Keyboard controller for the Sanyo MBC550/555

Background

The Sanyo MBC 550/555 was released in the early 80s, based on an 8088 CPU. Unfortunately, the computer doesn't use standard connectors for keyboards yet, but the protocol is based on a serial protocol and easy to emulate.

This software and hardware allow the connection of a modern PS2 or USB keyboard to the Sanyo.

Tested with an Arduiono Nano.

Keyboard protocol

According to the technical manual, the keyboard operates at 1200 baud, with 8 data bits, 2 stop bits and even parity. Key repeat is handled within the keyboard.

PS2 Connector

See this article for pinout: https://www.instructables.com/Connect-PS2-Keyboard-to-Arduino/

5V     :- Arduino 5V out
Ground :- Arduino GND
Clock  :- Arduino Pin 3  (has to be 3 as it's an interrupt pin)
Data   :- Arduino Pin 8  (can be changed if needed)

Sanyo Keyboard Connector

To connect the circuit to the Arduino, honor the following pinout (looking at the female connector):

    ==\/==   
  = o3  o1 =      
  = o5  o4 =
   =  o2  = 
     ====

  o1 = data (to Arduino TX)
  o2 = not used
  o3 = VCC (not used if Arduino powered externally)
  o4 = GND (to Arduino GND)
  o5 = reset (to Arduino pin 6) 

Flashing Firmware

Easily done via the USB connector. Do not flash firmware while connected to the Sanyo (conflict of the serial port).

Debug mode

The firmware contains a debug mode (see code to enable and reflash). This will trigger output of the PS2 scan codes to the serial console (1200 baud, 2 stopbits) and to the Sanyo if connected. Don't enable this if you want to use the keyboard.

Reset

The Sanyo uses a dedicated line that has to be pulled to GND to reset. This firmare triggers pin 6 to low if CTRL-ALT-DEL are pressed simultaneously which is not the Sanyo's original behavior but familiar behavior from other IBM clones.

Raw ASCIII mode

This mode allows submitting any ascii character code (0x00-0xFF) to the computer. Hitting CTRL-ALT-A followed by two ascii characters releases these characters to the computer.

Protocol

The scancodes received correspond mostly to ASCII, with keys producing different scancodes in different shift, control, and graph states. For example, key 1 produces 31h ('1') unshifted, but 21h ('!') shifted.

The funnest part is that the serial protocol uses the parity bit as part of the scan codes. The computer expects parity errors to trigger CTRL behavior on most characters.

Graph Mode (future release)

This firmware will support the GRAPH mode using the right ALT key on the keyboard. Unlike the original MBC, ALT doesn't toggle, keeping right alt (AltGr) pushed with the appropriate character will send the graphical character:

Graph

Graph-Shift

Graph-Ctrl

Limitations

• Sanyo graphics characters not yet implemented (including graph-keymode)