Texas Instruments TI99/4A Clone(ish) Project

Note that this is not a fully tested project at this point. . . .

This is based Rob Krenicki's design that is based on the HackMac KiCAD schematics. I have made several changes to the original design including:

• Placing 32K of 16bit wide fast ram on the board to fully populate the address space
• Placing an EPROM on the main board that can be used as an on-board cartridge ROM
• adding a header to allow a microcontroller to replace the GROM chips (hopefully this can be moved on-board soon)
• replacing the video DRAM with SRAM
• Updating the Keyboard header to support a soft caps lock switch on the keyboard PCB
• designing a Cherry MX keyboard
• adding headers for the GROM and expansion port to support riser cards for these ports

If there are any questions, I can be reached at vic2020Dan at gmail dot com.

ToDo

• Cleanup/Documentation Possible Future Thoughts:

• Redo the Board in an ATX form factor with PEB slots on board

• Add Support for the Pico9918 on-board

• Add Support for a Tang Nano FPGA TMS9918 replacement

• Add Hex-bus port

• Add a DIN 5-pin Composite video/Audio out.

• Add built in speech synthesis

• Clone Disk IO card

• Relocate all the ports to the back of the motherboard

• It has been suggested that we also have a console style version of the next revision as not everyone has a desire for a larger PE style machine

• Also perhaps add some keys to the keyboard

DIP Switch Settings

The DIP switch on the main board allows one to configure which bank areas are populated with RAM.

RAM0 enables RAM to addresses >2000-3FFF (Lo Memory Expansion) RAM1 enables RAM to addresses >A000-BFFF (Hi Memory Expansion) RAM2 enables RAM to addresses >C000-DFFF (Hi Memory Expansion) RAM3 enables RAM to addresses >E000-FFFF (Hi Memory Expansion) RAM4 enables RAM to addresses >8000-83FF (Normal TI Scratch Pad RAM)

Note that I have never actually tested off-board 8 bit RAM, so this is new territory and might need some debugging 😊
also on-board RAM is 16 bit – so all of those external RAM sources will be slower.

Bugs

• on rev 0.95 R36-40 are listed as 100K ohm, they are actually 100 ohm

• on rev 0.95 the joystick and cassette connectors are mirrored and need to be mounted on the bottom side of the board to function

• Both of these are corrected in 0.96

• The reset button J1 should be attached to pin 5 of U1 not Pin 4

Bill of Materials

Console

Keyboard

Cartridge Riser

IO Riser

UNICORN BOARD

License

The original project was licensed under the Creative Commons - Attribution - ShareAlike 3.0 License

Attribution

This board was derrived from works by, uses design elements from, or contains sofware writen by the following:

• Rob Krenicki (https://github.com/rkrenicki/TI99-Motherboard)
• HackMac (https://bitbucket.org/hackmac/ti-schematics/wiki/Home)