[ViperSan]

This will have very limited appeal or interest I guess.
I wanted diagnostic facilities for repairing VIC-20 computers ...and realised I could possily modify a super Expander cartridge to do this ...but wanted to retain the super expander functions.
Well this is the result .
Maybe someone will find it useful ?
Now I need to make a harness to fully use the diagnostics.

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So..
How to convert your VIC-20 super expander cartridge for dual use ?
Basically I wanted a VIC 20 diagnostic cart but these (originals) are very rare..
Luckilly it is possible to to retrofit a super expander cart as a diagnostic cart ..but I wanted to retain Super Expander functionallity and the 3k ram expansion it provides.
Looking at the cart pcb there is provision for a second ROM immediately below the Expander ROM.
This can be used to fit your Diagnostic ROM.
So first job was to fit a low profile 24 pin socket ..
read the 326323-01 Super Expander Rom with my GQ-4X4 programmer then download the Diagnostic ROM bin file
So ..now I have 2 binary files one for each rom ..
I found some old TMS 2532JL 45n 4k eproms which required UV erasing.
Blank checked and programmed these were fitted with the S-EX rom in the top position and the Diag ROM in the lower socket.
Now to make them both useable.
Looking at the cart PCB again there are a series of circular pads which can be cut in the middle to make semi circular pads and isolate some pins on the expansion card edge strip and importantly pin 20 of the 2532 eproms .
Pin 20 is the PDP-GM pin which when set high disbles the eprom
but there is another task which needs attention circular pad (1) needs the link cut and is only linked when in Super expander mode
also pad 5 needs to be cut ...
5 wires (I used a short section of ribbon cable) are then soldered to the 2 halves of pad 1
2 wires to either half of pad (5) ...
and the last wire to any 5 volt point
Last job ...obtain a dual pole single throw DPDT slide switch to connect to those 5 wires and allow switching between Super Expander or Diagnostic mode
You need to wire it so that when switched to one position it links both halves of pad (1) and the other half of the switch joins pin 20 of the diag eprom (pin 20 of the diag eprom to 5volts)
so pin 20 of diag eprom is the middle or common pin. one side of the switch goes to +5v and the other to pin 10 of the expansion port edge connector ...effectively
This disables the diag eprom an puts the cart in super expander mode
now to start in diag mode the other pole of the switch is effectively open isolating the 2 halves of pad (1) and pin 20 of the super expander eprom is connected to 5v through the switch (disables super expander eprom)
In short
To enable diagnostic mode pin 20 of s-exp eprom pin is isolated from pin 10 of the expansion port card edge connector and is then connected to 5 volts
To enable Super Expander mode pin 20 of the diag eprom connected to expansion port card edge connector pin 10 and pin 20 of super expander eprom to the expansion port card edge connector pin 13
rgds
VS