On Fri, 12 Mar 2004, Robert DiRosario wrote:

> OK, this is my understanding of the problem, based on what I have
> learned from my hobbyist VAX cluster at home.  I am not an expert on
> VAX/VMS systems.
>
> A DSSI device (disk or HSD controller) is not like a SCSI disk or MSCP
> controller.  A DSSI device is a "node" just like a VAX or Alpha system
> or CI controller, such as the HSC50.  The DSSI device does not use MSCP,
> but uses the SCS protocol, the same protocol that VAX and Alpha systems
> and CI controllers use in a cluster.

Actually no. DSSI is not a protocol on the same level as MSCP. DSSI is
more of the physical layer, like SCSI or SDI (or CI).
You talk MSCP inside DSSI, just like you talk MSCP inside SDI, or CI.
SCS is a middle layer, shared by both DSSI and CI, but not SDI.

So all disks (RA and RF) talk MSCP. Exactly how these MSCP packets reach
the disk differ, however. On an RA disk directly connected, the MSCP
packets are send over the DSI. If you have an HSC, the MSCP packets are
placed in SCS, sent to the HSC, which removes the SCS layer, and sends the
MSCP packet further onto DSI.
For RF disks, you send the MSCP packets using SCS to the disk.

But it's all MSCP.

Obviously, ST-512 and SCSI disks do it a bit differently. There, the MSCP
packets are instead handled totally within the controller, which then
performs the required transactions with the disk in order to fulfill the
MSCP request. (I'm talking about ST-512 and SCSI as used in controllers on
a PDP-11, VAX or Alpha which looks like an MSCP disk here.)

	Johnny

Johnny Billquist                  || "I'm on a bus
                                  ||  on a psychedelic trip
email: bqt@update.uu.se           ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol