Nice simple 1200XL OS upgrade using Channel Select switch

[ACML]

I've attached the ROM files for both U12 and U13 which are the two OS chips on the 1200XL motherboard. The stock 1200XL uses two 24 pin ROMs that are 8K each. In Atari's infinite wisdom, they included the traces for 28 pin EPROMs. The upgrade is not too hard and cosmetically leaves the 1200XL looking stock.

 

Steps:

1) Remove the 24 pin ROMs (U12 and U13).

 

2) If the socket is 24 pin you need to add the two pins on each side or replace the whole socket with a 28 pin socket. The former is the easiest since you don't have to remove the existing socket. You just need a SIP socket (see photo) and just cut off two from the link to add to each side.

3) Remove R6, R7, R8 and R9.

 

4) Install a wire or 0 OHM resistor in R11, R12 and R13.

 

5) Insert the new EPROMS in the 28 pin sockets U12 and U13, but remember to bend pin 26 up slightly on both EPROMs so it does not go in the socket. Pin 26 is the high address bit that selects which 8K bank (or OS) to read.

 

6) Jumper a wire from pin 26 on U13 to pin 26 of U12.

 

7) Solder a wire from pin 26 of U12 and run the wire to the center post of the channel select switch. You want to run the wire though a hole in the top of the PCB and attach it from the bottom side of the center post of the channel selector switch. The selector switch is already conveniently (GND - Middle post - 5VDC).

 

8 ) Reassemble.

 

The channel selector switch has been repurposed to select the operating system. Remember not to flip the switch with the power on.

 

The OSs I've attached are the 600/800XL OS and the Omniview 80 OS. The XL OS gives you the best compatibility and retains all the 1200XL extended keyboard and LED functions. The Omniview 80 is based on the 400/800 OSB, but allows 64K RAM usage. It also has the fast math routines and 80 column output through S-video.

 

Atari 1200XL U12 CRC 000192B29.BIN

Atari 1200XL U13 CRC 0001ABB5C.BIN

Edited August 2, 2017 by ACML

[+DrVenkman]

Interesting mod! Just FYI, both my 1200XL’s actually have 28 pin factory sockets even though the OS roms are 24 pin.

 

Also, in your instructions you mention installing 0 ohm resisters (jumpers basically). Those are actually designated at “W” on the silkscreens, not “R.”

[ACML]

fixed the "R" to correct "W" reference. Thanks DrVenkman!

I've attached the ROM files for both U12 and U13 which are the two OS chips on the 1200XL motherboard. The stock 1200XL uses two 24 pin ROMs that are 8K each. In Atari's infinite wisdom, they included the traces for 28 pin EPROMs. The upgrade is not too hard and cosmetically leaves the 1200XL looking stock.

Steps:

1) Remove the 24 pin ROMs (U12 and U13).

2) If the socket is 24 pin you need to add the two pins on each side or replace the whole socket with a 28 pin socket. The former is the easiest since you don't have to remove the existing socket. You just need a SIP socket (see photo) and just cut off two from the link to add to each side.

3) Remove W6, W7, W8 and W9.

4) Install a wire or 0 OHM resistor in W11, W12 and W13.

5) Insert the new EPROMS in the 28 pin sockets U12 and U13, but remember to bend pin 26 up slightly on both EPROMs so it does not go in the socket. Pin 26 is the high address bit that selects which 8K bank (or OS) to read.

6) Jumper a wire from pin 26 on U13 to pin 26 of U12.

7) Solder a wire from pin 26 of U12 and run the wire to the center post of the channel select switch. You want to run the wire though a hole in the top of the PCB and attach it from the bottom side of the center post of the channel selector switch. The selector switch is already conveniently (GND - Middle post - 5VDC).

8 ) Reassemble.

The channel selector switch has been repurposed to select the operating system. Remember not to flip the switch with the power on.

The OSs I've attached are the 600/800XL OS and the Omniview 80 OS. The XL OS gives you the best compatibility and retains all the 1200XL extended keyboard and LED functions. The Omniview 80 is based on the 400/800 OSB, but allows 64K RAM usage. It also has the fast math routines and 80 column output through S-video.

Atari 1200XL U12 CRC 000192B29.BIN 16KB 4 downloads

Atari 1200XL U13 CRC 0001ABB5C.BIN 16KB 3 downloads

[Brentarian]

I purchased a 1200XL from ACML a few years back with this mod and it works great!

[Osgeld]

ohh I have been eyeing a 1200, I think you just made my mind up

[+slx]

Now I am curious whether my 1200XLs have sockets.

[+DrVenkman]

Now I am curious whether my 1200XLs have sockets.

Your 1200XLs will have every chip socketed from the factory. This is before Atari, Inc. fell apart and the transition to Tramiel’s Atari Corp. occurred and the penny pinching started. The question is whether your OS sockets will be 24 pin or 28 pin. My two units were made two weeks apart in 1983 (week 15 and week 17) and both have 28-pin OS sockets.