Like many projects, this board has considerably evolved. The original intent was to take the Atari 800XL system board (C061851 Revision D) and add some upgrades. The original upgrades were as follows:

1. Chroma video output on monitor port pin 5.
2. +5VDC on PBI pins 47 and 48.
3. /HALT on PBI pin 33.
4. The optional blur capacitor (C56, 180pF) was re-added. (Note: This blur capacitor was later reduced to 33pF as per the video circuitry used in the Atari 800.)
5. An enhanced MMU with the following changes: a. Open collector PBI Refresh to prevent ANTIC’s Refresh signal from being shorted out due to the actions of some PBI upgrades. b. PBI RAM at $D600-$D7FF. (Inspired by Atari’s original XL Sweet 16 design document.) c. ROM access on read only.
6. Improved DB-9 joystick ports.
7. Full copper ground planes on both sides.
8. Isolated audio and isolated video ground planes.

After some discussion, on the Atari Age forums, many additional changes were made. These include:

1. Reduced the component count by making the following changes: a. Replaced all DRAM circuits with SRAM circuits. b. Replaced the OS and BASIC ROMs with a single chip. c. Combine the functions of the 74LS138 and memory control into a single PLD. d. Eliminate the ferrite beads and unnecessary 1nF capacitors. e. Simplify the power circuitry. f. Replace many resistors with resistor arrays and networks. g. Replace the audio circuit with the audio circuit designed by mytek on the AtariAge forums. h. Remove the RF video in favor of composite and S-Video. i. Use the simplified clock circuit used by mytek on the AtariAge forums.
2. Multiple memory options that include 64k only, 128k XE, and 320k RAMBO.
3. Addition of the jumpers as per Atari’s original XL Sweet 16 design document.
   (These are optional.)
4. Addition of three breadboard regions for user customization.
5. Channel select switch has been re-purposed as a ROM bank select switch to permit two possible OS/BASIC configurations.
6. Addition of separate pads for console keys.
7. Most components have values and/or part numbers on the board to aid in assembly and troubleshooting.
8. Addition of two locations for a fuse and a zener diode to protect against power supplies going over-voltage. (This is experimental.)

This revision, Revision 1.2, includes the following changes:

1. The re-settable fuse was removed because there isn’t a reasonable fuse rating available for this purpose.
2. A small connection breadboard was added to the left of the SIO connector to facilitate custom SIO upgrades, such as S-Drive Simple.
3. Minor video trace improvements were made to improve the layout of the video components.
4. Composite video components are now designated with a “C” on the system board.
5. +5v and GND power supply pads are now indicated on the system board.
6. The 100nF capacitor, for the cartridge, will now only provide service to the cartridge.
7. Footprints have been added for the ~20pF capacitors required for use with a non-OEM oscillator crystal.
8. A minor fix was added for on-board shielding of the clock signal near the PBI.
9. MapRAM support was added so as to gain access to the full 2k of memory located at $D000-$D7FF.

The board is available as a bare board, a mostly bare board with the programmable logic chips in their sockets, or as a completed board. Please note that the buyer will have to supply a non-returnable and functional "donor board" as some of the components are no longer made and will be used to create your new finished and tested re-make board.

If you purchase a completed board, due to Tindie's vendor payment policy, your order will be marked as being "shipped" when your donor board is received. Assuming that I need to order some additional parts, completion and testing of a new board takes approximately two weeks after the board has been received.

The manual, ROM image, PLD code, etc., is here: Atari Age Forum Post