Minitel

Hardware

The integrated electronics in a Minitel terminal has four functionally distinct parts:

●The subset visualization responsible for producing the TV images from the signals received by the Minitel or hit on its keyboard.

●The subset keymap recognizes the keyboard and sends their code to the subset visualization and to the telephone line keys.

●The subset serial interface provides signals to the DIN socket (computer peripherals) to devices.

●The MODEM (modulator-demodulator) transforms internal digital signals of Minitel. These subsets are grouped on three printed circuit boards.

The keyboard of the Minitel is managed by an 8048 microprocessor at Intel. It is a circuit that performs several functions in one housing:

  • The heart of the microcontroller: ●an 8-bit CPU.
  • ●64 bytes of RAM.
  • ●1 KB of ROM.
  • ●An 8-bit timer counter.
  • ●27 lines of inputs/outputs.

Software integrated in the ROM 8048 provides scanning of 65 keys. The keystroke produces a code or a code sequence corresponding to the character or function indicated. If two or more keys are pressed simultaneously, no action occurs until only one key is pressed.

The 8048, through its inputs/outputs, determines the key pressed and is also responsible for serialization of the code since there is no standard UART function integrated. The series is produced by the integrated software.

The keyboard unit connects via a six-pin connector carrying power, ground, and the RESET signal issued by the serial data 8048.