Magic Desk 1 8k multi-ROM modification by Clockmeister (c) 2009 V1.0

- Theory of operation

On a standard single 8k cart the /EXROM is pulled low and the  /ROML
lines connected to the /CE and /OE lines of the ROM are pulled low to enable the ROM at $8000.

On the Magic Desk Cartridge the /CE lines on all the ROMs are connected to ground and so pulled low
by default and the /OE line of each ROM is selected and pulled low as required by the 74xxxx
logic on the board.

The modification bypasses that logic through the use of manually selectable switches to
manipulate which ROM is selected by /ROML. 

It's not quite that simple though as we have to make sure that the other 3 ROMs are NOT selected
so /OE on those ROMs must be pulled high. This is achieved through using pull-up resistors which make sure
that the ROMs that are not selected are always pulled high and only pulled low by /ROML through the switch.

It's important to make sure that only one ROM is selected at one time to avoid damage to the ROMs or the C64,
though I doubt that any actual damage is likely there is always a possibility.
Any questions can be addressed to clockmeister@internode.on.net

Have fun,

Clockmeister.

http://www.users.on.net/~clockmeister



Notes:

* No responsibility is taken for any damage caused to any equipment or persons from using any information
  on any of my web pages. 

* Note on the capacitors C1,C2 and C3:
  When the ROMs are removed and a 28 pin socket is fitted the capacitors C1, C2 and C3 occupy what would
  be pin 1 of the 28 pin ROM socket  therefore the capacitors need to be removed. You can refit the capacitors
  by using the alternative  mounting location for the leg of the capacitor if you desolder the caps. 
  Leaving the caps off entirely is unlikely to have negative impact so that is another option.