Hello Greg!

  ASB> ...why fight the natural inclination of warm air to
     > rise, when it could actually help?

  GAW> You mean in the 9U 12-slot floor chassis and the
     > like?

Well, any machine with a vertical card cage.

  GAW> My only guess is that it's to avoid picking up too
     > much dust and dirt.  They do anyway, but not as badly
     > as if they were pulling it right off the floor in
     > some places!

That's the best argument I've heard so far for sucking down
through the cage.  I guess we'll know whether I'm convinced
when I get to build one ;-)

  GAW> (Especially those with cats! :-)

Hmm... I wonder whether cats were a design consideration?!
<cheshire grin>

  GAW> There are no air filters of any kind on those systems
     > -- only fanblade guards.

Eeek!  I guess they expected you to use them in clean-rooms!

  GAW> Most older rack cabinets intended for stand-alone
     > environments pull air in from the top.

It's been a while, but I seem to remember that the Vaxen at
one place I worked had a big (12"?) fan, sucking air in the
front, blowing it up through the card cage to the PSU at
the top and then straight out the front.  That said, the
inlet was around waist hight, high enough not to pick up
gunk off the floor.  Perhaps the disk drives were in the
lower half.

Either way, my own feeling is that a vertical card-cage,
with either convection or forced-air cooling, is preferable
to the majority of PCs I've seen, where air is dragged in
via any available orifice, swills about the case haphazardly
(possibly being sucked through the microprocessor cooling
fan a few times) before being blown out of the PSU's exhaust
port.  If any breeze gets between the cards, it's more often
by luck than good judgement.  I even hear horror stories of
machines that drag air in through the PSU, and then blast
hot air over the mainboard.  :-(

  GAW> When I had my 3/280 running in a big old digital
     > PDP11 rack I ducted (with cardboard) the twin 10"
     > fans at the top of the cabinet down onto the 280
     > chassis (which was missing its fan tray) and let me
     > tell you it sure did keep the concrete floor warm!
     > :-)

Hmm... I wonder if I can offset the cost of the power
consumed by a VME machine against savings on my heating
bill!  (great in Winter, I don't really want to think about
summer ;o)  A 3/x80 appealed to me a lot because I could
bung it in a rack with other equipment.  It would take a lot
more room than a 3/60 though, and presumably drink more
current (what are the Volt and Amp ratings stamped on your
3/280?)  There's probably a point where the lines cross
though, and a x80 chassis would start saving me space and
inconvenience (assuming I could find one).

  GAW> Of course on a big cabinet there's usually a holder
     > slot over the fans where you're supposed to put a
     > foam filter (eg. like those for air conditioners),
     > so that's how I kept even the floating cat hair out
     > of the system.

Not to mention the occasional floating cat.  That reminds me
of a tale, but I'll save it for a month when the list is
really, really desperate for content ;o)

  GAW> VE3TCP

73 DE KB9YLW

Regards,
  - Andy.