ATARI 1200XL OS MODIFICATION #1 (Modify to Atari 800 OS):
 

This mod was created by Brent Borghese of the ACE of Columbus, Ohio.
Updated by Curt Vendel of Atari Historical Museum.
 
 

                          Follow these simple instructions to replace the nasty 1200XL OS with
                          the more favorable Atari 400/800 OS.

                          1. Pull the Rev.B ROMs out of the motherboard of a 400, or from the
                          10K ROM board of an 800. Make sure that the chips function properly!

                          2. If you can't find a set of ROMs, order them from:

                          B&C Computervisions: Atari Sales & Service
                            916-295-9270
 

                           BEST Electronics
                            408-243-6950
 

                          3. At this point you should have three chips, marked C012399B,
                          C012499B and C014599B, respectively.

                          4. Place the C014599B piggyback on the C012499B chip with the
                          notched ends facing the same direction. Solder pin 1 to pin 1, pin 2 to
                          pin 2, etc. for all 24 pairs of pins.

                          5. Open the 1200XL using a small Phillips-head driver on the screws,
                          and a needlenose pliers to pull the pop rivets that hold the RF shield
                          together. This step is completely non-destructive: there is nothing to
                          cut, unsolder or mutilate.

                          6. Locate the two 24-pin ROM chips on the 1200XL board near the
                          cartridge slot. Notice that they are marked U12 and U13, respectively,
                          in white letters on the board. Notice also which way the notched ends
                          of the chips are facing.

                          7. Remove these ROM chips and give them to someone you don't like.

                          8. Pop the chip marked C012399B into the slot marked U12, with the
                          notch facing in the proper direction. Place the piggyback chips in slot
                          U13, with the notches properly oriented.

                          9. Test the board before putting the 1200XL together again.

                          You now have an 800 with a 1200 keyboard, the best of both worlds.

OS Modification #2 (800XL OS & Basic upgrade):

 

Condensed OS/BASIC upgrade instructions for 1200XL (originally posted by Bob Woolley)

Modified by Adam Brannon using procedures used in AtariMax�s 32in1 OS Upgrade

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Parts needed

1-XL/XE OS (CO61598B)

1-XL/XE MMU (CO61618)

1-2764 EPROM (Programmed with BASIC RevC)

2-28 pin sockets

2- 5� length wire (wire wrap type)

 

1. Remove the current 1200XL OS roms at U12 & U13

2. Desolder and remove 24 pin sockets at U12 & U13

3. Make sure all 28 pin sites at U12 & U13 are cleared of solder. Install and solder 28 pin sockets at U12 & U13

4. Remove the jumper at W6. Set aside for later.

5. Remove current MMU at U14 (CO60609). Replace with XL/XE MMU (CO61618)

6. Remove all jumpers except W11, W12 and W13

7. Solder wire from pin 18 of U14 (MMU) to pin 11 of U23 (PIA)

8. Solder wire from pin 23 of U21 (CPU) to the top pad of W6

9. Install XL/XE OS at U13

10. Install programmed 2764 at U12. Make sure pin 26 is lifted or removed.

11. Bonus Mod!! Locate R63 near SIO connector. Remove R63 and replace with jumper removed from W6. You now have +5 to the SIO for using SIO powered hardware not normally compatible with the 1200XL.

 

Put everything back together and test by booting without a drive attached. If the upgrade went right, you should see the READY prompt. Type PRINT PEEK(43234) and press return. A value of �234� should be returned. This tells you that the revision of BASIC is C.

 

You now have a fully 600XL/800XL compatible 1200XL with built in BASIC. Enjoy!!