RE: (Fwd) 65816->6502

Halo allemaal,


Mia schreef:
> On PET (except 8096/8296), VIC-20 and 1541, it's as we all know not that nice. Therefore
> IMHO a 65816 add-on for those computers should preferable also contain ram and a
> mechanism to bank switch between that ram and the computers built in stuff.

IMHO it isn't that hard. I expanded a VIC-20 with an ISA interface so I could use an old RAM expansion card as extra memory. This card is equipped with 128 KB SRAMs so no problems with refresh. The interface was made in a quick and dirty way by piggybacking the ICs on top of the existing ones of the VIC-20. It worked more or less because I found out I made a mistake due to the lack of knowledge at that time: RDY did not work as I expected. Even if RDY is negated, the 65816 keeps on outputting address lines A16..23 on the data bus. I did find a solution for it at the end but didn't implement it here because of the work involved.

What is needed:
- a 573 for creating the address lines A16..23
- two 541s for generating the address bus towards the original system + pull-up resistors
- one 125 gate for the R/W line + pull-up resistor
- a 245 between the 65816 and the original system
- a 688 for detecting if bank 0  = original system is accessed
- four NAND gates = one 00 for handling RDY
- some glue logic

The idea is simple: as long as the 65816 accesses the original system, the 541s forward the address lines A0..15 towards it. Accessing any other bank will tri-state the address lines and R/W line. The resistors will make the original system think that address $FFFF needs to be read. Save enough IMHO. The 245 makes sure that the outputted data does not reach the 65816.
I'll see what is on schematic and I will upload it to my site.


Met vriendelijke groet / With kind regards, Ruud Baltissen

www.Baltissen.org




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