Commodore 1541: Encyclopedia II - Commodore 1541 - The serial computer interface

Commodore 1541 - The serial computer interface

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.

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. [1]

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 disk—10 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/sec. The most common of these third-party 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 publishing Sizzle in 1987.

Because each 1541 had its own onboard disk controller and disk operating system, it was possible for a user to command two 1541 drives to copy a disk (one drive reading and the other writing), and 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 spend the next 35 seconds copying the entire disk.

Adapted from the Wikipedia article "The serial computer interface", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki