I've been wanting to get involved in the TI-99/4A discussions for a while now, and thought it would be good to do now that I got my messy TI-99/4A hack working.
So I've only got a plain TI-99/4A console, nothing more apart from a few cartridges. I took one cartridge apart and started to use it as a basis for hardware hacking. I put in a socket from a ROM chip and implemented a GROM emulator with a NXP LPC1114 ARM micro controller. I also wired it up to a printed circuit board I made for another TMS9995 computer, to enable page switching of the ROM (initially just 2 pages to get extended basic running, there is 64K of ROM now so I could have 8 pages). The final touch was the addition of a Pipistrello FPGA board, which creates a 32K RAM expansion. This is just the basis to start doing something interesting - these boards combined have pretty significant hardware resources in the TI-99/4A scale.
I spent a while debugging the very simple FPGA bus connection, finally I found out there was bug in the schematics of the buffer board I'm using on top of the FPGA for 5V level conversion.
To summarise, in addition to creating this mess of wires and boards, the actual creations are:
- GROM emulator with the ARM processor, written in "bare metal" C, i.e. no libraries
- A few lines of VHDL for the CPLD to implement bank switching logic
- A few more lines of VHDL in the FPGA to use a small portion of its resources to create the 32K RAM
I hope the writing I put to hackaday explains things in a meaningful way. I hope you guys find this interesting!