Q-bus (was Re: Wirin' up blinkenlights)

From: Ethan Dicks <ethan_dicks_at_yahoo.com>
Date: Thu Jun 8 22:41:18 2000

--- allisonp <allisonp_at_world.std.com> wrote:
> >Interesting idea, a Q-bus front panel.
> >
> >signals are there as well. A DMA cycle can take forever so you could build
> nope, it will bus timout on you.

Boy did we find that out the hard way. The Software Results Q-Board was built
back in the days of the MicroVAX-I and MicroPDP-11. The designers picked some
time (by tapping off of a stage on a divide-by-N counter) to timeout while
waiting for a DMA cycle. It was well outside the Q-bus spec of the day. This
board worked in MicroVAX-II and -III designs with no complaint. Eventually,
someone wanted to stick a Q-Board in a VAX 4000 with a TLZ04 interface. By
the time this machine came out, the Q-bus was no longer implemented in the
way it once was. Certain cards were no longer "in spec". The upshot of this
was that the TLZ04 interface could grab the bus longer than the Q-Board would
wait. The fix was to redesign the bus timeout circuit (a cut and a jump and
a new PAL) because DEC sure wasn't going to change their interface to work in
an older Qbus box.

It took us quite a while to find this one because our board didn't change and
there were no problems in 99% of the VAX 4000s out there. It was a piggy


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Received on Thu Jun 08 2000 - 22:41:18 BST

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