Re: SCSI I/O errors galore, PART II
Just to add a bit more confusion ;-)
On 20 Jun, this message from Michael Schmitz echoed through cyberspace:
>> > Seems in this case it can't. My external disk only runs at 5 MB (of
>> > course).
>>
>> External is different. Since Apple wisely decided on a 25-pin scsi
>> cable scheme for external disks, they can only get up to 5MB/s
>> there. [Sarcasm] So they used a separate controller for that. The
>
> Nope, not on a Powerbook at least.
Well, here are a few explanations as far as I know about SCSI on Macs:
- external SCSI connectors have always been 25-pin Dsub connectors,
except the Powerbooks with their square connector
- maximum speed has always been 5 MB/s, conforming to either the SCSI-1
standard on very old hardware, or SCSI-2 (from some more advanced 68k
designs onwards)
- older designs _may_ not have been capable of synchronous operation
(which provides speeds at or above 5 MB/s)
- Macs always used either 5380 (very old 68ks), some variant of 53c9x
(later 68k and external on some first- and
second-generation PowerMacs), or MESH (late PCI-based designs)
- the exception to the 5 MB/s rule are the second busses on some PCI
PowerMacs (73/75/76/85/86/95/9600), which are internal-only, and
implement Fast SCSI-2 (i.e. 10 MB/s) in a MESH design. Hence the label
'Fast' on the internal connector ;-)
- all other MESH implementations are 5 MB/s only.
Why not faster on the 25-pin connector? Well, every SCSI signal should
have a seperate ground return pin (which means twisted-pair transmission
lines end-to-end). That's not possible on the 25-pin designs, so signal
quality is rather poor. Hence the speed limit...
Now we only need to find out whether their choice of low-speed SCSI
drives influenced the hardware designs, or the other way round ;-)
Cheers
Michel
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