[Info-vax] DLT drives and antique SCSI compatibility

[Terry Kennedy]

On Friday, July 17, 2020 at 1:57:41 AM UTC-4, Michael Moroney wrote:
> Dave Froble <davef at tsoft-inc.com> writes:
> 
> >It's my guess that some of what DSSI could do might still be useful, 
> >but, as mentioned, there might not be the volume to support specialized 
> >equipment.
> 
> >Think about it for a bit and one will realize there was all kinds of 
> >good ideas in the past.  Including Alpha ....
> 
> DSSI was enough like SCSI that on a certain VAX box part of the configuration
> was telling it whether an IO board should talk DSSI or SCSI.

DSSI was physically SCSI, as far as signaling went. The connectors were of the opposite gender in order to prevent inadvertent mismatches. However, the command set was MSCP/CI, not SCSI. That initially let DEC control the whole ecosystem, although later on a number of 3rd-party manufacturers created bridge boards. Some were standalone boxes, while others held one or more SCSI drives as well. DSSI drives don't seem to have held up as well as SCSI drives from the same era. A number of systems I've rebuilt for museums have needed replacement of some or all of their DSSI drives.

And this being DEC, there was a nearly infinite combination of incompatible mounting kits, so you couldn't take a bare half height drive and use full height brackets to mount it. But what can you expect from a company that managed to get up to Revision F, ECO level 3 on a simple cardboard box...

There was code for clustering over SCSI as well as DSSI, but it required a working target mode (as well as the vastly more common initiator mode) on the SCSI controllers, which not all DEC controllers implemented. I don't remember if this ever became a supported configuration.