Use fast driver with old slow bus?

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I want to find proper driver chips for an old bus (Q22Bus). The protocol
says: rise/fall times must be no faster than 10ns. The chip I am evaluating
is 74lvt125. It meets all other requirements but the rise/fall time is much
faster than 10ns. It is not stated in the datasheet, but it would be around
1 ns if we look at the propegating delay time (1ns~4ns). I know there is
potentially signal reflection problem. But how bit the risk is? The Q22 bus
is a terminated bus, with resistors of 178 to 5v and 383 to gound for one
type and 330 to 5v and 680 to gound for another type. The length of the
backplane is 10 inches and maximum total length of 16 ft cables can be used
to connect several backplanes. I wonder if I use this chip to drive the
bus, is the total length of cables the only facter I need to sacrifice to
avoid problems? Are there other problems that I am not aware of? Thanks.


--
http://www.yahoo.com/mscpscsi

Re: Use fast driver with old slow bus?
I'm intrigued regarding the who, what, why of a Q-bus build (?) project in
the 21st century.

Firstly, a comment on the 16ft of cables, I don't believe that bit.  Once
upon a time I ran pairs of BA23s buttoned together with DEC cables.  Many
system crashes later I took to stuffing it all into one BA123, and the
'mileage' improved.  Not proof, but a caution.  Are you certain 16ft was
permitted, it's well outside my experience; i.e. I'm curious.

As my docs are in a boneyard, I can't comment on what the old (qualified)
chips were for several days at best.  However, I would be inclined if at all
possible to continue to use them.  Alternatively, even with the
terminations, you will see funny waveforms on the Q-bus; 1 ns rise/fall
needs careful termination once the signal path exceeds ~7"; cf 70" for 10ns
Tr.  In other words I doubt you can physically shorten the bus sufficiently.
And, the careful termination would start with source resistors dor signals
with signiicant edges ...  Other issues which might keep one awake with 1ns
Tr/f include: edge sensitive (which may include level signalled interupts
etc) signals exhibiting multiple transitions at the receiver and ...; signal
mangling by the backplane / connector assembly's impedance discontinuities;
anything else in Johnson's book of Black Magic (ISBN 0-13-395724-1).

I'd be interested to hear what works.

Regards

DrB

Cross posted to vmsnet.pdp-11 to tap their expertise.

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evaluating
much
around
bus
used



Re: Use fast driver with old slow bus?
DrB,


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I plan to work on a SCSI controller project because SCSI QBUS controllers
still carry high prices.

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It is stated in the Qbus protocal (page A34, item 8,
<http://www.chd.dyndns.org/qbus_ide/q22-bus-23-33.pdf and
<http://www.chd.dyndns.org/qbus_ide/q22-bus-34-45.pdf ).

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They are hard to find and still carry high prices. That's why I am looking
for alternatives.

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It is really bad news. I borrowed that book months ago but didn't get the
time to read. Now it seems a good time.

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Thanks. I will post once I find the proper chip.

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vax3900
--
http://www.yahoo.com/mscpscsi

Re: Use fast driver with old slow bus?
I'd like to learn a little more regarding your project, but
http://www.yahoo.com/mscpscsi appears broken.

Has any work been done on interfacing IDE disks to RQDX3's by bridging the
ST-506 bus to IDE ?

DrB


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in
Once
Many
(qualified)
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and
protocol
is
one
to
Thanks.



Re: Use fast driver with old slow bus?
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sorry it is http://www.geocities.com/mscpscsi

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not that I heard of.

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vax, 3900
--
http://www.geocities.com/mscpscsi

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