Vic 20 Charcter ROM EPROM PCB adapter

Discussions for CBM PET, VIC-20, SX64, C16/+4 and Commodore 128 systems
banman
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Vic 20 Charcter ROM EPROM PCB adapter

Post by banman »

Hi Philip L,

Apologies for not carrying on with this VIC 20 Character ROM adapter PCB project. I have been a bit lazy.

I will now try to explain my take on what I did. Please don't hesitate to add or comment. The more information on this the better I feel.


First of all I would like to thank eslapion for providing me with much technical background information on the subject. Thank you to C64istanbul for creating a one of the nicest and most complete C64/ Vic 20 EPROM adapter designs out there. I think it might actually fit in the tight spaces of the TI/99A.
He is a very talented and kind person. I think he made the design up in a day or so after a suggestion. He makes some other cool stuff on PCBway.com too (this is not a paid promotion by the way).


I have a CR (cost reduced) NTSC Vic 20 that has the Character ROM missing. The only issue is that there is a RF shield that stops most EPROM adapter PCB's from being used.


One suitable PCB, I will focus on exists at...


https://www.pcbway.com/project/sharepro ... d_C64.html




There are others that are variants of this that should be able to be converted as well.



This particular PCB needed to have a small wiring mod done to it, which made it work for me in the NTSC Vic 20. I have tried it in a Japanese VIc 20 OK as well.
Attachments
IMG_20200904_094223.jpg
IMG_20200904_094203.jpg


banman
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Re: Vic 20 Charcter ROM EPROM PCB adapter

Post by banman »

Here is a suggested mod to this PCB.



IMG_20201026_144521.jpg
IMG_20210719_152437.jpg
IMG_20210719_163118.jpg




If you look carefully You will see that I used cut off resistor wires to make pins that one can use to plug a jumper wire between a standard C64 ROM configuration and the Vic 20 Character ROM configuration.
I lifted the socket leg up of pin 22 where 27c512 EPROM sits and connected the red jumper wire. I added a turned pin socket to the end of the red wire to act as a plug.

A careful maker could possibly use precision turned pin headers and melt out these 2 pins replacing then with a 2 longer pins to achieve what one sees in the pictures. I am sure there are plenty of ways to do it. Hard wiring it work work fine too. Especially if you were planning on making it permanent.


What I have made here will of course make the whole rig tall. It may have issues with the top keyboard cover closing properly I'm not really sure here. One can of course just wire the PCB directly and solder it directly into the mainboard.
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Re: Vic 20 Charcter ROM EPROM PCB adapter

Post by banman »

I have here is a version using a 4Kb 2732 EPROM (I assume a 27c32 will work as well).

Have a close look at the wiring and compare them to the schematic to get a clear look. I have lifted the pins of the topmost socket to alow rewiring to the bottom pins which connect the the VIc 20 mainboard. I placed small peices of insulating tape between these lifted opins and the adjacent bottom socket.

When viewed along side the Vic 20 27C512 EPROM Character ROM PCB there are very similar connections. There are some more lines on the 27C512 EPROM that are used to control the switchable memory banks.

IMG_20201026_193354.png
IMG_20210715_172344 Untitled-1 2332 to 2732 4Kb VIc 20 Char ROM adapter wiring.jpg
IMG_20201024_185304.jpg
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Re: Vic 20 Charcter ROM EPROM PCB adapter

Post by banman »

Pins 18 and pin 20 of the 2732 EPROM socket (the top socket) have been lifted so that there is no direct connection to the pins directly below.


IMG_20210719_172328.jpg
IMG_20201024_185308.jpg
IMG_20201026_143027.jpg
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eslapion
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Re: Vic 20 Charcter ROM EPROM PCB adapter

Post by eslapion »

banman wrote: Mon Jul 19, 2021 7:43 am First of all I would like to thank eslapion for providing me with much technical background information on the subject.
Glad I could help!
Thank you to C64istanbul for creating a one of the nicest and most complete C64/ Vic 20 EPROM adapter designs out there.
I looked up the adapter on PCBWAY's web site and I think it may not be such a good idea to use it on a VIC-20 unless I missed something in the information you posted here.

On the C64, the 2332 Character ROM CS1 is active low and CS2 is active high and just connected to 5V. Only one condition has to be met for this IC to start signaling on the bus and that is for the activation line from the PLA to be low.

On the VIC-20, both CS1 and CS2 are active low and both conditions must be met for the signaling to start. You redirected one of them to /CE and the other to /OE. If that's not what you did then expect trouble.
This particular PCB needed to have a small wiring mod done to it, which made it work for me in the NTSC Vic 20. I have tried it in a Japanese VIc 20 OK as well.
I suppose that's the mod in question.

I don't understand why you'd use a chip with a capacity as large as a 27C512 as this would allow you to have 16 different character sets. I suppose you could always have the standard VIC-20 set, the C64 set, the PET set and finally the VIC-1001 set. Even then a 27C256 will house all of these nicely too.
Wealth, like happiness, is never attained directly. It comes as a by-product of providing a useful service. -Harland D. Sanders
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Re: Vic 20 Charcter ROM EPROM PCB adapter

Post by banman »

Hi eslapion,



Oops, I made an error on the 2332 to 2732 hand wired image. :oops:

I have pin 21 CE2 on the 2332 side pointed to pin22. The blue pointer should infact be moved over and pointed towards the real pin21.
I even got the spelling for the thread's title wrong!
IMG_20210721_055023.jpg

I think 64instanbul's PCB board was something else to start off (possiblily a C64 ROM adapter). Then was modified to fit the tight space of a CR (cost reduced) NTSC Vic 20. The board has yet again been revised and reimagined again as a C64 ROM adapter. I think he's up to version 3.0.

There is a large amount of storage space in 27c512 EPROM. I think they're easier and a bit cheaper to buy, especially the dodgy Windbond varieties.

I had version 1.0 which had a small wiring error. I think from memory the A11,12,13,14 lines were not tied to 5v and were essentially floating (I think I got this the right way around).
I was getting some really great effects on the screen when I gently rubbed the EPROM...
I think seeing these things really sparked my interest in TTL voltage level thresholds. So much unpredictable behaviour occurs between say .5V and 2.7V.

Again I appreciate your conversations. :D
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Re: Vic 20 Charcter ROM EPROM PCB adapter

Post by nama »

Hi Banman, Thanks for posting that.
I found your video very helpful...however what I did was a little different than your description.
Please see attached. One trace cut and a jumper added. In the photo, the cut trace is very small, just by the pad, and the wire is also hard to see as it's green and tucked away near the SIP pins...I like doing things real neat!
Anyway, have I made a mistake, if so, why does it work?

Thanks again
Phil
Mod.png
Mod 2.jpg
Mod 2.jpg (20.3 KiB) Viewed 3354 times
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Re: Vic 20 Charcter ROM EPROM PCB adapter

Post by nama »

Seems I'm connecting:
- 27512's CS pin to CE2
- 27512's OE pin to CE1

...which I'm assuming is the opposite of your setup, which seems to be:
- 27512's CS pin to CE1
- 27512's OE pin to CE2

Anyway, it seems to work great!

Thanks again
Phil
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Re: Vic 20 Charcter ROM EPROM PCB adapter

Post by nama »

...or possibly you have OE feeding both CE1 and CE2?
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Re: Vic 20 Charcter ROM EPROM PCB adapter

Post by banman »

Hi nama,


WOW!

That's a fantastic rework of the PCB. I can hardly see the bodge wire. I can't see the cut trace. Where exactly did you nick the track? Can you send a closeup photo where you cut?


It looks like you have the opposite wiring to what I have. It does appear to work. Please, let me know how it goes in the long term. I ran mine for 3 and 1/5 hours before I got bored.
I think I might try your wiring schematic out. It's always nice to have other ways of doing things.


I have been reviewing eslapions notes on this subject. Your solution may indeed be a valid option. Here is what I think, and I could be totally wrong here.

Let us take this PCB in its original form, untouched by us. Both pin 20 /CE (Chip Enable) and pin 22 /OE (Output Enable) of the 27c512 EPROM are tied together to pin 20 /CE (Chip Enable) on the VIC 20's mainboard socket side. Careful viewing of the PCB will reveal this.

Looking further and we see nothing is actually physically connected to pin 21 /CE of the VIC 20's mainboard.

That's fine in situ in a C64. The C64's PLA handles the required signalling. That is what this PCB is designed for a C64 circuit.



On the Vic 20, Pin 20 /CS1 (Chip Select1) AND Pin 21 /CS2 (Chip Select2) are active low. For the VIC 20's CHARACTER ROM chip to start signalling these lines must be both low.
What you have done is completed this wiring. Successfully too it appears.
It's because the PCB's traces go all over the place it's very hard for the mind to get a grip on where the traces are going. At lest I find this to be case. It's always getting me trouble.



** A side note when one sees a / (slash) or ! (exclamation) or a ~ (bar above a symbol) I believe it means the element is active low.


https://en.wikipedia.org/wiki/Logic_level
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