Acorn Winchester Disks

The Acorn System Computers never had the option of Winchester disks, given the high cost of Floppy drives at that time I wonder what a Winchester disk drive would have cost ?  However Winchester disk based products did become available for the BBC/Master Computers and the FileStore.


Acorn produced two Winchester disk drive products for the BBC and Master Computers:

These units were interfaced to the computer using the 1MHz Bus and were accessed using the ADFS Advanced Disk Filing System.

The Winchester drives use the Seagate ST-412 interface and were controlled via an Adaptec ACB-4000 Winchester disc controller, this provides a SCSI bus interface.  The connection between the SCSI bus and the Acorn 1MHz bus was then provided by Acorn's own SCSI Host Adapter Board.  The units also included their own mains power supply.

The following documentation applies to these products:

(Disc or Disk - convention for computer products is generally Disk and that is what I prefer to use however Acorn seem to have used both spellings !)


FileStore was Acorn's Level 3 Econet File Server, it came in two variants: the E01 and E01S.  The E01 came as a basic machine with Floppy drives only but could be expanded by adding an E20 20Mbyte Winchester expansion unit.  The E01S base unit included the SCSI interface and could have up to four Winchester drives connected, two drive capacities were available:

Acorn SCSI Host Adapter Boards

Acorn have several designs of SCSI Host for interfacing between a processor bus and SCSI though they are all based on the same circuit:

Here are the schematics for each board:

Whilst the three circuits are functionally similar there are some important differences:

Winchester Disk Format

Winchester disk's provide a number of disk surfaces laid out in tracks and sectors in much the same way as Floppy disks.

I've tried to determine the disk parameters for the various drives used by Acorn and included them in the following table (still draft):


Product Nominal Capacity
No. of surfaces (heads) No.  of cylinders (tracks
per surface)
No.  of sectors
per track
No. of bytes
per sector
Formatted Capacity
Drive BeebSCSI Image
Winchester Disc 110 10 4 306 33 256 10,340,352    
FileStore E20 20 4 306 61 256 19,113,984 RODIME RO652  
Winchester Disc 130 30 6 640 32 256 31,457,2801 RODIME RO203E  
FileStore E40S 40 5 680 47 256 40,908,800 RODIME RO3057S  
FileStore E60S 60 7 750 ? 256 ? RODIME R3085S  





The Acorn Model 110/130 included a suite of utilities that were included on the drive:

I believe all except WEDITOR are the same as the version on the Master Welcome Disk, I'm still looking for a copy of WEDITOR ?


Acorn SCSI Implementation


For at least their 8-bit computers Acorn used an early implementation of SCSI, SASI (Shugart Associates System Interface) which was then apparently renamed SCSI (to avoid using a company name) later becoming SCSI-1 when later implementations were added.  SCSI-1 includes a subset of SASI so should be backwards compatible.


There are a number of features of SASI that vary from the more widely used SCSI protocols.


In particular SASI allows for non-arbitration in a single Initiator (computer) system, this means that the usual Arbitration Phase can be omitted and the Initiator can start communications with a Selection phase.  This means that you will see communications start by activation of the SEL line rather than the BSY line, the BSY line being asserted by the Target in response to its Id on the bus.


ADFS appears to output an ID of 2 in the Selection Phase regardless of which drive (0 to 3) is being accessed.  (The Adaptec Controller in the Winchester 130 is set to SCSI ID 0 i.e. no links fitted.)










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