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Vintage Computers Any vintage computer systems, calculators, video games etc., but with an emphasis on 1980s and earlier equipment.

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Old 13th Jun 2019, 3:50 pm   #1
ViperSan
Triode
 
Join Date: Jun 2019
Location: Manchester, UK
Posts: 30
Default Dual Function for VIC20 super expander cartridge

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.

-----------------------------------------------------------------------------------
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
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Old 13th Jun 2019, 6:12 pm   #2
SiriusHardware
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Default Re: Dual Function for VIC20 super expander cartridge

Nice write-up, I'm sure that will be viewed with grateful interest.

I take it the 'super expander' cartridge is to the VIC20 what the '16K Ram Pack' would be to a ZX81, and is therefore probably far more common / easy to find than a genuine diagnostic cartridge?
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Old 13th Jun 2019, 6:22 pm   #3
ViperSan
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Default Re: Dual Function for VIC20 super expander cartridge

Indeed it is for the VIC20 ..though 16k memory expansion cartridges were also made for the VIC20.
The reason I went for the super expander for modding is that the PCB is in part pre made for a second chip ..so why not use it ?
..and yes I am aware that modern equivalent to disgnostic cartridges do exist and are available ..I think from Tynemouth software (Dave Curran) ...I wanted to see if I could do it thats all ..
regards
VS
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Old 13th Jun 2019, 6:30 pm   #4
SiriusHardware
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Default Re: Dual Function for VIC20 super expander cartridge

Do you have a particular affinity for the VIC-20, or is that just one of many?

I have to admit I don't know that machine well as I was a member of the Sinclair tribe, although a friend did have a C64 for a few years before he upscaled to an Amiga so I have some (but not much) familiarity with the '64.
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Old 13th Jun 2019, 6:46 pm   #5
electronicskip
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Default Re: Dual Function for VIC20 super expander cartridge

Quote:
Originally Posted by ViperSan View Post
Indeed it is for the VIC20 ..though 16k memory expansion cartridges were also made for the VIC20.
The reason I went for the super expander for modding is that the PCB is in part pre made for a second chip ..so why not use it ?
..and yes I am aware that modern equivalent to disgnostic cartridges do exist and are available ..I think from Tynemouth software (Dave Curran) ...I wanted to see if I could do it thats all ..
regards
VS
Wow!
That's impressive.
Im an avid VIC20/64/16/+4 User myself so this is of great interest to me although most of it is over my head as im not very Computer minded when it comes to the inner workings.
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Old 13th Jun 2019, 6:51 pm   #6
ViperSan
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Default Re: Dual Function for VIC20 super expander cartridge

Not really Sirius ...I have a Commodore Bias I suppose ..since I once wrote commercially sold games for Commodore machines through a local company Mr Micro.
6502 was my CPU of choice.
But for me all 8 bit computers are interesting and deserve reverence.
and now I've also extended my interests to include Amiga 16 bit machines.
All that said ..
My list so far of restored compters include
A PET 4032 and 8050 dual floppy drive
4x Commodore 64s and 2 floppy drives (1540 and 1541)
3 VIC20s and if I can find another damaged unit ..I have a spare motherboard
2 Commodore Plus 4s
1 Commodore C16
1 x Apple IIe
1 x Apple II Europlus
and several floppy drives
The latter is particularly interesting as I converted it back to produce colour via NTSC monitor ...and I literally built a keyboard for it using non standard parts.
I'll replace this if I ever find a genuine Keyboard

and a couple of rebuilt speccy 48ks ...one of which has a proper keyboard..and dual boot original and gosh wonderful.

I have many many laptops from DOS to win XP
One in particular is ..or was a Toshiba T3200SX which had a smashed plasma display.
So to get it in working order I fitted it with a modern LCD screen

I also have a Toshiba T5200/100 amber plasma screen which was heavily modded to include an extra 16 bit ISA slot
A later floppy drive 1.44 as opposed to the original 720k
A dual PCMCIA card and ports
More ram using a modded EDO 72 pin sim as the original ram is rare as hens teth
A faster CPU ..to make it capable of running win95 (originally win 3.1 was barely do-able)
etc
so you see ..all is fair game to me ..
I prefer originality hardware unmodded...but sometimes needs must.
rgds
VS
I'll post another thread with some photos at some point if anyone want to see anything specific ?

Last edited by ViperSan; 13th Jun 2019 at 6:59 pm.
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