Re: Mac HD #M0135

[pubpc1@library.ucla.edu]

shack@deimos.frii.com (Randy Shackelford) wrote:
>In article <5iqh6k$bvl$1@news.vanderbilt.edu>,
>Tilghman Lesher <NO%lesh%SPAM%erjt%HERE@vuse.vanderbilt.edu> wrote:
>>SCSI may be SCSI, but there are several types of SCSI.  The one used
>>most frequently in Apple II's in SCSI-II.  I think there's now a SCSI-III
>>standard.  Of course, there is also SCSI-I, and SASI (pre-SCSI).
>
>Who told you that Apple IIs use SCSI-II? They can use the drives, but they
>fall back to SCSI-I when  you connect 'em up to a SCSI card. Apple IIs are
>not  fast enough to keep up with SCSI-I throughput so it would be utterly
>pointless to implement SCSI-II on 'em.

Read what he said, Randy.  He said that the one USED MOST
FREQUENTLY in the II is SCSI-II, which is the truth of the
matter since they pretty much don't make SCSI-I drives anymore.

As for speed, the throughput of SCSI-I is theoretically 1MB/sec.
That's the same as the maximum of the Apple bus.  SCSI-I's real
world performance is somewhat worse because of noise and errors,
so the II (with Apple II High Speed and RamFAST SCSI cards) can
more than keep up.  There's no need to insult the Apple II.

Implementing SCSI-II on the II would be good because it would
eliminate incompatibilities with some CD-ROMs and hard drives.
Remember how some SCSI-II drives won't work on the RamFAST?
Also, a caching controller can accept data faster than the
maximum of the computer's bus.  This is advantageous because
data in RAM can be accessed faster (no R/W head positioning)
than the physical hard disk.

>
>>The HD20 may be SCSI-I or SASI.
>
>When I was looking to buy one, someone told me they use the MFM style hard
>disk like PCs from the same era.

The HD20 is like the other floppy port hard drives of the era,
like the ProAPP series (10 or 20 megs).  The ProAPPs work with
the //e and //c, though, while the HD20 will only work with a
UDC.

-Scott G.