VickyTwenty Build Question on RAM Upgrades

[robert_sissco]

Now that my Clone SixtyFour is up and running, I decided to do a Vicky Twenty build. But I have a question, I know the factory stock Vic-20's came with 5K of RAM, and that the memory mapping would allow upto 8K before affecting how some software operates due to the memory map manager in the Kernal (thanks to the 8-Bit Guy for me knowing that), so my question is, would it be possible to just use 8k directly on the board without needing any mods to the board or Kernal, and if so, anyone recommend any drop in RAM chip replacements? I will have the expected RAM on hand since I ordered a parts kit, but I was just curious on this to do something to make this build a bit more custom and more my build that just a factory clone of the system. I am not to worried about breaking software either since most of what I use the Vic-20 is games on cartridges and a small handful of cassettes I own for it.

 

[giobbi]

Well, on my Vicky Twenty and on another VIC-20 too I did a small board plugged into the BASIC ROM socket (the same board also comes with the adapter for the BASIC eprom). I added 32 Kb of RAM with 4 x 8 kB chip (but of course you can see just 24 kB from the BASIC).

This is a brutal schematics I've drawn to help me while I was painting and etching the board.
If you want to add a 2nd chip of 8 kB RAM (total 16 kB more) you must to connect the pins #20 and #22 to the UC5 -> pin #13.
3rd RAM chip (24 Kb): pin UC5 -> pin #12.
4th RAM chip (32 Kb): pin UC5 -> pin #10

Yep, there was the covid-19 lockdown and I was really bored, so I needed something useless to do LOL

BEWARE: in this schematic lacks the Vcc and the Vss connections to the RAM chip

 

[Eudimorphodon]

I have to admit I'm a little bit baffled by these PCB remakes, are damaged PCBs really that big of a problem compared to dead MOS-proprietary chips? I guess maybe I could see it if the PCB actually included provisions for substituting more modern parts or room for a built-in memory expansion, but it seems like it really doesn't? Kind of a missed opportunity they didn't just slap a single socket down for a CY7C199 or equivalent and an encoder (say a 74HCT148) to allow it to emulate most of the standard RAM expansions.

The problem with trying to slap more memory (IE, bigger chips) into the RAM sockets of the original VIC-20 design is the VIC uses this weird split data bus system for the RAM that lives where the VIC chip can see it. (That bus also has the character ROM on it, but not the BASIC ROMs.) You would have to modify the circuit that switches the buffer between that "private" bus and the unbuffered bus that goes to the cartridge port to "know" you've got the additional RAM in there, and then make further modifications to the decoder circuitry because only 14 bits worth of address bus make it into that area as well. These would be pretty trivial problems to have worked around if instead of doing a "remake" they'd redesigned the board to take advantage of modern stuff, but... is what it is, I guess. As it is that idea of piggybacking off the ROM sockets is probably your best option if you don't want to use the expansion slot the "canonical" way.

 

[robert_sissco]

I have to admit I'm a little bit baffled by these PCB remakes, are damaged PCBs really that big of a problem compared to dead MOS-proprietary chips? I guess maybe I could see it if the PCB actually included provisions for substituting more modern parts or room for a built-in memory expansion, but it seems like it really doesn't? Kind of a missed opportunity they didn't just slap a single socket down for a CY7C199 or equivalent and an encoder (say a 74HCT148) to allow it to emulate most of the standard RAM expansions.

For me it is most of a fun project. I have a working CR VIC-20 already, and a unsure longboard vic-20 I do not have a power supply for, nor one I trust due to the head issues that these board tend to have, so I am migrating from that longboard to the modern CR board reproduction.

But, yes, the repro board did miss several opportunities. The one I am most annoyed by is that the footprint of the board is the same as the CR board. I know pennies and everything, but making it the same size as a viC-20 or C64 longboard would make mounting it in a case a heck of a lot easier without needed to source or build the under shielding, or modifying the case to use the CR vic-20's footprint.

 

[giobbi]

I have to admit I'm a little bit baffled by these PCB remakes, are damaged PCBs really that big of a problem compared to dead MOS-proprietary chips? I guess maybe I could see it if the PCB actually included provisions for substituting more modern parts or room for a built-in memory expansion, but it seems like it really doesn't? Kind of a missed opportunity they didn't just slap a single socket down for a CY7C199 or equivalent and an encoder (say a 74HCT148) to allow it to emulate most of the standard RAM expansions.

Like Robert, it's just for fun for me too. I have 12 working VIC-20 and the same amount of C=64 in many flavors, so I don't really need a Vicky Twenty or a Sixty Clone. I built both because I had enough spare parts to do that. A board with modern parts as replacement isn't funny to me (also because mostly I play at C=64 games on VICE, so I don't need a modern hardware to do that. It's the smell of vintage that drives me LOL -- Also, I enjoy to do that during the covid-19 lockdown, a lot of free time and nothing to do. I love to do that, I just built a couple of 1581 drives and, of course, I don't really need any of them.

About the Vicky Twenty and the Sixty Clone, I wish they had, at least, put an option for using original ROMs or (E)EPROMs, with a jumper; I know that other replica boards have this option. But I respect the choice they made to keep the board a 100% original replica... and it gives me the chance to build the adapters too LOL