4 Jun
2003
4 Jun
'03
7:31 a.m.
--- "Peter C. Wallace" <pcw(a)mesanet.com> wrote:
Full specs of the Qbus are available to evaluate a discrete replacement,
but I know that some boards used 74LS240s in lieu of DC005 chips, etc.
There are two aspects of attaching third-party devices to a DEC bus:
bus drivers and bus receivers. Receivers are the more critical item
if you are planning on deploying the device in a marginal backplane.
Some of DEC's solutions included hand-selecting 74-series chips for
low leakage current as well as a fistful of custom bus chips. In a
single 9-slot "standard" Qbus backplane, you can get away with murder.
It's when you want to have two or three loaded backplanes chained
together that things get "interesting". Same goes for Unibus and
OMNIBUS. At 25% of "max expansion", out of spec parts might not
drag you down. Start filling the box and attaching it to long cables
and you will probably have a different experience.
I have plenty of 8641s, but not many of the other chips of the day
(8881s, 8837s, etc.)
I'm more interested in OMNIBUS attachment than Qbus attachment. I
do have plenty of Unibus interface ICs (new and used), from my massive
COMBOARD stockpile.
-ethan
But it would
be nice to stick a nice "small" 1-10GB disk on a QBUS...
...As I remember though, the hard part about making QBUS things was
finding the bus driver chips - unless you wanted to sever them off of
working boards... which I'm not inclined to do at the moment.
There arent that many QBUS lines to drive. If I was to make a new QBUS
card I would use SOT-23 MOSFETs as the bus drivers, maybe with a 10 ohm
series resistor for output protection against VCC shorts and slew rate
limited gate drive...