Alternate CPU in the Atari XL / XE series

By Guus Assmann
Preview -- More Coming Soon!

Features of the complete PCB design:
* A 65C02 CPU with buffered address and data bus.
* A second PIA, the first port with a Real Time Clock, the second with a print buffer.
* The PCB may be cut to only use either CPU or PIA, or both.
* Device driver software for the Real Time Clock.
* Driver software for the printer may be available too (still searching...)
* Board replaces the original CPU and PIA, which must be removed. The new PCB occupies their previous sites; the PIA can be reused on the new PCB.
* The CPU may also be replaced by a 65C816 with some minor modifications.

Instructions:(no soldering required if chips are already socketed)
1. Remove the old CPU and PIA.
2. Replace the old PIA in the board.
3. Plug the board in place of the old IC's
4. Done!
Note: the ribbon cable in the pictures is to support a memory upgrade, and not necessary for this mod

Why the second PIA?
To be able to use the RTC and add a printer port, without compromising compatibility. If the extension is not wanted, don't load the drivers and it's not there.

Why replace the CPU?
The original 6502 and also the ATARI 6502C have some minor bugs. The 65C02 fixes these and also adds some very useful instructions. Also, now the CPU address and data bus are buffered. The 65C02 is CMOS and draws less current. Some Atari CPU's, manufactured in Mexico are defective. They are temperature sensitive and make the computer hang. Test: In Basic: 10 FOR X=0 TO 100000: ?X; " Atari": NEXT X and use a hair dryer to heat up the processor (CO14608) If the screen freezes, there's your problem. And finally, it's fun to build hardware...

Will the PCB be made available?
The PCB schematics are now being formatted for publication on the web. If there is enough interest in this mod, it may be possible to produce a small run of boards.

Design history:
The schematics of the old 400 / 800 contain the CPU part already. So this was just simple to do. And back in 1986 it was expensive to buy an Atari CPU. The 65C02 was less expensive and could also be buffered. This is useful if you want to add more hardware to the system, and more hardware is always welcome, right? And when a PCB has to be designed, why not do something more? So a Real Time Clock was thrown in, but this called for an extra PIA. This could be done in the cartridge port, but at the cost of compatibility. The extra PIA had one port left, so I decided it would be good to use this port for a Centronics printer driver. This driver was produced in a coding contest for ATARI Magazine (published in the Netherlands).

This mod was originally produced by Guus Assmann and published in ATARI Magazine, by the SAG (Foundation of Atari Users in the Netherlands)

html-ized by Dan Vernon -- August 2002