====== Introduction ======
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.
====== Registers ======

^Address^Name  ^Description^
|E7DE|SCR/SSDR |Control (write)/Status (read)|
|E7DF|STDR/SRDR|Data transmit/receive        |

====== Keyboad communication protocol (TO9) ======

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

===== Reading =====
==== When no peripherals are connected ====
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.

==== With peripherals ====
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).