Editing Commodore 1541 (section)

== Design ==
=== Hardware ===
The 1541 does not have [[DIP switch]]es to change the device number. If a user adds more than one drive to a system, the user has to cut a trace in the circuit board to permanently change the drive's device number, or hand-wire an external switch to allow it to be changed externally.<ref>{{cite web |url=https://archive.org/stream/run-magazine-28/Run_Issue_28_1986_Apr#page/n83/mode/2up |title=RUN Magazine issue 28|date=April 1986}}</ref>  It is also possible to change the drive number via a software command, which is temporary and would be erased as soon as the drive was powered off.

1541 drives at power up always default to device #8. If multiple drives in a chain are used, then the startup procedure is to power on the first drive in the chain, alter its device number via a software command to the highest number in the chain (if three drives were used, then the first drive in the chain would be set to device #10), then power on the next drive, alter its device number to the next lowest, and repeat the procedure until the final drive at the end of the chain was powered on and left as device #8.

Unlike the [[Apple II]], where support for two drives is normal, it is relatively uncommon for Commodore software to support this setup, and the CBM DOS copy file command is not able to copy files between drives – a third party copy utility is necessary.

The pre-II 1541s also have an internal power source, which generates a lot of heat. The heat generation was a frequent source of humour. For example, ''[[Compute!]]'' stated in 1988 that "Commodore 64s used to be a favorite with amateur and professional chefs since they could compute and cook on top of their 1500-series disk drives at the same time".<ref name="levitan198812">{{cite news |url=https://archive.org/stream/1988-12-compute-magazine/Compute_Issue_103_1988_Dec#page/n105/mode/2up |title=Levitations |work=Compute! | date=December 1988 |access-date=2013-11-10 |author-last=Levitan |author-first=Arlan |pages=104}}</ref> A series of humorous tips in ''[[MikroBitti]]'' in 1989 said "When programming late, coffee and kebab keep nicely warm on top of the 1541."<ref>{{cite magazine |author=<!--Staff writer(s); no by-line.--> |title=NiksiBitti |magazine=MikroBitti |location=Helsinki, Finland |issue=5/89 |page=80 |publisher=Tecnopress Oy |date=May 1989 |issn=0781-2078 }}</ref> The ''MikroBitti'' review of the 1541-II said that its external power source "should end the jokes about toasters".

The drive-head mechanism installed in the early production years is notoriously easy to misalign. The most common cause of the 1541's drive head knocking and subsequent misalignment is [[copy-protection]] schemes on commercial software.{{r|info19860506}} The main cause of the problem is that the disk drive itself does not feature any means of detecting when the [[read/write head]] reaches track zero. Accordingly, when a disk is not formatted or a disk error occurs, the unit tries to move the head 40 times in the direction of track zero (although the 1541 DOS only uses 35 tracks, the drive mechanism itself is a 40-track unit, so this ensured track zero would be reached no matter where the head was before). Once track zero is reached, every further attempt to move the head in that direction would cause it to be rammed against a solid stop: for example, if the head happened to be on track 18 (where the directory is located) before this procedure, the head would be actually moved 18 times, and then rammed against the stop 22 times. This ramming gives the characteristic "machine gun" noise and sooner or later throws the head out of alignment.

A defective head-alignment part likely caused many of the reliability issues in early 1541 drives; one dealer told ''Compute!{{'}}s Gazette'' in 1983 that the part had caused all but three of several hundred drive failures that he had repaired.{{r|halfhill198312}} The drives were so unreliable that ''[[.info (magazine)|Info]]'' magazine joked, "Sometimes it seems as if one of the original design specs ... must have said '[[Mean time between failure]]: 10 accesses.'" Users can realign the drive themselves with a software program and a calibration disk. The user can remove the drive from its case and then loosen the screws holding the [[stepper motor]] that move the head, then with the calibration disk in the drive gently turn the stepper motor back and forth until the program shows a good alignment. The screws are then tightened and the drive is put back into its case.<ref name="info19860506">{{cite news |url=https://archive.org/stream/info-magazine-10/Info_Issue_10_1986_May-Jun#page/n57/mode/2up |title=Physical Exam |work=Info |date=May–June 1986 |access-date=2013-10-06 |pages=57}}</ref>

A third-party fix for the 1541 appeared in which the solid head stop was replaced by a sprung stop, giving the head a much easier life.{{r|info19860506}} The later [[Commodore 1571|1571]] drive (which is 1541-compatible) incorporates track-zero detection by photo-interrupter and is thus immune to the problem. Also, a software solution, which resides in the drive controller's ROM, prevents the rereads from occurring, though this can cause problems when genuine errors do occur.

Due to the alignment issues on the Alps drive mechanisms, Commodore switched suppliers to Newtronics in 1984. The Newtronics mechanism drives have a lever rather than a pull-down tab to close the drive door. Although the alignment issues were resolved after the switch, the Newtronics drives add a new reliability problem in that many of the read/write heads are improperly sealed, causing moisture to penetrate the head and short it out.

The 1541's PCB consists mainly of a 6502 CPU, two [[MOS Technology 6522|6522 VIA]] chips, and 2k of work RAM. Up to 48k of RAM can be added; this is mainly useful for defeating copy protection schemes since an entire disk track could be loaded into drive RAM, while the standard 2k only accommodates a few sectors (theoretically eight, but some of the RAM was used by CBM DOS as work space). Some Commodore users use 1541s as an impromptu [[math coprocessor]] by uploading math-intensive code to the drive for [[background process]]ing.<ref>{{cite web|url=https://www.lemon64.com/forum/viewtopic.php?t=27911 |title=The Masque/Oxyron uses the 1541 to calculate the 3D math in the endpart. / Panta Rhei/Instinct+Plush+Oxyron uses the 1541 to do stuff for part 1 and 2. / altered states and digital world both use the disk drive for 3d math.|access-date= 6 May 2020}}</ref>
<ref>{{cite web|url=https://csdb.dk/release/?id=820|title=The Masque (1994) |access-date= 6 May 2020}}</ref>
<ref>{{cite web|url=https://csdb.dk/release/?id=15745|title=Panta Rhei (2004) |access-date= 6 May 2020}}</ref>
<ref>{{cite web|url=https://csdb.dk/release/?id=11610|title=Altered States 50% (1994) |access-date= 6 May 2020}}</ref>
<ref>{{cite web|url=https://csdb.dk/release/?id=11610|title=Digital World (1998) |access-date= 6 May 2020}}</ref>

=== Interface ===
The 1541 uses a [[Commodore bus|proprietary serialized derivative]] of the [[IEEE-488]] parallel interface, found in previous disk drives for the [[Commodore PET|PET/CBM]] range of personal and business computers,<ref>{{cite web|url=https://www.thegeekpub.com/9473/sd2iec-manual-use-sd2iec-c64/|title=SD2IEC MANUAL (HOW TO USE THE SD2IEC ON THE C64)|date=26 July 2017|publisher=The Geek Pub|access-date= December 8, 2017}}</ref> but when the VIC-20 was in development, a cheaper alternative to the expensive IEEE-488 cables was sought. To ensure a ready supply of inexpensive cabling for its home computer peripherals, Commodore chose standard [[DIN connector]]s for the serial interface. Disk drives and other peripherals such as printers connect to the computer via a [[daisy chain (electrical engineering)|daisy chain]] setup, necessitating only a single connector on the computer itself.

=== Control ===
{{further|topic=the 1541's DOS|Commodore DOS#Technical overview|}}