SCSI connectivity

From: Eric J. Korpela <korpela_at_ellie.ssl.berkeley.edu>
Date: Sun Mar 11 14:42:00 2001

I'll put my two cents into the discussion. A lot of the discussion depends
upon the busses in question.

Either an 8 or 16 bit ISA SCSI-1 or 2 card will saturate an ISA bus even at
5 MHz SCSI rate unless the ISA bus is overclocked. Of course the data rate
at which the 16 bit card saturates the bus will be double the tranfer rate
of the 8 bit card. In this case, if you're looking for max transfer rate you
should probably choose polled. If you want best system performance (and you're
running an OS that will take advantage of it) interrupt driven I/O is your
best bet, but your max transfer rate will likely be lower than PIO. A bus
master should be able to squeak out a bit more performance, to where you might
get closer to the max ISA rates.

The midrange busses (EISA, MCA) should be able to handle Fast Wide SCSI (20
MB/s), but they'll be near saturation there. The high end busses (VLB,PCI,
SBUS,etc) should be able to handle any SCSI you want to throw at them. For
our high end machines we tend to spread controllers on multiple PCI busses,
but that's not exactly classic-cmp stuff.

The classic-cmp question I have is at what point is a machine too slow for
SCSI, and how do things degrade as a machine gets slower. My Apple IIgs
doesn't seem to have any problem, but I doubt it can really handle transfers
at even 1/10 the SCSI-1 rate. But I assume there is a point at which things
break if bytes don't move fast enough. Anyone know what that point is? I've
been thinking about sticking an 8 bit SCSI card in my old Epson PC which,
because of brain damaged bus design can only pull 150 kB/s off of its drives.
Think it will work?

Eric
Received on Sun Mar 11 2001 - 14:42:00 GMT