This project provides a simple firmware that can be used to make an adapter to connect a Sharp X68000 keyboard on a NEC PC-8801 Type-A computer that supports Type-A keyboard (FH/MH models or later, excluding the PC-88VA series).
The firmware is made for the Atmel/Microchip ATTiny25 microcontrollers, and should be compatible with any AVR family MCU that have 2kB of flash memory and at least 3 available I/O pins.
Luckily, since both the X68000 and the Type-A PC-8801 keyboards uses the JIS layout, the vast majority of keys are exactly where they should be.
However, the PC-8801 has some proprietary keys, which are mapped on the X68000 keyboard like this:
| PC-8801 key | X68000 key |
|---|---|
| GRAPH | XF1 |
| PC | XF5 |
| F6 to F10 | Unsupported |
The 登録 key (on the top-right corner of the X68000) has been selected to be the PANIC button: upon release, the firmware will reset its FIFO and clear every row on the PC-8801, which can be handy if the latter has released-but-still-repeating-key problems.
PC-8801 Type-A keyboards added "extended" keys, which were only available with a modifier on the original PC-8801 keyboards. These keys are not supported for now. The most visible case are the F6 through F10 keys, which were available as Shift+F1 through Shift+F5. For now, you will have to use the key combinations.
Since the ATTiny25 does not have a hardware UART, all serial communications are performed by software.
The firmware reads the keyboard's keycodes on pin PB0, and store the keycodes in a 8 bytes FIFO buffer.
A loop then reads the keycodes from the FIFO, translates them to the PC-8801 format using the built-in keymap, and sends them to the PC-8801 using pin PB2.
In the event where the FIFO buffer gets full, the keyboard's READY signal connected on pin PB1 will be de-asserted, preventing the keyboard to send new keycodes until the next translated keycode is sent to the computer.
The X68000 keyboards have a simple protocol, consisting of single bytes transmitted through an UART at 2400bps in the 8N1 format. No handshaking is performed, except for the active high READY signal that forbids the keyboard to send new keycodes. No MCU-to-keyboard communication is required for the adapter operation (PC-8801 keyboards does not have any LEDs), and the keyboard does not require any initialization.
Any hardware UART should be able to understand the keyboard frame format.
The PC-8801 historically used a 13 pin DIN connector for its keyboards, using 4 bits to select the keyboard row, and 8 bits to read which keys were pressed on that row (information known as column bits). The computer cycled through each row, read the column bits, and knew which keys were pressed on the whole keyboard through polling.
PC-8801 with Type-A keyboards replaces this parallel communication system by a serial one: when a key is pressed or released, the 4 bits of the row and the 8 column bits are sent.
The frame format is non-standard (UART format with 12 data bits, an even parity bit and one stop bit), and the frames are sent on a non-standard speed (20800bps). This requires a software implementation, since AVR family MCUs doesn't support these parameters.