cbm-hackers mailing list: Professional-1541-DOS (was: Re: Big-PET)
http://www.softwolves.com/arkiv/cbm-hackers/0/0727.html
"The "Professional-1541-DOS" from Mikrotronic System is an add-on for the 1541 and a C64 or a C128 in C64 mode, which is supposed to speedup all read- and write-operations of the drive. The Professional-DOS is an expansion for an already installed speeder with a parallel cable, like SpeedDOS, SpeedDOS Plus, Floppy Flash User System and Floppy Flash Toolkit System."