multi scsi questions

[Karl-Heinz Herrmann]

Hello Justin, thanks for the explanation below.

On 07-Jul-99 Justin T. Gibbs wrote:
> Think of the arbitration like this:
>         Low Byte High Byte
>         XXXXXXXX XXXXXXXX
>         -----------------
>         01000000 10000000
> 
> This is what an arbitration for the bus might look like. The device
> at id 1 cannot see that id 8 is also arbitrating.  So if they both
> pop up during the same arbitration cycle, id 8 will bow out. 

It seem I have no real idea how this arbitration works.
Do the devices look from time to time if there is something? 
I thought of it more like: The adapter sends a request, all are
listening but only the one with the right ID is responding.

So what happens when the adapter asked for a high ID and the devices
bows out? Has the adapter to repeat the request? Is there a delay in
transfer? 
And why doesn't the narrow device react. It sees only the Low Byte --
so there is his own ID  number, and the Wide device left the Bus to the
narrow device, so the Bus is not busy.

> Luckily, SCSI devices don't want to have the bus all the time, so ID
> 8 will win some other time and then take the bus.  The BSY and SEL
> lines are seen by all devices, so ID 1 can tell if the bus is busy
> or that it is too late to participate in an arbitration cycle, so
> there is no conflict.

Does the SEL ine tell the devices that there is some request? And then
they all have a look if it's for them?
 
>>Could I indeed switch both wide harddrives from their ID's 1 and 6 to
>>ones above 8 and still use the (ID-8) by a narrow device?
> Certainly.

If my narrow devices keep piling up like now, I will soon have to try
that. The cabling is quite short (everything internal) so it will
hopefully work from that side.
I will keep in mind the 1.5m and the 10cm(?) in between. :-)

Karl-Heinz


----------------------------------------------------------
E-Mail: Karl-Heinz Herrmann <k.-h.herrmann at fz-juelich.de>
Date: 07-Jul-99      Time: 22:42:54
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