Path: utzoo!yunexus!geac!syntron!jtsv16!uunet!vsi!friedl
From: friedl@vsi.COM (Stephen J. Friedl)
Newsgroups: comp.sys.att
Subject: Re: First impressions: 3B2 120MB SCSI tape drive
Summary: 3B2 SCSI explained
Message-ID: <901@vsi.COM>
Date: 25 Oct 88 02:27:20 GMT
Article-I.D.: vsi.901
References: <899@vsi.COM> <539@ctisbv.UUCP>
Organization: V-Systems, Inc. -- Santa Ana, CA
Lines: 72

In article <539@ctisbv.UUCP>, pim@ctisbv.UUCP (Pim Zandbergen) writes:
> What I'd like to know is how the 120MB tape drive compares to
> to the 60MB tape drive.

It holds twice as much :-).

> While I'm on the subject of comparing SCSI devices for the 3B2,
> AT&T sells both native SCSI hard disks and ESDI hard disks that
> need a SCSI-to-ESDI thing like the ones that come with the
> 3B2/600. Does anybody have experience with both types?
> How do they compare in speed?

They are the same thing; *all* the drives are ESDI.

I think this is the typical SCSI-vs-ESDI confusion that pops up a
lot; ESDI is a controller-to-drive interface specification, while
SCSI is a host-to-peripheral spec.  The following little picture
shows what is happening here:


                  *------------*
                  | disk drive |<------ ESDI ------*
                  *------------*                   |
                                                   V
*----------*      *---------*                *-----+------*
|          |      |  SCSI   |                | disk       |
| 3B2/600  |<---->|  host   |<---SCSI bus--->| controller |
|          |      | adaptor |                |            |
*----------*      *---------*                *------------*


You can hang other peripherals (CTC, 9-track, etc.) on the
SCSI bus, because this is processor and device independent.
This particular disk controller is ESDI, but this is defined
by the *drive*, not by SCSI.  An industrious person could take
an industry-standard SMD (or ST-506, or IPI, or ...) controller
and hang it on the SCSI bus as well, as long as the proper drive
was used.

Part of this confusion is caused by the appearence of drives
with SCSI interfaces.  What this means is that the drive has
a built-in controller, so the controller-to-interface link is
hidden to you.

All the 3B2s with SCSI start with the `SCSI host adaptor'.  This
converts the machine-specific bus (the 3B2 backplane) into a SCSI
signal.  From there it can go two ways: external or internal.

An EXTERNAL drive is connected via the `Disk Controller Module' (DCM).
This is a box that holds the controller board and power supply, and it
has SCSI going in and ESDI coming out.  From the ESDI ports it goes
to a box that holds the drive itself; I think that a single DCM can
talk to up to four ESDI drives.

An INTERNAL drive has the controller card -- probably the same
one in the DCM box above -- mounted internally somewhere, and the
SCSI hookup is a little ribbon cable.  From this little board it
goes to the same ESDI drives that are external, except it too has
a little ribbon cable doing the talking.  Note that if you're
adventurous, you can probably hook up two more drives "internally"
if you want to wire up an external power supply with a ribbon cable
from the internal controller.

*Whew*.  A little of this is speculation, so those In The Know are
encouraged to post/mail their updates.

     Steve

-- 
Steve Friedl    V-Systems, Inc.  +1 714 545 6442    3B2-kind-of-guy
friedl@vsi.com     {backbones}!vsi.com!friedl    attmail!vsi!friedl
----Nancy Reagan on 120MB SCSI cartridge tape: "Just say *now*"----