So you might perchance well well per chance own bought your fingers on a slack 1980s Minitel 1B (or something a minute bit of later) and then you sign that the whole Minitel network shut down in 2012. What manufacture you manufacture? Neatly, clearly, you snatch that the Minitel 1B (and varied later items) own two modes of operation: the utilization of the modem to connect to the Minitel provider or as a serial terminal by capability of the “peri-informatique” port. With the latter you might perchance well well exhaust the total thing as a serial terminal and connect it to something (equivalent to a Raspberry Pi) and browse the gain.

Fee: there are varied piquant modes (equivalent to gaze-to-gaze Minitels) nonetheless I might well well no longer hurry into that here.

The peri-informatique port on the benefit of the Minitel is a five pin DIN socket which helps starting up collector TTL common sense ranges with as a lot as 4,800 baud serial (later items supported sooner transmission).

The pinout (prove the ordering!) is pin 1 (RX), pin 2 (0V reference), pin 3 (TX), pin 4 (willing) and pin 5 (8.5V at 1A). With approved three pins (1, 2, 4) or no longer it is relatively straightforward to save collectively a cable that turns the Minitel staunch into a serial terminal.

And since the Minitel has an ASCII mode it will work relatively mighty out of doors of its customary Videotex structure with the qualified combination of keys. It is wanted to own a Minitel with the FNCT key attributable to it controls moderately a pair of settings.

A straightforward test

A transient procedure to check that your Minitel is chuffed being in peri-informatique mode is to connect TX to RX within the DIN socket and explore if characters gather echoed to the quilt. I did this with a tiny fragment of paper clip connecting pin 1 (RX) to pin 3 (TX).

By default the peri-informatique port is enchanting and so something else typed is sent out of it, and something else obtained is displayed. I carry to dwelling the demonstrate mode to US ASCII with FNCT+T followed by A (whenever you occur to love to own a French model of ASCII exhaust FNCT+T followed by F).

In the event you starting up typing you might perchance well well peaceable explore the whole lot you sort twice: as soon as attributable to you typed it and the Minitel echoed it and as soon as attributable to it bought sent and receive thru the fragment of paperclip. To turn off local echo and approved explore what went from TX to RX, or no longer it is FNCT+T then E.

Assuming that worked then you are in real form to expose this straightforward serial port into something that can even be plugged into USB. For that you can have a USB-to-serial adapter, three resistors, one transistor and a trot for the DIN socket.

USB-to-Minitel Cable

Sooner than attending to the USB half of the connection or no longer it is worth taking an tour thru the details of the Minitel’s serial port. The legitimate documentation describes it as “smartly qualified with TTL starting up collector” common sense and presents the next constraints:

Before the whole lot, we be taught that common sense stage 0 is below 0.4V and common sense stage 1 is above 2.5V (and below 15V). In the event you stare upon the RX intention on the pinnacle left you might perchance well well explore that as soon as receiving a common sense 1 (voltage bigger than 2.5V) recent might well well float from the Minitel and might well well be below 150uA. It is probably going you’ll well well be in a neighborhood to be asking yourself why the receive signal might well well be a recent supply. That is long-established TTL common sense style.

In TTL common sense inputs are on the total the emitter of a transistor. Right here is a simplified TTL NAND gate. Be taught about how the inputs are emitters in a unfamiliar double-emitter transistor.

Imagine the case where A and B are tied to ground (common sense 0): recent will float thru VT1 all around the noxious-emitter junction and attributable to this fact out A and B. So, an input is a recent supply when pushed low.

OK, apt, nonetheless what does the Minitel in actuality put into effect since there are no TTL common sense gates on the PCB wherever shut to the serial port. I ran all over a Minitel schematic on line that has the serial port circruit. Right here is allowed the RX and TX half.

The TX half is one of the best. If the processor is transmitting a 1 then the 4069 hex inverter drives the noxious of the transistor low which procedure the transistor is off and the TX port floats. Right here is why in Pila’s USB-to-Minitel connector they pull the TX line to 5V the utilization of a resistor:

If the processor needs to send a 0 then the noxious of the transistor T03 is pulled to 5V and so the transistor conducts (as the noxious-emitter of the NPN transistor is ahead biased). This implies the TX line is pulled low (by capability of the transistor) and as a lot as date flows into the Minitel. Nothing particular there.

RX is a whole varied kettle of fish. No longer a transistor in look nonetheless three resistors and two diodes. The two hex inverters assist to make certain that that no topic signal they’re presented with they create it benefit to sure common sense ranges. We will ignore them.

Let’s first think in regards to the scenario where we are transmitting a 0 into the Minitel and pull the RX line low. This appears to be like love:

V1 is the intention vitality (5V), V2 is the voltage supply for no topic is linked to the Minitel’s serial port. XMM4 measures the voltage presented to the two inverters.

So, with S1 dwelling to pull the RX input to ground the inverters gather 1.165V which they’ll clarify is low. Extensive. But why 1.165V?

Let’s hurry element by element. D4 is reverse biased and attributable to this fact would no longer habits and does nothing. D3 is ahead biased and so recent flows thru D3/R3 to the RX pin. So, when the Minitel receives records it will most likely well well supply recent.

R1 and R2 make a voltage divider which creates exactly 1.165V and furthermore recent can float thru them too.

Receiving 1

When receiving a 1 the RX line needs to be dwelling between 2.5V and 15V based mostly fully on the documentation. Let’s analyze some varied voltages presented on RX. 

We will starting up with 2.5V. Right here is amazingly equal to the 0 case above. D4 does nothing, recent flows thru D3/R3 out the RX pin and R1/R2 make a voltage divider this time giving a voltage of 3.082V. The inverters explore that as excessive and all is correctly.

How about 15V? Things are a minute bit of varied. D3/R3 does nothing as the diode is reverse biased. D4 is now ahead biased and conducts, clamping the voltage presented to 5V + the diode ahead voltage (which is about 0.5V). In the event you test the datasheet of the 4069 inverter it says that its input voltage can even be Vdd + 0.5V in narrate that diode keeps voltages in vary.

There’s no equal diode clamping unfavorable voltages, which is a minute bit weird as the 4069 will easiest settle for down to -0.5V. Perchance or no longer it is pointless attributable to unfavorable voltages are illegal in France or something.

In the rupture, what occurs whenever you occur to approved let the RX input hurry alongside with the float and do not connect it the least bit? Right here is foremost for Pila’s accumulate. It in actuality works piquant attributable to R4 pulls up the output to 5V.

Extensive, so Pila’s accumulate makes total sense and works within the particular world.

You would specialize within the yarn would discontinue there nonetheless I might well well no longer be taught the designate of R01 within the scan of the schematic so figured I might well well well hurry stare upon the particular motherboard and gather that resistor. At this point I found that my Minitel did no longer correspond to the schematic the least bit. To starting up out with I believed I spied the resistors and diodes here:

But that is a 220k resistor no longer 270k and careful following of the traces (below ultraviolet mild for clarity) revealed that the RX circuit is:

A conventional PNP swap. Pull it down to expose the transistor on, pull it up or hurry away it floating otherwise. The TX line is amazingly equal to the schematic. 

So, by some means, I even own happy myself that Pila’s accumulate is gleaming.

Narrator: he will own approved constructed the object and no longer obsessed over it.

The exhaust of the USB-to-Minitel cable

So, I constructed the object and here it is a long way:

It is probably going you’ll well well per chance be taught all about it on Pila’s blog nonetheless there might be a male DIN connector which I’ve stuffed the total substances interior and a USB to serial converter. Pila suggests opening the connector to check the colors of the connectors in narrate that RX and TX are seemingly to be no longer getting confused. I’m very grateful to the one who laid out the miniature PCB interior the USB connector for labelling the whole lot:

I managed to stuff the transistor and three resistors interior the DIN trot itself as Pila suggests. Tight fit and a minute bit of heatshrink for insulation.

As soon as carried out I attempted talking to the Minitel the utilization of Python miniterm.py (most incessantly my utility of selection for stuff love this) and observed the next on conceal:

In the event you staunch away acknowledge that the motive a, d, g, h, h, m, n, … are showing as blocks is attributable to I’ve bought the misguided assortment of bits dwelling for the serial transmission then you are a wide nerd. The Minitel is awaiting 7 bits plus even parity. Because the pinnacle bit for all these characters is 0 for one of the most most letters the parity shall be “qualified” nonetheless for these characters which own an weird assortment of 1s in their binary model the parity is misguided and the Minitel shows a white rectangle.

To in actuality exhaust the Minitel as a terminal I plugged it staunch into a Raspberry Pi and it looked as /dev/ttyUSB0: