Subject: Re: SCSI wiring
To: None <>
From: Jochen Kunz <>
List: tech-kern
Date: 08/10/2005 10:38:10
On Tue, 9 Aug 2005 14:04:37 -0400 (EDT)
der Mouse <mouse@Rodents.Montreal.QC.CA> wrote:

> I was thinking about connecting two SCSI buses with the data wires
> switched around, so that one bus's D7 (and therefore ID 7) isn't the
> same as the other's.  This would allow me to take two machines with
> internal disks and plug them together through their external SCSI
> connectors.
There is no need for such hacks. A SCSI host adapter is just an other
SCSI device and it can be programed to any SCSI ID you want. IIRC many
(older) SCSI adapter chips like NCR 53C80 (?) can be used as host
adapter and "target" in e.g. a disk or tape drive. The setup (two hosts
using the same bus and same disk) you describe can be found on Tru64
(aka OSF/1 aka DEC) UNIX clusters. All cluster nodes need to share a
SCSI bus with a quorum disk used by all cluster members. AFAIK there is
even in the SRM firmware of alphas support for this. You can set the
SCSI adapter ID via firmware variables. There is no need for a SCSI host
adapter to be at ID 7. SGIs use ID 0 for the host adapter. Some VAXen
use ID 6 in the firmware. The later confused me some years ago as the
kernel later changed the host adapter ID to 7...