worried about violating the AC spec that reads in part:
>
> Rise/Fall Times (from 10% to 90% and 90% to 10%)
> must be no faster than 10ns.
>
> This parameter relates to the 120 ohm bus termination and indicates that
> the R/C termination constant will critically damp signals that switch at
> 10ns. If you drive it faster you will get a bit of bounce in your edges as
> the termination network isn't quite fast enough to absorb the energy and
> thus some of your signal gets reflected back. Anyway, you could do a couple
> of things. You could load the capacitance on your bus drivers a bit and
> slow them down, you could run them through some other gates to slow them
> down, or you could just use them and look at it with a scope once you're
> running. If you want to "check ahead of time" then kludge together a card
> with some on it, switch them on and off and watch the ring.
>
> Now how much should you be worried about? If the signal overshoots by more
> than about 10% of Vcc I'd worry about it or under shoots by a similar
> amount. Or if the ground bounce induced by switching everything is more
> than about .2 volts.
>
> Clearly you are a worthy designer unlike a PC guy who would said, "Hey if
> it breaks they'll blame the BIOS or something so no worries!"
>
> --Chuck
>
> Then again, I've not done transmission line design in about 10yrs so if I'm
> completely wrong you get your money back :-)
>
>
>
Received on Fri Jun 29 2001 - 08:01:07 BST