This project is an attempt to revive a 30+ year old GTAC-2 motherboard back to a functioning Apple II clone. The project required some minimal set of missing hardware including a power supply, keyboard, and ROM. The power supply was was adapted to use a modern ATX supply with the -5v rail generated by a 4.7v Zener from the -12v ATX power rail. The ROM card was rebuilt as a plug-in board using a modern EEPROM programmed with original GTAC-2 firmware (or Apple II firmware). The missing keyboard was replaced by a PS2 keyboard interfaced to the system with an AVR ATtiny84.
The font rom in the GTAC 2 clone has the wiring from the FONT ROM to the shift register altered from the way in a standard Apple II. Because of this you cannot take an font rom image for an Apple II such as the Dan Paymar image with LowerCase and use it on the GTAC 2 without modifications.
On the GTAC 2 the following bits of the ROM are tied to the shiftregister in this order:
FONT ROM LS166 SHIFT REGISTER
- PIN 11 - O3 PIN 3 - B
- PIN 17 - 08 PIN 4 - C
- PIN 16 - 07 PIN 5 - D
- PIN 15 - O6 PIN 10 - E
- PIN 14 - O5 PIN 11 - F
- PIN 13 - O4 PIN 12 - G
- PIN 10 - O2 PIN 14 - H
- PIN 09 - Flash/Inverse Hardware
The bits are shifted out H to A. A is tied to the flash/inverse hardware.
The program reversebits.c will take the input of an Apple standard font rom image and modify it and output the version good for GTAC use on stdout. The program showfont.c will read the GTAC image and display the font chars as acii images.