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Domesday Master AIV SCSI Card.
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.
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.When looking for a BBC Master for building a domesday system the most likely system to have the correct connector are those sold as Master Turbo's complete with an internal co-processor.
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
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 it is possible to build 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.