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The integrated electronics in a Minitel terminal has four functionally distinct parts:
1. The subset visualization responsible for producing the TV images from the signals received by the Minitel or hit on ones keyboard.
2. The subset keymap loaded recognize the keyboard and hit send their code to the subset visualization and to the telephone line keys.
3. The subset serial interface responsible for providing signals to the DIN socket (computer peripherals) to devices.
4. The MODEM (modulator-demodulator) that transforms internal digital signals to the Minitel.
These various 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.
• 64 bytes of RAM.
• 1 KB of ROM.
• An 8-bit timer counter.
• 27 lines of inputs / outputs.
• The heart of the microcontroller: the central processing unit or CPU, of 8 bits.
• Software integrated in the ROM 8048 providing the scanning of 65 keys.
The keystroke causes the production of a code or a code sequence corresponding to the character or indicated on this function. If two keys (or more) simultaneously depresses no action occurs. One of these keys will be taken into account as soon as the others released.
The 8048, two inlets / outlets will provide the determination of the key, is also responsible for the serialization code since there is no standard UART function integrated in the microcontroller and the series is produced by the integrated software.
The connection of the keyboard unit is via a six-pin connector carrying the power, mass, and the RESET signal issued by the serial data 8048.
