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The Disk Drives: SFD 1001, 1541D, 1542, 1543, 1551 (SFS 481), 1561, 1563, 1565, 1570, 1571-II, 1571CR, 1572, 1590/1591, 8060, 8061, 8062, 8280

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SFD 1001

.jpg Image of the SFD 1001 (24K, courtesy Marko Mäkelä and George Page)

(Some corrections courtesy Nicolas Welte.)

The SFD 1001 is a single-drive, low profile version of the better known 8250 IEEE-488 disk drive capable of storing 1MB per floppy disk, powered by CBM DOS 2.7. According to Nicolas Welte and Andre Fachat, many PET-era drives, including the SFD, are powered by two CPUs, one for the floppy drive controller and the other for file handling and bus transfer. The SFD, as well as the 8050 and 8250 disk drives, have twin 6502s; the high profile 2040, 3040 and 4040 have a 6502 and a 6504. Vestiges of this are still in the modern single CPU Commodore drives; the 6502 alternately operates in FDC or command processing mode, switching modes on IRQs. (The PET-era exception is the oddball 2031, which has only one CPU and is suspiciously similar to the 1541. In fact, Nicolas points out that you can turn a 1541 into a 2031 merely by putting in 2031 ROMs and exchanging the IEC serial bus interface for an IEEE-488. The 2031's position on the disk drive timeline is somewhat murky. The 4032 drive is merely a high-profile version of the 2031.)

The SFD 1001 has a single drive head, 4K RAM for buffer storage, 77 tracks per side with 23 to 29 sectors per track for a total of 4,166 sectors, BAM in four sectors on track 38 (0, 3, 6 and 9), and directory entries on track 39 with a total of 224 free file slots (29 sectors). 4,133 blocks (!) are free on a freshly formatted disk. SFD disks are quite unreadable in other drives except the 8050 and 8250, but disks written by the 170K-capacity 2031/4040 drives can be read by the later DOS in your 1541 or 1571. However, it is strongly recommended you don't try to write to them with your 1541 or 1571 unless you regard file recovery as an enjoyable pastime, as the write gap between sectors is too different between the two DOSes.

Remember to feed your SFD quad density disks (i.e., 96tpi dual density): not traditional 48tpi dual density, and not modern 96tpi high density. They are not equivalent.

Traditionally, SFD stands for "Super Floppy Drive".

Nicolas Welte has provided an analysis of the 8050/8250 disk format.

CBM8050/8250 Format

29sectors/track for track 1-39/78-116
27sectors/track for track 40-53/117-130
25sectors/track for track 54-64/131-141
23sectors/track for track 65-77/142-154

BAM on track 38 (8050 2 blocks, 8250 4 blocks, rest is free for files)
DIR on track 39 (1 block for header, 28 blocks for dir entries)


$00: 26 00 43 00 00 00  (linker to 38,0, format 'C', three $00 bytes)
$06: name, padded with $A0
$18: ID1 ID2 $A0 '2C' $A0 $A0 $A0 $A0
$21: 00 ....

$00: 26 03 43 00 01 33 (linker to 38,3, format 'C', $00, first track contained
                        in BAM sector, last track + 1 in BAM sector)
$06: FREE M1 M2 M3 M4  (Track 1 entry: Nr. of free sectors on Track, Bitmap of
                        free sectors (1: free) M1: S7 S6 S5 S4 S3 S2 S1 S0, M2:
                        S15..S8, M3: S23..S16, M4: S32..S24
$0B: track 2-50 BAM entries

38,3: (8050)
$00: 27 01 43 00 33 4E
$06: track 51-77 BAM entries

38,3: (8250)
$00: 26 06 43 00 33 65
$06: track 51-100 BAM entries

38,6 (8250 only)
$00: 26 09 43 00 65 97
$06: track 100-150 BAM entries

38,9 (8250 only)
$00: 27 01 43 00 97 9B
$06: track 151-154 BAM entries

39,1: dir entries, just like 1541


A dual drive 1541.


This charcoal grey 1541 incarnation appeared briefly at CES 1984 with the 264 series. It may have been intended as a more compatible alternative to the 264 series' 1551, which was introduced as the SFS 481 at the same show, since unlike the 1551 the 1542 is a serial device. Never released.


A high-density 1541 (also called the 1541H). Unknown storage capacity.

1551 (SFS 481)

Complete Views of the 1551 (.jpg, courtesy Moise Sunda)
Portrait of Unit and Interface (10K) | Backplate (20K) | Mainboard (24K) | Mechanism (21K) | TCBM Interface Board (17K)

.jpg Image of the SFS 481 (32K, courtesy Antonio Pagliaro)

.jpg Image of Jim Butterfield Demonstrating the SFS 481 (36K, RUN 4/84)

Cover of 1541/1551 German User's Manual (39K, .jpg) and 1551 Insert Page (29K, .gif). It is interesting that this manual is a regular 1541 manual with a 1551 sticker on the front, and an update sheet (shown here in its entirety). The manual is otherwise a standard German 1541 manual.

First introduced by none other than Jim Butterfield at CES 1984 as the SFS 481 (as depicted in Antonio's picture which is probably of a very early prototype), the 1551 is the flagship disk drive for the Plus/4, 16 and 264 series. Unlike all other Commodore disk drives, it does not use the IEEE-488 or IEC busses, instead plugging directly into the 264 series' expansion port. This in and of itself is a much faster arrangement, but even more importantly the lines on the expansion port allow the 1551's TIA 6523 (this particular unit has a 6523T; some have a 6523A), a triport interface adaptor located in the drive's interface, to map directly into the 264 series' computer's addressing space and be manipulated directly. The 1551's drive number (8 or 9) causes the TIA to appear at corresponding memory addresses in the computer; address decoding is handled by the interface itself. The TIA port A is used as a direct 8-bit data connection between the computer and the drive, called the TCBM bus in the 1551 schematic; the other TIA ports are used for other TCBM bus signals such as handshaking. The 1551, like all other Commodore disk drives, is an intelligent peripheral; it is based on the 6510T CPU, a specialised version of the 6510 found only in this drive.

The TCBM system was originally going to completely replace the Commodore serial bus, which had been severely crippled for compatibility reasons in the 64. In fact, there were plans to allow the 64 itself to use the TCBM series of drives. A prototype interface, which would have plugged into the expansion port, was mulled over for awhile. Then, says Dave Haynie, the RandD labs at Commodore figured out fast serial transfers and built that into the 1571 instead, since it was just as quick and didn't tie up expansion port lines. The TCBM standard was dropped.

Interestingly, Jim Butterfield does not recall the SFS 481 scheduled as part of the 264 series initial rollout; as corroboration, the SFS 481 did not emerge (and when it did, it was renumbered as the 1551) until winter 1985.


.jpg Image of the 1561 (12K, courtesy Jan Neuvians)
.gif Closeup Image of the 1561 (24K, courtesy Christian Janoff)

The 1561 is a 3.5" drive similar to the 1581, demonstrated an unknown period before the 1581, but never released. Intended for the Commodore LCD. 720K; battery-powered.


Prototype 720K 3.5" drive for early 128s. The built-in 3.5" drive in the 128D/81 (and possibly the Commodore 256) is a 1563.


Views of the 1565 Disk Drive (.jpg, courtesy Bo Zimmerman)
Portrait (28K) | Mainboard (32K) | Ports (20K)

The 1565 is a 3.5" drive intended for use with the C65. Never officially released. The 1565 is not a regular Commodore serial drive; it is a Shugart drive, according to Nicolas Welte. Too bad, I would have liked to see it use TCBM, myself. :-)


Views of the 1570 Disk Drive (courtesy Nicolas Welte)
Front (100K, .gif) | Rear Ports (97K, .gif) | Internal Board (146K, .gif) | Internal Board Closeup (163K, .gif) | Power Supply (162K, .gif)

The 1570 is a single-sided version of the 1571, apparently produced in Europe (according to Nicolas Welte) and Australia (Marc Walters). Nicolas notes that it has a rewired 1571 board in a 1541 case with 1541 transformer and drive mechanism (Alps but with a track 0 sensor and an index hole sensor for handling MFM disks). The power supply, however, is on a small, separate board near the serial bus connectors on the rear.

Stefan adds that the 1570 was only intended to be a stopgap product -- Commodore had intended to release the 1571 with the 128 from day one. However, being the butterfingered dopes they were, Commodore managed to foul the manufacturing schedule and the 1571s were close to several months late. Complaints came in droves from reviewers and magazines; finally, the 1571s made it into stores and the 1570 was discontinued. At least two production runs are known, surprising for a product that was only intended to last for a brief period; Stefan's drive is actually a brown unit that was painted white. After significant use, the underlying brown started to show through! Other production runs are actually entirely white plastic; these may well have been cases stolen from the 1541C.

The 1570 is known to be patched over the 1571 (weird, since the 1571 emerged after); one key example is in proper handling of locked files in fast serial mode, according to Nicolas. However, it has pretty much all of the other 1571 bugs, and you cannot use a 1571 upgrade ROM to upgrade a 1570.


Not known to exist, but there is a mention of it in a very strange place: 1541-II power supplies, as reported by Ron Slaminko, so Commodore must have for a time considered producing such a unit. If it existed at all, it would have most likely resembled a 1541-II with drive electronics based on the 128DCR's internals, according to Nicolas Welte; this means a single chip 5710 instead of the regular WD1770/6526 controller combination and an integrated R/W amplifier chip like the 1541-II. As this is only one extra chip on the board, it would probably have fit into the same case without difficulty, but no one can confirm if Commodore even got it to prototype state.


Images of the 1571CR (.jpg, courtesy Wolfgang Moser [from unknown eBay sale])
Board Notation (15K) | Mainboard (18K) | Label (6K)

The "1571 Cost Reduced" disk drive is, analogous to the 128DCR and other cost-reduced computer models, a modified 1571 with a smaller part count for cheaper mass production. One unit is known in the wild with an unknown owner, verified by an edition of the Commodore 1571 service manual showing a 1571CR with the same board layout provided by Spiro Trikaliotis. Like the 128DCR's internals, the 1571CR uses the integrated 5710 FDC instead of the WD1770/6526 combination, and an integrated R/W amplifier. Other than a single solitary unit, no others are known to exist.


Jan Neuvians' Commodore Curiosity Page (defunct, via Wayback Machine; promotional information on the 1572)

.jpg Image of the 1572 (19K, courtesy Jan Neuvians)

1571/1572 Announcement Clip (.jpg, 119K, from Commodore Microcomputers, April 1985, courtesy Anthony Beckett). Some interesting data and statistics on the '71 and '72.

The 1572 was first introduced CES 1985, according to Jan Neuvians, and is essentially two single 1571s stuck together in an oversized case. Unlike regular dual drives like the 8050, it has two controllers (not one) as well as two mechanisms, which seems to imply that again unlike regular dual drives the drives were addressed as device 8 and device 9, not as device 8 drive 0 and device 8 drive 1. It also has somewhat more RAM (8K) and ROM (a whopping 64K), and was intended for fast backups with a maximum capacity of 410K. Note that released 1571s have only 2KB RAM apiece, not 4KB, but the manual states 4KB which explains the discrepancy. Interestingly, version 3.0 1571s may have a remnant of the 1572's fast backup capability in them. According to Martin Brunner, two 3.0 1571s can be connected and a backup from eight to nine completed within a few seconds. Unfortunately, the version 3.1 ROM in the DCR and later 1571s no longer has this feature, and it is not tripped with the old PET-era DUPLICATE command, so no one is quite sure how to kick it off. It is not likely that the 1572 ever saw the light of day.

1582, 1590, 1591

The 1591 aka 1590 is apparently an HD 1581. ROMs for the 1591 are known to exist, but it is uncertain if the drive itself ever even entered prototype status. A replica 1591 has been constructed from the 1591 ROMs and the A4000 drive mechanism by Steve Croucher, although Steve no longer possesses it.

The 1591, were it produced, would have been very unique for the way it dealt with the issues of high-density storage, according to Nicolas Welte. Amiga FD controllers can only cope with data at a fixed rate. For the lucky Amiga drives that can use HD media, and presumably for the 1591, Commodore slowed the disk spin to 150rpm instead of the normal 300rpm to read in the extra data at the same rate.

James Crook reports a 1582 based on correspondence with one confirmed owner. This appears to be a unknown HD 3.5" mechanism in a A1010 case and upgraded 1581 firmware on a slightly modified 1581 mainboard, and like the 1590/1, can apparently use 3.5" HD media. The owner states that it only reached prototype status, his unit being #2 of six, but no additional information was ever provided. It is also not known whether there is any relationship between the 1582 and the 1590/1.

8060, 8061, 8062, 8280

.jpg Image of the 8061 (41K, courtesy Leo LaFlamme; from a George Page picture)

The 8060 series and 8280 were Commodore's only line of 8" disk drives, all IEEE-488. The original, virtually unknown 8060 was a single drive that stored 750K per disk; the later 8061, 8062 and 8280 were all dual drives and could read IBM 3740 as well as Commodore GCR format. The 8061 had a total capacity of 1.6MB, the 8062 3.2MB and the 8280 1MB. The slimline-styled 8280, the final 8" in the series, was powered by DOS 3.0 (the same DOS used in the 9060/9090).