🚀 go-pugleaf

>Nonsense. On an IBM you start with a drive that already speaks the
>SCSI interface standard. I'll repeat that, because it is important.
>Drives for the IBM PC market already speak the SCSI interface
>standard. 
>
>Now, this isn't absolutely true, some of the older ones only
>speak SASI, the precursor of SCSI. SASI differs from SCSI in that SCSI
>has improved daisy-chaining, but if you don't need to hang multiple
>hard disks off your mac SASI works just as well.
>

That is interesting, because I was reading in an old Circuit Cellar
article from Byte, that SASI was modeled after the IBM i/o channel.
  
After heavy editing...
>Today, designers use special purpose
>chips that decode the header. In the old days we just used to use a
>Parallel Interface Adapter (PIA), the same chip that was used to
>implement the "IBM" (really Centronics, remember them?) parallel
>printer interface. And the header was decoded in software rather than
>on the fancy NCR chip. I myself have written hard disk drivers for
>SASI drives using this technique. Apple used to sell a single parallel
>interface adapter card for the Lisa that would take either a printer
>or a disk. 

    What chip are you refering to?   I'm writing code for a custom SCSI
    controller built from a NCR5380 interfaced to a HD64180 microprocessor.
    I was aware there was a more sophisticated chip in the works, but
    didn't know it was being sold at the moment.  (You can't be refering
    to the 5380 because it most definately doesn't decode command descriptor
    blocks.)

>
>All you need to connect such a drive to a mac is an appropriate cable.
>(The mac has a 25 pin connector, the drive a 50. Half of the drive's
>pins are tied to ground.)  A bare bones SCSI driver is trivial. The
>ROMS use the SCSI standard to read the first few sectors into memory,
>these should contain your MAC disk driver. 

It seems kind of strange that one protocol could support two different
physical cables.  According to the ANSI description, a SCSI cable can
be 50-conductor flat cable, or a 25-signal twisted pair cable.  This
implies that there must be 50 pin connections.  I wonder why the mac
has a 25 pin connector?  Are the cable and connector requirements
of SCSI just suggestions?  (from American National Standard X3.131-
1986, pgs. 16,19) 


-- 
Michael S. Czeiszperger
College of the Arts Computer Lab
The Ohio State University
Columbus, OH 43210
{decvax,ucbvax}!cbosgd!osupyr!artsvax!czei