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Commodore 1541 by jekky
(2232, 'Disk capacity br Each side of 170 kBs was split into 683 sectors on 35 tracks each of the sectors holding 256 bytes the file system made each sector individually rewritable br However since one track had to be used by the drive itself for directory and file allocation information so called BAM Block Allocation Map and two bytes of each physical sector were used as a block pointer to the next physical track and sector as to implement interleave for faster access a logical block would hold 254 out of 256 bytes br If the disk was not otherwise prepared with a custom format e g for data disks 664 blocks would be free after formatting giving 168 656 bytes or almost 165 kB for user data br By using custom formatting and load save routines sometimes included in third party DOSes see below all of the mechanically possible 40 tracks could be used The reason why Commodore decided not to use the upper five tracks by default or at least more than 35 was the bad quality of some of the drive mechanisms which did not always work reliably at the highest tracks So by reducing the number of tracks used and thus capacity it was possible to further reduce cost in contrast to Double Density drives used e g in IBM PC computers of the day which saved 180 kB on one side by using a 40 tracks format br The 1541 did not have an index hole sensor making it straightforward to use the reverse side of a disk by flipping it A disc could be converted to a flippy disk by simply cutting punching a notch on the left hand side causing the drive to recognize both sides as writable This would effectively double the storage capacity The notch could be made with a knife single hole paper punch or disk notcher tool that was specifically designed for this task To achieve the same effect on other drives would normally have also required an extra cut out for the index hole a harder modification br For compatibility and ease of implementation the 1571 s Double Sided format of one logical disk side with 70 tracks later was created by putting together the 35 physical tracks on each of the physical sides rather than using two times 40 or e g 38 tracks even though there were no more quality problems with the mechanisms of those drives The 1571 CP M format however uses the full 360 kB available on that two R W head drive br Introduction and early problems br 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 and they struggled to produce the drive in adequate quantities br Failure rates on the 1541 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 four of the seven drives the magazine had in its editorial offices had failed Eventually the problems subsided and the drive became nearly as widely available as the C64 itself br The 1541 did not have dip switches to change the drive number If a user added more than one drive to a system the user had to open the case and cut a trace in the circuit board to permanently change the drive s number or hand wire an external switch to allow it to be changed externally It was also possible to change the number temporarily from the operating system br The 1541 also had an internal power source which generated some heat The heat generation was a frequent source of humour For example series of humorous tips in MikroBitti 5 1989 said When programming late coffee and kebab keep nicely warm on top of 1541 MikroBitti review of 1541 II said that its external power source should end the jokes about toasters br The early 1541 s had a spring eject mechanism Alps Drive and the disc s often failed to release This style of drive had the popular nickname Toaster Drive because it required the use of a knife or other thin object to pry out the stuck media just like a piece of toast stuck in a real toaster This was fixed later when Commodore chaged the vendor of the drive mechanism Mitsumi Drive and went to a flip handle that allowed extraction of the disc using your fingers and no longer requiring the spring br Versions and third party clones br 1541C the first upgrade version br 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 br Commodore 1541 II the second of two upgraded versions of the CBM 1541 The 1541 II has the more modern radial handle locking mechanism br 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 br 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 br The serial computer interface br The 1541 used a bit serial version of the IEEE 488 interface the speedier parallel version of which was used on Commodore s earlier drives for the PET CBM range of personal business computers To ensure a ready supply of inexpensive cabling for its home computer peripherals Commodore chose standard DIN connectors for the serial interface Disk drives and other peripherals such as printers were connected to the computer via a daisy chain scheme necessitating only a single connector on the computer itself br Initially Commodore intended to use a hardware shift register the 6522 VIA to maintain relatively brisk drive speeds with the new serial interface However a hardware bug with this chip prevented the initial design from working as anticipated and the ROM code was hastily rewritten to handle the entire operation in software According to Jim Butterfield this caused a speed reduction by a factor of five br As implemented on the VIC 20 and Commodore 64 CBM DOS could transfer only about 300 bytes per second which translated to about 20 minutes to copy one disk10 minutes of reading time and 10 minutes of writing time However since both the computer and the drive could easily be reprogrammed third parties quickly wrote more efficient firmware that would speed up drive operations drastically Without hardware modifications some fast loader utilities managed to achieve speeds of up to 4 kB s The most common of these products were the Epyx FastLoad the Final Cartridge and the Action Replay plug in cartridges which all had machine code monitor and disk editor software on board as well The popular Commodore computer magazines of the era also entered the arena with type in fast load utilities with Compute s Gazette publishing TurboDisk in 1985 and RUN magazine publishing Sizzle in 1987 br Even though each 1541 had its own on board disk controller and disk operating system it was not possible for a user to command two 1541 drives to copy a disk one drive reading and the other writing as with older dual drives like the 4040 and 8050 that were often found with the PET computer and which the 1541 was backward compatible to it could read 4040 disks but not write to them since its internal Operating System was essentially the same Unfortunately however the routines in the 1541 disk operating system to enable disk copy were removed as it was intended to be a stand alone unit To copy from drive to drive software running on the C64 was needed and it would first read from one drive into memory then write out to the other Only later when first Fast Hack em then other disk backup programs were released was true drive to drive copying possible One could then unplug the C64 itself from the drives i e from the first drive in the daisy chain and do something else with the computer as the drives proceeded to copy the entire disk br Copy protection by read error br Early copy protection 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 Few of these schemes had much deterrent effect as various software companies soon released nibbler utilities that enabled protected disks to be copied br The drive head misalignment issue br 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 Daisy Bell Amazing Grace and perhaps most appropriately When I m Sixty Four The most common cause of the 1541 s drive head knocking and subsequent misalignment however was copy protection schemes on commercial software br The main cause of the problem was that the disk drive itself did not feature any means of detecting when the read write head had reached track zero Accordingly when a disk was formatted or a disk error occurred the unit would physically drive the head 40 tracks to track zero although the 1541 DOS 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 br There were at least one or two home remedy software products on the market during the heyday of the 1541 drive and Commodore 64 computer that could be used to re align the drive yourself and save a costly trip to a repair center and down time They generally consisted of a software program and a calibration disk What the user would do is remove the drive from its case and then loosen the screws holding the stepper motor that moved the head Then with the calibration disk in the drive gently turn the stepper motor back and forth until the program showed a good alignment The screws were then tightened and the drive put back in its case The real problem actually was that the sprocket which moved the head would slip on the stepper motor shaft after repeated hammering and this would put the drive gradually out of alignment br The later 1571 drive which was 1541 compatible incorporated track zero detection and was thus immune to the problem A third party fix for the 1541 appeared where the solid head stop was replaced by a sprung stop giving the head a much easier life Also a software solution which resided in the drive controller s ROM could prevent the re reads from occurring though this could cause problems when genuine errors did occur br Commodore s successor products br 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 3 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 32 bit Amiga and the 1581 was mostly sold to remaining GEOS users br References br CBM 1982 VIC 1541 Single Drive Floppy Disk User s Manual 2nd ed Commodore Business Machines Inc P N 1540031 02 br 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 br Immers Richard Neufeld Gerald G 1984 Inside Commodore DOS The Complete Guide to the 1541 Disk Operating System DATAMOST Inc amp Reston Publishing Company Inc Prentice Hall ISBN 0 8359 3091 2 br Englisch Lothar Szczepanowski Norbert 1984 The Anatomy of the 1541 Disk Drive Grand Rapids MI Abacus Software translated from the original 1983 German edition Dsseldorf Data Becker GmbH ISBN 0 916439 01 1 br External links br C64 Preservation Project Discusses internal drive mechanics and copy protection br Undocumented 1541 drive functions From the Project 64 website br v 160 160 d 160 160 e br Floppy disk drives for Commodore 8 bit systems br IEEE br 160 160 8 160 160 160 160 5 br 8060 160 8061 160 8062 160 8280 160 160 160 2031 160 2040 160 3040 160 4031 160 4040 160 8050 160 8250 160 SFD 1001 br Serial')
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