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From: toddpw@tybalt.caltech.edu (Todd P. Whitesel)
Newsgroups: comp.sys.apple2
Subject: Re: HD Problems, another question
Message-ID: <1990Oct3.082855.7591@nntp-server.caltech.edu>
Date: 3 Oct 90 08:28:55 GMT
References: <111012@tiger.oxy.edu> <7451@darkstar.ucsc.edu>
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unknown@ucscb.UCSC.EDU (The Unknown User) writes:

>In article <111012@tiger.oxy.edu> prophet@oxy.edu (Dale Bruce LaFountain) writes:

>	What I believe to be the case is that the GS is 'polling' the SCSI
>card... Since there's no card there, it's hung up (for a while)..

>	If you let it sit for somewhere around 15-30 seconds (I forget for
>sure), it will then start rebooting slot 5 or whatever slot you have
>set to boot from.

This is correct. When the SCSI card is first turned on it has to wait a minimum
of 10 seconds to give SCSI devices time to warm up. The SCSI specification says
that a device does not have to be ready to answer ANY requests for ten seconds
but must be able to respond after that time.

Some peripherals, most notably Apple's Scanner, take about 12 seconds to power
up (violating the SCSI spec) so you are supposed to set the DMA SCSI's priority
number to 1 or 2 so the card will know you have a scanner and will wait an
extra few seconds. (This is a little option added to the card by Apple to make
supporting the scanner easier. Beta drivers are mentioned on the cover of the
new APDAlog.)

This is one of many cases in which the Apple II is fully standard compliant and
people wonder what the problem is because Macs AREN'T standard compliant -- and
perform better because of it.

One performance hack in the Macintosh SCSI manager is the lack of support for
arbitration -- the SCSI bus CAN be used as a high speed eight device network,
which essentially means that you can connect up to eight computers, scanners,
hard drives, or whatever together (make sure there are only two terminators,
one at each end of the chain) and all the computers on the chain will be
able to share all the devices on the chain. Note that the permanent built in
termination on the DMA SCSI card means that only two DMA SCSI cards can be used
in such a configuration -- any number of older SCSI cards could be added in
between them.

This shared bus scheme only works properly if ALL the computers on the network
support arbitration, because once the bus has an owner no error checking is
made on the transmitted data. I know for a fact that the DMA SCSI supports
arbitration, I am not sure if the Rev. C SCSI card does, but I do know for
a fact that NO Mac supports arbitration (at least through the built in SCSI
port -- third party cards hopefully will be more sensible). This means that
a GS and Mac can share hard drives BUT if the Mac wants to use the bus it
will simply start using the bus without looking to see if anything else is
going on, whether it is a GS or an automatic tape backup unit or even another
Mac. Since SCSI does not use data error checking this means corrupted data for
anyone using the bus when the Mac wants on, WITH NO ERROR MESSAGE. While the
Mac gets faster SCSI performance, it sacrifices the ability to share its SCSI
bus with anything other than passive devices. For most people this tradeoff is
preferable, however my roommate and I found it annoying as we wanted to share
hard drives (he had to clean out parts of his drives to install the stuff from
my drive's Mac partition that we wanted to use).

While I am bashing the Mac SCSI implementation I suppose I should also mention
that Macs will not recognize any SCSI hard disk as formatted unless there is a
tiny partition on it called 'APPLE.PARTITION' (or something like that) which
contains the actual SCSI driver. So if you want to be able to create Mac
partitions create them with Mac HD software and then format the prodos ones
from finder. If you partitioned it with the DMA SCSI utilities then the
APPLE.PARTITION will be created but it won't have a driver so the Mac software
will still insist that the drive is totally blank.

The above information was obtained from conversations with Matt Gulick (he
designed the DMA SCSI card) and from personal experience and experimentation.
I have not dared to attempt actual drive sharing yet.

To be fair to the Mac, I realize that these design decisions were probably
made by Apple in order to get the Mac Plus's SCSI to work, however I can't
understand why they haven't fixed the SCSI manager since then (backwards
compatibility maybe?).

Todd Whitesel
toddpw @ tybalt.caltech.edu