The 6850 is a serial port controller. It is used on Thomson TO9 to connect the keyboard to other parts of the system, allowing a detachable keyboard. The joystick/mouse connector is on the detachable keyboard, so it is scanned through the same serial protocol.

Later machines use bit-banging on the system 6846 to implement the keyboard scan, to avoid using this chip and reduce costs.

The communication happens at a rate of 9600 bauds. It is asynchronous, there is no clock signal on the keyboard connector.

This always uses an odd parity

• When a single key is pressed, its key code is sent every 70 milliseconds (keyboard repeat)
• When more than one key are pressed, they key codes are sent only once (on press), until only one is pressed.
• When no key is pressed, nothing is sent.

When a joystick or mouse is connected, the protocol is made of 4 bytes every 10 milliseconds.

• The first byte has odd parity and holds the key code as above. If no key is pressed, it has value 0. If key repeat is active, it is 0 6 times out of 7, and the key code of the repeated key for the 7th time, leading to the same 70ms key repeat delay.

The 3 other bytes have even parity and hold data for the peripheral:

• The second code is the relative X coordinate (from 0 to 255).
• The third code is the relative Y coordinate.
• The last byte is the buttons (bits 0 and 2) and X and Y overflows (bits 6 and 7, when the mouse or joystick moves too far, this is set).