Hello everyone,

Recentely, I was interested in turning an old Minitel into an 8 bit computer powered by a Z80. Of course, I want to use as many parts from the original Minitel as possible, but there was a catch. My minitel is a Minitel 9 manufactured by RTIC, which is NOT really documented, so I have to build my knowledge on this machine through reverse-engineering.

I’ve started to work on the keyboard because it seemed to be the simplest part to understand (and it is). Luckily for me, I wasn’t the first trying to understand the inner workings of Minitels, and Matthieu Benoit’s work and Éric Sérandour’s work gave me a solid idea on the internal of the keyboard. I won’t go into any details, since I’m not an expert, but it’s a membrane keyboard, so it works like a matrix keypad, just with more keys.

Knowing that, I just had to understand the pinout of the keybord’s membrane connector.

Looking at the PCB, I realized that the 9 pins on the left were connected to a a BCD-to-Decimal decoder(SN74LS145N), and 9 pins on the right were connected to an 8-bit shift register (HEF4014BP), which contributed to my understanding of the pinout. Therefore, all I had to do was to connect the different pins to my Raspberry Pi Pico, running MicroPython, turn some pins on, and hope the values were correct. And after a few hours, I was finally able to understand the connector’s pinout.

As you can see, the cut in the middle doesn’t reveal much about the pinout of the connector. And now that I know how each pin interacts, I was able to build the following table which describe which pin to read and write to in order to check if a key is pressed :

Input pin \ Output pins	0	1	2	3	4	5	6	7	8
0	Left Yellow key	W	B	N	Right Yellow key	V	C	X
1		D	G	J	L	7	8	9	Q
2		S	F	H	K	M	P	O	Control
3		Z	E	R	T	Y	U	I	A
4		,	.	’	;	-	:	?	Esc
5	Connexion	Guide	Correction	Suite	Envoi	4	5	6
6	Fnct	Sommaire		Retour	Répéteur	1	2	3
7	Up	Down	Left	Right	Return	*	0	#	Space

For comparaison, here is a picture of the keyboard :