Commodore 1541

The Commodore 1541 (originally called VIC-1541), made by Commodore Int'l, was the best-known floppy disk drive for the Commodore 64 home computer. The 1541 was a single-sided 170 kilobyte drive for 5¼" disks. The 1541 followed the previous Commodore 1540 (meant for the VIC-20).

Description

There were two versions of the 1541 mechanics. Early models used a drive mechanism made by Alps Electric, distinguishable by its push-down drive door. Later models utilized a drive mechanism manufactured by Newtronics (Mitsumi), which used a lever release. All but the very earliest 1541s can use either the Alps or Newtronics mechanism. Visually, the first models, of the VIC-1541 denomination, had an off-white color like the VIC-20 and VIC-1540. Then, to match the look of the C64, CBM changed the drive's color to brown-beige and the name to Commodore 1541.

Use of "flippy disks" ("made" by cutting/punching a notch on the left-hand side of a single-sided disk, opposite the factory-made write-protect hole) would give access to the 'reverse' side of the disks, doubling the storage capacity. Each side, of 170 KB, was split into 664 'blocks' holding 256 bytes each; the file system made each block its own cluster.

The disk drive used Group Code Recording and contained a MOS Technology 6502 microprocessor, doubling as a disk controller and on-board disk operating system processor. The number of sectors per track varied from 17 to 21 (an early implementation of Zone Bit Recording). The drive's built-in disk operating system was CBM DOS 2.6.

The drive-head mechanism was notoriously easy to misalign, and had a tendency to make a 'machine-gun' rattle when out of alignment or when formatting a new disk. Some people even wrote code to vibrate the head at different frequencies to play simple tunes such as Amazing Grace. The most common cause of the 1541's drive head knocking and subsequent misalignment, however, was copy prevention schemes on commercial software.

The main cause of the problem was that the disk drive itself did not feature any means of detecting when the read/write head returned to track zero. Accordingly when a disk was formatted or a disk error occured, the unit would physically drive the head 40 tracks to track zero (although the 1541 only used 35 tracks, the drive itself was a 40 track unit). Once track zero was reached, the head would be physically rammed against a solid stop. This ramming gave the characteristic 'machine gun' noise and would, sooner or later, throw the head out of alignment.

Early copy prevention schemes deliberately introduced read errors on the disk, the software refusing to load unless the correct error message was returned. The general idea was that the inbuilt disk copy command was incapable of copying the errors. When one of these errors was encountered, the disk drive (as do all disk drives) would attempt one or more re-read attempts after first resetting the head to track zero. The 1571 drive (which was 1541 compatible) incorporated track zero detection and was thus immune from the problem. A third party fix for the 1541 appeared where the solid head stop was replaced by a sprung stop which gave the head a much easier life. Also a software solution which resided in the drive controller's RAM, prevented the re-reads from occuring, though this could cause problems when genuine errors did occur.

The 1541 used a bit-serial version of the IEEE-488 (GPIB) parallel protocol. The simple protocol that the built-in DOS used supported only about 300 bytes/s. Some third-party speed-ups, however, could transfer about 4 kilobytes per second over the interface, and some "fast loaders" managed up to 10 kbytes/s. The most common of these third-party products were the Epyx FastLoad, The Final Cartridge, and Action Replay cartridges, which all had machine code monitor and disk editor software on board as well.

History

Priced at under US$400 at its introduction, the 1541 became widely popular. Although expensive by today's standards, a C64 plus a 1541 cost about $900, while an Apple II with no disk drive cost $1395. The demand caught Commodore by surprise, who struggled to produce the drive in adequate quantities. Failure rates initially were very high, and the drives were virtually impossible to find. The lead editorial in the December 1983 issue of Compute!'s Gazette lamented that of seven drives the magazine had in its editorial offices, four had failed. Eventually the problems subsided and the drive became nearly as widely available as the C64 itself.

The 1541's numerous shortcomings opened a market for a number of third-party clones of the disk drive, a situation that continued for the lifetime of the C64. Well-known clones were the Oceanic OC-118 aka Excelerator+, MSD 's single and dual drives, the Enhancer 2000, the Indus GT, and CMD 's FD-2000 and FD-4000. Nevertheless, the 1541 became the first disk drive to see widespread use in the home and Commodore sold millions of the units.

In 1986, Commodore released the 1541C, a revised version that offered quieter and slightly more reliable operation and a light beige case matching the color scheme of the Commodore 64C. It was replaced in 1988 by the 1541-II, which used an external power supply to provide cooler operation and allow the drive to have a smaller desktop footprint (the power supply "brick" being placed elsewhere, typically on the floor).

The Commodore 1570 was an upgraded 1541 for use with the Commodore 128, available in Europe. It offered MFM capability for accessing CP/M disks, improved speed, and somewhat quieter operation, but was only manufactured until Commodore got its production lines going with the double-sided 1571. Finally, the small, external power supply, MFM-based Commodore 1581 3½" drive was made, giving 800 KB access to the C128 and C64. By this time, however, many CBM users had shifted their attention to the 16-bit Amiga, and the 1581 was mostly sold to remaining GEOS users.

References

• Neufeld, Gerald G. (1985). 1541 User's Guide. The Complete Guide to Commodore's 1541 Disk Drive. Second Printing, June 1985. 413 pp. Copyright © 1984 by DATAMOST, Inc. (Brady). ISBN 0-89303-738-9.
• Immers, Richard; Neufeld, Gerald G. (1984). Inside Commodore DOS. The Complete Guide to the 1541 Disk Operating System. DATAMOST, Inc & Reston Publishing Company, Inc. (Prentice-Hall). ISBN 0-8359-3091-2.
• Englisch, Lothar; Szczepanowski, Norbert (1984). The Anatomy of the 1541 Disk Drive. Grand Rapids, MI: Abacus Software (translated from the original 1983 German edition, Düsseldorf: Data Becker GmbH). ISBN 0-916439-01-1.