[Sunhelp] VME/sbus/PCI/QBUS/other, advantages/dis-advantages

[James Lockwood]

On Fri, 17 Sep 1999, Gregory Leblanc wrote:

> I'm going to admit to being a PC guy now, and show my ignorance.  I've got a
> very good knowledge of ISA/EISA/PCI, and what advantages each offers over
> the others on PC hardware.  I know exactly how they communicate with the
> rest of the system, and all that jazz.  But I don't really know all that
> much about these "other" bus architectures.  I've got VME on my 4/330, which
> seems pretty cool, and actually looks a lot like NuBUS to me.  Sbus is
> newer, right?  I know that it's what my SS20 uses, along with my friends
> SS2.  PCI is the (only?) bus available on Ultra based Suns right now, and
> it's got some nice features.  QBUS is a total mystery, but it's on that VAX
> 4000-400 in the back room.  :)  Can people share what they know about these

Ok, here's the story.

Ultra based Suns have come with both SBus and PCI (the first PCI-based Sun
was the Ultra 30).  The Exx00 servers can take expansion cards that will
give them either sbus or PCI slots.

VME is a bus with a long and varied history.  The original spec was that
cards used the Motorola VersaModule pinouts and the EuroCard (6U) form
factor, but many other variants have appeared.  The one used by Sun (and
SGI on some machines, such as the Crimson) has 3 connectors per card and a
larger form factor (EuroCard has 2 connectors).  This allowed for more
complicated cards that use more power, the VME signalling scheme is still
the same.  It also introduced an auxiliary bus ("P2") usually used for a
high-speed memory interconnect.  The 4/330 actually has a pair of EuroCard
slots that can take 6U cards directly, it is the only Sun so equipped
(though Sun sold adapter cards to fit 6U cards into 9U slots). 

Maximum VME bandwidth is around 25MB/sec, IIRC.  It was really designed as
a backplane bus for embedded applications, and not as an "expansion" bus
(such as PCI/SBus/ISA/etc).

Nubus uses similar connectors to VME (though with an extra 3 pins like the
Sun P4 bus connector).  I don't have details on the signalling used,
though I have heard that it was derived from VME as well.

SBus was introduced on the Sparc 1 in 1989 as an extension of the system
memory bus.  In this sense it was fairly analogous to VLB in the PC world,
but it included device identification and the concept of a device
hierarchy from the start, more like PCI.  Later SBus-based Suns used bus
interface chips to divorce the SBus from other internal busses ala PCI.

SBus speeds vary from 16MHz to 25MHz and are either 32 or 64 bits wide.
64 bit cards are rare, though, so typical bandwidth is 64-100MB/sec
depending on the system.  Larger systems have multiple independant SBus
controllers.

Sun was already moving away from VME by the early 90's, the last VME
machine produced was the 4/600 (which also had SBus slots).  I don't
believe that Sun really expected SBus to go as far as it did, I think that
it was fueled by the enormous success of the Sparc 1.  The last Sun
produced with purely SBus expansion (as opposed to the Enterprise servers
which take option cards) is/was the Ultra-2.

QBus is a whole other kettle of fish.  It also has a long history, but I
don't have enough information on the particulars to give an accurate
account.  I'm sure Tim Shoppa or one of the other PDP-heads on the net
could do it, though.

> busses.  Oh, one other question on VME.  Supposedly there are people working
> on support for VME still, using a PCI-VME bridge.  Would that be the same
> VME as on early SPARC machines? Thanks a bundle,

VME is VME, if drivers are written then almost any devices can
communicate.  VME is still big in the embedded world, though CompactPCI is
displacing some of it.

Fun trivia fact of the day:  The Netra T1 ("giant purple waffle") is
actually a SparcEngine CP (Sun's CompactPCI product for embedded apps) in
a special case with an adaptor to break out a PCI slot from the CompactPCI
bus.  Neat idea, good product reuse.  Now if only there was an affordable
Sun produced AXi-based system...

-James