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Making new ROMs for the Vic20 / Vicky Twenty

M

y Vicky Twenty is very nearly complete.  As things stand, the board and every single component is new*.



The processor and VIAs are newly-manufactured (W65C02 and W65C22).  Obviously the Vic1 chip isn't manufactured today, but there is 'new old' stock about. I have been able to buy a Vic 1, date code 1987 (which seems very late). It obviously hasn't been in a computer before, passes the acetone test and works.

The same goes for two of the ROMs - character and BASIC. But I haven't been able to buy a new-old Kernal ROM (901486-07). I am able to borrow one - all of the boards I have, have this particular ROM socketed. I don't know whether all of this indicates that the Kernal has proved less reliable than the other two.

I recently bought a TL866 for another project. Of all the retro-computing hardware things I've had to learn to do, making ROMs has been one of the simplest. So far, everything has been very easy and worked first time. 

I'm not sure that it's possible to buy programmable ROM chips in the right package (2364, a 24-pin DIP) but it is possible to buy 2764's which is pretty close. The pinout is slightly different, and there are 4 extra pins, but adapting them is pretty straightforward. 

You can make an adaptor using sockets and bodge wires. But I found these very neat adaptor boards. 

They put the overhang at the back, which makes no difference to me because there's plenty of room on my Vicky Twenty at both ends of the chip but I think it may matter with some revisions of the board.

So here goes. The kernal images are available at zimmers.net, although I guess it's easy to read it off an existing chip.


Being a Mac user occasionally means that Windows-only software isn't an option, so to avoid having to look up the options for minipro each time I use my TL866, I've previously made myself GUMP, a graphic user interface for minipro. 
First I made up a 'test adaptor' with a socket on top to test my new ROM in a known-working Vic board and hurray!! 
So then I made up another adaptor and soldered the new ROM onto it. It keeps the profile down a bit and looks a bit neater than a 'leaning tower of dip sockets'.


** at that point, everything was new. But when I first switched on, it didn't work, turns out that the W65C02 is not a drop-in replacement for the 6502 as I then thought. (the 6522s are drop-in replacements). For the first few months I used Vicky with a reclaimed 6502 (seems that there are no NOS around any more) and have since made a W65C02 -> 6502 adaptor using Byte Attic's circuit and one of those is now running happily in Vicky.  
 


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Comments

  1. Virtvic8 May 2021 at 13:57

    Nice work. Is the Mac software for the Minipro available? I hate having to use a Windows computer to burn roms, but all 3 of my burners only use Windows programs..

    ReplyDelete
    Replies
    1. peahen8 May 2021 at 14:24

      Thanks for asking. At present it's at the point where I can use it but I would want to do a bit more work before releasing it. As you can see on the blog post I wrote about it, I invited people to get in touch if they were interested but despite many views to the blog post, you're the first to ask. I will tidy it a bit and then release it. Keep in touch and ask me again if necessary.

      Delete
  2. robcar16 July 2022 at 04:59

    Hi, where did you get the Eprom? Did you have to erase them first? I'd like if you could share your experience

    ReplyDelete
  3. peahen16 July 2022 at 06:17

    Hi robcar. I used 2764 eproms. I used erasable (with the windows) but there are also one-shot / one-time programmable. They seem to have become a bit more expensive and harder to find, but there are still plenty on offer on eBay for about a fiver each. Other 27-series eproms will work, with a suitable adaptor board.
    So my erasable ones came ready to program, but if you need to you can erase them by exposing to UV light through that window on the top. I think the software for the programmer should tell you whether they're ready to write.
    Other than that, I think my experiences are in this and other blog posts.
    I should mention that I had trouble with one of my home-made roms, the character ROM started to give trouble, and I've currently got a genuine MOS rom in Vicky at the moment. I don't know the reason yet.

    ReplyDelete

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