On Wednesday 16 February 2005 04:27, Geert Uytterhoeven wrote:
>On Tue, 15 Feb 2005, Gene Heskett wrote:
>> On Tuesday 15 February 2005 10:02, Eric Siegerman wrote:
>> >On Tue, Feb 15, 2005 at 10:53:55AM +0100, Geert Uytterhoeven 
wrote:
>> >> I thought[*] 7 was the highest priority, and 0 the lowest (on a
>> >> narrow channel).
>> >
>> >That's what I recall too.
>> >
>> >> Wide devices have an even lower priority: 15 to 8.
>> >
>> >This sounds vaguely familiar too, but I'm *far* less certain
>> >about it.
>>
>> The bus scan at bootup starts at 0, thats true, but I'm not aware
>> that any device number has a 'higher priority'.  As far as the
>> driver scanning the bus to see who yelled when the ATN line gets
>> pulled down, I don't think its done that way at all as the ACK or
>> whatever that comes back then causes the device thats waving its
>> hand to put its address on the bus.  So there isn't even a
>> priority there that I know of.  If another device also puts up its
>> hand, then I think it gets read for the device number and
>> otherwise queued, and will be serviced as soon as the first device
>> has been told what to do.  The scsi bus multitasks very well
>> indeed in well written code.  Every device, including the
>> controller, should have collision avoidance built in since thats a
>> basic part of the scsi specs.
>
>A device puts its address on the bus by asserting the data line that
>corresponds to its own ID. If multiple devices do that at the same
> time, multiple data lines become asserted and the highest priority
> device wins.
>
>For a wide bus, if none of the 8 lowest data lines are asserted, the
> bus master will check the 8 highest data lines. Hence the lower
> priority of devices 8-15.

That makes sense Geert. I'd forgotten the exact details since its been 
nearly a decade since there was more than one device, a tape drive, 
on my own scsi bus.  There were 3 disk drives and a tape drive in my 
big box amiga at one time, but all were regular scsi-2, no wider 
devices were usable on an a2091 even with a guru-rom on it.  The fact 
that a scsi bus is a transmission line, and must be properly 
terminated on both ends, was and is a much larger problem in 
practice, leading to comments about sacrificing chickens, goats, 
virgins etc to make the darned things work right.  The whole 
termination scheme should have been 100% active from the gitgo as 
resistive is way too sensitive to any sags in the 5v line, and drops 
in the host isolation diodes.  Resistive, fed a solid 5 volts works 
fine, add the .7 volt loss of the si isolation diode and you have 4.3 
to 4.4 volts left to run the terms, and the whole thing goes up in a 
cloud of smoke and invective.  Unforch, by the time active terms 
became available, scsi had been carved in stone for 20 years.

>Gr{oetje,eeting}s,
>
>      Geert
>
>--
>Geert Uytterhoeven -- There's lots of Linux beyond ia32 --
> geert AT linux-m68k DOT org
>
>In personal conversations with technical people, I call myself a
> hacker. But when I'm talking to journalists I just say "programmer"
> or something like that. -- Linus Torvalds

-- 
Cheers, Gene
"There are four boxes to be used in defense of liberty:
 soap, ballot, jury, and ammo. Please use in that order."
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Copyright 2005 by Maurice Eugene Heskett, all rights reserved.