At 2:02 PM -0600 3/15/02, Charles Shannon Hendrix wrote:
>On Fri, Mar 15, 2002 at 12:08:38PM -0600, Peter Seebach wrote:
>> In message <E16lvff-0000Ip-00@chiark.greenend.org.uk>, Ben Harris writes:
>> >>Those Macs had a single SCSI bus.  The oustide connector was just an
>> >>external continuation of the whole thing.
>>
>> >Ahem.  On my 9600:
>>
>> Also on the 7500, at least.  The 7200 had one SCSI bus; the 7500 had two,
>> and the internal one was fast.  I think that's also true of the 9500.
>
>OK... :)
>
>Is the second bus on the motherboard or a card?
>
>I could have sworn that my 7500 at work (in 1998) had only a single
>SCSI controller, and I remember adding SCSI to a few of them to get
>another bus.  In my defense, it was a Mac-zealot's recommendation,
>and I was running MacOS at the time.

Both scsi busses are on the motherboard. The external bus is a Slow-SCSI
bus handled by an onboard NCR53c94 chip (iirc this bus also has an internal
connector, which usually goes unused). The internal bus is a Fast-10 bus
handled by an apple-custom ASIC known as MESH, also located onboard.

Just to throw some extra spice into the mix, many SGIs are also available
with two SCSI busses on the motherboard, such as my Indigo2 (dual Fast-10
WD33c93B), or the O2 (dual UW Adaptec 7880), or Octane (dual UW QLogic or
something) and so on. To throw a bit of an oddity into the mix, the Fuel
workstation has two internal U160 busses on the mobo, but no external bus.


Cheers - Tony 'Nicoya' Mantler :)


--
Tony "Nicoya" Mantler - Renaissance Nerd Extraordinaire - nicoya@apia.dhs.org
Winnipeg, Manitoba, Canada           --           http://nicoya.feline.pp.se/