5200 EPROM -> Memory mapping

[calamari]

Hi,

 

A 32k .bin uses $4000-$BFFF. I am fairly sure that a 16k .bin uses $8000-$BFFF. My EPROM board isn't working (not sure why yet), but I suspect that the 7408 chip might be a problem, so I want to try without it.

 

If I take it out, I will be limited to 16k, which will be fine for testing the rest of the board. Since I only have 27256 EPROMs, that leads to my question: Do I burn the 16k image in the 32k space from $0000-$3FFF or $4000-$7FFF?

 

Thanks,

calamari

[calamari]

I believe I may have been able to answer my own question, because of the EPROM. Even though I won't be using the A14 line on the EPROM, I can't just leave it floating. My guess is that if I tie it to GND, then I'll burn $0000-3FFF, If I tie it high, I'll need to burn $4000-7FFF. If the address lines use negative logic, those two might be reversed, but I can just burn two EPROMs and figure out which one works.

 

calamari

[calamari]

Well.. hrm.. burned two EPROMs, a $0000 and a $4000, neither worked.. tried it with A14 tied to either GND or +5v.

 

So now I'm not sure at all what is wrong with the board. I tested the connections from the cart edge to the EPROM pins, all the connections seem OK.

 

calamari

[AlanD]

Man, you sure like to talk to yourself

 

Anyway, what I did was just go ahead and wire it up as the docs say then burn the 16K game into the eprom twice. That way no matter where it looks it finds the data it needs.

 

for ex. (assuming millipede.bin is 16K)

 

copy /b millipede.bin + millipede.bin 32kmillipede.bin

 

AlanD

 

PS Feel free to give me a call and we'll work out the troubles

[calamari]

Finally found the problem with my EPROM board. I was using an AMD 27C256 EPROM. Today, because I was desperate for other things to try, I bought an Intel 27256 EPROM. It worked!

 

So, I dunno if its the brand difference or the CMOS thats doing it. It would be great if someone with a wider variety of EPROMs can do some tests for me!

 

Moderator Edit: The 5200 cart rewiring schematic is located here:http://lilly.csoft.net/~jeffryj/5200bas/52...es/5200res.html

[mos6507]

Can't the 5200 only address 16K at a time on the cart port?

 

That's the way the Atari 8-bits work.

[calamari]

Yep.. only 16k at a time, but 32k is possible.

 

There is a $40-7F enable line and a $80-$BF enable on the 5200 connector, as well as A0 through A13. To get 32k you tie $40-$7F to A14.. but you can't just tie either plain $40-$7F or $80-$BF to the /CE line (because then the chip would be disabled for half the addresses). So you put both enable lines as inputs on an 7408 (2-Input AND Gate). You'd really want an OR gate, but since these lines use negative logic, the AND turns into an OR. Then if either enable is activated, the chip is enabled.

 

Hope that made sense. I am a beginner with electronics, so if there are errors, please let me know

 

calamari

[calamari]

UPDATE!

 

After modding the cart back to 32k (realized I had the 7408 wired wrong.. I was comparing it to another modded cart when I should have been using my spec. Apparently the 7408 used in the another cart used a different pin layout.)

 

Anyways, I got the 32k working, so I thought I'd try with the 27c256 eproms just for fun.. they actually work now! Couldn't tell you why, but I'm glad, because that means I can test and erase more efficiently.

 

calamari