
                   Parallel IEEE 488 interface for VC1541
                                      
   (c) 64er magazin
   
   Someone asked me about the parallel IEEE 488 interface for the VC1541,
   that has been published in an early issue of the famous german "64er"
   magazin. Well, I don't know if this magazin still exists (long out of
   the scene) but then I have this old issues in case I need to repair my
   interface.
   
   I have redrawn the schematics now, so there is a nice schematic
   instead of my sketchy hand-drawn one. People who know what they are
   doing can see how to produce such a board.
   
  Disclaimer
  
   There is absolutely NO WARRANTY, neither expressed nor implied! Noone
   can be held responsible for any damage arising from the use or
   building of this interface or even from reading these pages! I do not
   claim nor guarantee that the hardware and software can be used for
   anything useful! 
   
   The nicely drawn schematics are available in vc1541-ieee.png and
   vc1541-ieee.ps. The socket on the right is the IEEE488 connector. In
   vc1541-ieee.desc.txt you find the schematics in a text file together
   with some info how to build it. Hint: The ouput lines PA0-7, PB0-7
   (Pins 2-17) and CA1 and CA2 (Pins 40,39) are not connected through to
   the motherboard! Note that the IC and socket labeled "UC3" has to be
   the VIA 6522 that is connected to the IEC bus. This might be different
   on different versions, it is UC3 on my 1540048-01 circuit board(short
   board). The interface is basically modelled after the 2031 schematics,
   the IEEE488 sister drive to the VC1541.
   
   To use the interface the VC1541 needs a new ROM. In the 64er magazin
   they listed a program vc1541-ieee.mkrom.prg to read and patch the
   original VC1541 ROM. You can read an ASCII listing in
   vc1541-ieee.mkrom.txt. Because I was too lazy to type in the whole
   program, only the very necessary parts are there. So before you start,
   you have to do a "POKE 56,80" to protect the area above $5000, where
   the modified ROM is saved. When the program has finished, you have to
   save the area from $5000 to $6fff with a machine language monitor.
   This file has to be burned into an EPROM that replaces the ROM with
   the upper half of the VC1541 DOS. For the exact location please see
   http://www.funet.fi/pub/cbm/schematics/drives/new/1541/index.html.
   The original 2031 ROM is different from the VC1541 ROM in both ROM
   chips, but the patch manages to get the changes into one ROM chip
   only, so only one EPROM is needed.
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