Domesday Master AIV SCSI Card.
INTRODUCTION
------------
The original domesday
system was based on a special customized version of the Acorn BBC
master
called the Master AIV. The
computer is essentially a BBC Mater Turbo but with an extra custom
SCSI interface.
CONNECTION
----------
A 20 way ribbon cable
plugs into the connector labeled PL12 on the Master motherboard.
The connector is located
in the far left just next to SK1, the left hand co-processor
connector. The PL12 connector was originally designed
for an analogue dial-up modem, but is simply a connection to the I/O bus
used for other storage systems such as the Floppy Disc and Tape interfaces.
The BBC Master internal bus structure is split in to three by an MSI Peripheral Bus Controller (CF30049) custom gate array. The purpose of this controller is to split the system's data bus into three sections. This is to reduce output loading on any active device from inactive devices, as well as separating slow (1MHz) peripherals from the fast (2MHz) CPU bus.
Not all BBC masters
have the correct connector for connecting the SCSI interface. Some
have the pads on the PCB but no connector, while others have a row
of pins. To fit the SCSI interface the connector must be a female single
in-line connection.
THE SCSI INTERFACE
------------------
The BBC Master SCSI interface must be one of the hardest items to find. It was only ever sold as an integral part of the AIV Domesday Machine to provide the interfacing between the BBC and Laserdisc Player. It is unclear why Acorn decided to include this custom interface inside the computer instead of the usual external Winchester Disk interface that would connect to the 1MHz bus. The SCSI interface is approximately 30cm x 4cm and is fitted in the bay between the power supply and the main board. The board is secured in position with five screws.
The SCSI interface
has an unusual arrangement where a separate oscillator circuit takes
its power from the Master power supply and feeds a signal to
the main board. The two power connectors which usually
go
to the
pins
in the
top left-hand
corner of the main board plug into the SCSI interface and a pair of flying
leads with spade connectors on replace the removed
connectors. The SCSI interface does not use this power connection it
receives the power for operation from the main board via the ribbon
cable.
The oscillator circuit generates a 16MHz clock signal that is fed to
the main board via a connection marked L6 on the BBC Master main board
using a
yellow wire.
VIDEO FILING SYSTEM
-------------------
In addition to the interface a ROM is required to drive the Videodisc player. As it stands the SCSI interface can only be used with the Videodisc player. It is not known if there is a way to run a hard disk with the interface.
BUILDING AN INTERFACE
---------------------
As the interface is hard to come by I am planning on building an interface from the Schematic of the SCSI interface. It should not be to difficult to construct as it has no custom IC's and is a simple double sided board. The IC's are just normal 74 series logic commonly available form any electronics wholesaler. If I manage to get that working I will make PCB layouts etc available for others to download and build their own.