Date   : Mon, 01 Jun 1992 16:49:54 BST
From   : Richard York <yorkr@...>
Subject: beeb SCSI interface card.

hi all,
       just a note to say that anyone who is interested in the above needs to
tell me fairly soon since I am having the boards made in my department and I
break up for the summer on the 19th of June, so getting the boards is more
awkward and also they will be increasing their prices at the end of the term so
I want to get the boards I need made before then.
       I have had quite a few expressions of interest but if you want one I
want a FIRM statement to this effect. Since I make very little profit I can't
afford to have anyone change their minds after I have built them. I hope
everyone understands.

Below is a description for new group members;

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A few questions have been made about what the SCSI interface card does.
Perhaps now is a good time to describe it a little more fully.
       Firstly Acorn ADFS is 'hard wired' for SCSI device 0. It is
easy to adjust so that is uses a different device, but the fixed device
number remains. Currently multiple drives are supported using the LUN
terminology. This system was chosen because in the days of the conception
of ADFS SCSI drives were very rare. The system instead used SASI (a subset
of SCSI, and infact what SCSI was based on) cards, which drove ST506
drives. This is how part of my system is built, although as I have said
previously I now have a SyQuest drive connected direct to the host card,
ie my SASI to ST506 card (Zebec 1410a for the record) is currently unused.
       Anyway these cards normally supported two drives (normally they
had to be of the same type and size), and the LUN was used to select between
them. Hense drive 0 is SCSI device 0, LUN 0, drive 1 is SCSI device 0, LUN 1
etc.
       I have done some exploritary work on the ADFS with SOLIMON and have
worked out the critical SCSI bus access code, so it COULD be possible to add
support for multiple SCSI drives. However it would not be easy since;
a) ADFS 1.3 is TOTALLY full.
b) SCSI access code is duplicated in several places.
c) How is multiple drive support added? (probably converting LUN to device
   number)

The question however must be asked; WHY whould you want more than one drive??
I certainly can't see any need for it. After all very cheap drives of around
20-40 Meg can be bould, which should be enough for any purpose I can think of.

Don't forget the drive can be partitioned for use with the Master 512, exactly
like with the PC emulator on the Arc. The program is I believe provided with
the 512, but I have a friend who has a program (published in Acorn User) which
allows partitions to be created of arbitary size, which is much better than
the Acorn offering.
       The partition is simply a file of the required size created in the
ADFS file structure, so it doesn't affect the normal ADFS operation at all.

TAPE STREAMER SUPPORT.
In principal it will work, in that the SCSI commands can be sent to the device,
just as with a normal drive. However tape streamers use a slightly modified
class of SCSI instructions which ADFS I don't think would be able to cope with.
I suppose a piece of software could be written to create an ADFS filing
system on the tape drive, but it would not be easy. Also the problem of 2
seperately addressable SCSI devices on the same bus would be a problem.

REMOVABLE HARD DRIVES.
These are obviously supported (I'm using one aren't I :-) ), but really this
doesn't mean Acorn went out of their way to do it; just that they look
exactly like a normal drive, except that certain commands respond slightly
differently. For example issuing *BYE spins down the cartridge ready to
eject it. I presume on a non-removable drive the particular command issued
causes the SCSI drive to park its head, though I'm not sure.
       Also I can issue a SCSI command to lock the cartridge and relase it,
though I can't see any benefit in doing this.

GENERAL SCSI NOTES.
Osword &72 is effectively the "issue a SCSI command" OS call, and it works
well. All the above calls (spin down drive etc) can be issued using this
route, just as long as you know what command to issue :-) A SCSI standard
helps!!!


Lots to chew over here. Basically its easy, and fairly foolproof. All you
need is the host card, PSU, SCSI drive and something to put it in.


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again replys before the middle of next week if possible please,

               Richard York.

:----------------------------------------------------------------------:
: Richard York                   :   E-mail : yorkr@...                :
: 2nd year Computer Engineering  :   Janet  : yorkr@...                :
: Manchester University          :                                     :
:----------------------------------------------------------------------: