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The BBC Microcomputer System (BBC Micro)

BBC Micro Floppy Disk Drives

In addition to the floppy disk drives distributed by Acorn Computers, a number of companies offered disk drive upgrades for the BBC Micro, including Akhter, Cumana, Opus, Torch and Watford Electronics. These companies did not manufacture the drives, but used a range of hardware from manufacturers such as Chinon, NEC and Teac.

Acorn's first floppy disk based file system for the BBC Micro was the ROM based Disk Filing System (DFS). This basic filing system is only aware of a single side of a floppy disk, double sided drives are treated as two logical drives, in a two drive system, the first physical drive would have logical disk numbers 0 and 2, with the second drive being 1 and 3. With compatible hardware, disks can be formatted as either 40 Track or 80 Track. Since an 80 Track drive could not automatically read a 40 Track disk, some drive vendors offered drives that featured a switch to select between 40T and 80T operation, a number of the Cumana drives that I have picked up have this feature.

DFS was first implemented on the BBC Micro using the Intel 8271 disk controller which used Frequency Modulation (FM) encoding which gave half the capacity of Modified Frequency Modulation (MFM) encoding leading to the terms Single Density (SD) and Double Density (DD). The data rates for the two encoding systems are 125 kbits/sec (FM) and 250 kbits/sec (MFM).

Acorn introduced the Advanced Disk Filing System (ADFS) to handle the DD format, this required the addition of a daughter board with an upgraded disk controller, the Western Digital WD1770 or later.

BBC Micro Drive Formats

 

Geometry

FM MFM

Sides

Tracks

spt* bps* Capacity spt* bps* Capacity
5.25", Single Sided, Single Density 1 40 10 256 100 kB      
5.25", Single Sided, Double Density# 1 40       16 256 160 kB
5.25", Single Sided, Single Density 1 80 10 256 200 kB      
5.25", Single Sided, Double Density# 1 80       16 256 320 kB
5.25", Double Sided, Single Density 2 40 10 256 400kB      
5.25", Double Sided, Double Density# 2 40       16 256 640kB

#   requires ADFS

*   spt = sectors per track, bps = bytes per sector, MFM = Modified Frequency Modulation

For compatibility with the various configurations, the drives sold for use with the BBC Micro ranged from SS/SD up to DS/HD, although, DS/HD drives that would give a formatted capacity of 1.2MB on a PC, would operate as DS/QD drives on a BBC Micro.

The same drives can also be used with other retro computers and I found that legacy BBC drives were a good source of replacement drives for my Memotech FDX system. However, the configuration of the drive for BBC use was not necessarily compatible with the FDX and often required the re-pinning of various jumpers. The purpose of the jumpers is not always clear from the bare drive and the reseller manuals did not provide the necessary details to reconfigure the drive.

As the drives are very old, at least 30 years in most cases, online manuals or other references were often no easy to come by, I have managed to find a number of old documents that have proved useful in when needing to reconfigure the drives, these documents are available on this page, they may save you some time. If you have other legacy floppy drive manuals or configuration information, particularly for 5.25" drives, I would appreciate it if you could send me a copy please. Drives known to work on a BBC Micro are shown in green, the other drives may be compatible, caveat emptor.

 

Doc Manufacturer Model Size Heads Tracks- Capacity+
Chinon FZ-506 5.25" 2 80 1.6MB
Mitsubishi M4851 5.25" 2 40 500kB
NEC FD-1165 5.25" 2 80 1.6MB
NEC FD-1036A 3.5" 2 80 640kB
Qume QumeTrak 142 5.25" 2 40 500kB
Sony MPF920-Z 3.5" 2 80 2.0MB
Teac FD235F (HF) 3.5" 2 80 1.0MB (2.0MB)
Teac FD-55F (E) 5.25" 2 (1) 80 (40) 1.0MB (500kB)
Teac FD-55G 5.25" 2 80 1.6MB
Teac FD-55GFR 5.25" 2 80 1.6MB
YE Data YD-380 (service) 5.25" 2 80 2.0MB

-   More accurately, the number of cylinders (per side)

+   Unformatted capacity from drive data sheets

 

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