Hello all,
I am attempting to upgrade my Apple //e to Enhanced. besides the 65c02 CPU and 27c32 DIP24 EPROM(for video ROM), I also purchased 2 27C256 DIP28 EPROM. I programmed 27C256 EPROM with a combined ROM image I downloaded from archive site. but after plugged them in, I only get 4 (or 5?) horizontal bars. So I am wondering if this single EPROM approach works at all - do I need 2 EPROMs for CD & EF?
Here are some additional information:
- I ran some BASIC program that verified the 65C02 with a new opcode, so it's not a fake 6502. this was done with original ROMs
- I put the original CD and EF ROMs back in, with the new Video ROM (27c32 EPROM), it works as well
Thanks!
You need two 27C64 EPROMs or 28C64 EEPROMs. I think you can probably use 27C128 or even 27C256, but you have to watch the unused pins bacause if they float or are at the wrong level you may get weird results. You can't use just one larger EPROM because the motherboard isn't wired to address them that way. If you use a larger capacity chip you probably should program the CD or EF images multiple times to fill it out so that no matter what the extra address pins are it gives the same result.
Thanks @softwarejanitor. the image I used is from
https://mirrors.apple2.org.za/Apple%20II%20Documentation%20Project/Computers/Apple%20II/Apple%20IIe/ROM%20Images/
note the directory is for //e. if a single ROM doesn't work, I wonder what's the image for then?
Anyway, I also tried to write 2 27c256 EPROMS, one for CD and one for EF, same result, just bunch of horizontal bars like
IMG_5255.jpeg
Also, when you say "If you use a larger capacity chip you probably should program the CD or EF images multiple times to fill it out so that no matter what the extra address pins are it gives the same result"
do you mean I should repeat the content of 2764 4 times in sequence? what would assume the additional pins causing reads always aligned with the 8K boundries right? I will try that nad let you know
I went and read the Apple II schematics and datasheet of 27C64, and confirms a single ROM chip won't work, as there is a 74LS138 "ROM selection" chip in front of the ROM chips. so it's not like all roms are directly connected to address bus. 27C64 shows one of the pins that used for address in 27C256 is not used, so it's floating. I then concat the images 4x and wrote the 2 EPROM with them, and it worked. Thank you for the repeating suggestion!
The "Platiunum" //e used a different motherboard layout which combined the ROMs, but that obviously as you've seen doesn't work on the original //e motherboard layout with two chips.
I'm glad that things worked for you. That's the great thing about Applefritter. It's pretty much the best place out there right now to quickly get answers to questions like this.