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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.
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