BUS-SCS

[Mapo]

Good morning forum,
I find myself needing to test a DC signal of 27V that is modulated with pulses of approximately 35μs and an amplitude of around 3V.
This communication is used by BTicino both for home automation and for the 2-wire door entry system, and is called BUS-SCS
How can I bring these pulses to the pin of the microcontroller (PIC)?
I'm attaching a photo of the signal.
If I reduce it with a resistive divider, I also reduce the pulse amplitude, and it becomes critical for recognition.

https://en.wikipedia.org/wiki/Bus_SCS

Any ideas?

[John Lawton]

Some sort of zener diode clamp, plus maybe a level shift?

[Mapo]

I had thought of a capacitor to remove the DC, then a 100k R between C and +3v. 
This should give me 3v when there is no variation and drop to 0v when there is a negative peak. 
I hope it works but it doesn't seem like a "nice" solution to me

[trastikata]

What exactly you want to measure?

You said resistive divider is not working, but it will proportionally scale down the amplitude too?

[John Lawton]

Mapo, if your scope trace showed where 0V was and the voltage per division then maybe we could assist better. Or was it AC coupled?

[RGV250]

Hi,
It sounds like you want to interface with the system, if so it would be worth reading here. 
https://michael.stapelberg.ch/posts/2020-09-28-nuki-scs-bticino-decoding/#connecting-the-nuki-opener-to-the-bticino-344212

Also on the second page he show a circuit to interface with a 3.3v Teensy micro. 
https://michael.stapelberg.ch/posts/2020-11-30-scs-processing-microcontroller/
Bob

[Pepe]

Assemble the next circuit

[Stephen Moss]

Mapo, if you have a supply voltage greater than 27 volts available that shares a common ground with the PIC, then can you not simply use a differential amplifier with a gain of 1?

It looks like your unmodulated voltage is 27V and drop to around 24V when modulated, so if you set the inverting input at 24V and connect your signal to the Non-Inverting input, the with no modulation you should get 3V out, dropping to 0V when modulated. Then run the output of amplifier it into an I/O pin with a Schmitt input to clean it up a bit.

[Mapo]

Thanks everyone for the various ideas. 
My goal is to decode this data and use it accordingly. 
Bob thanks for the links, very interesting and will save me a lot of time. 
Pepe, it seems to me a simple and functional solution, as soon as possible I will do some tests with this circuit. 
Stephen, excellent solution if I use an op amp with open collector I can clean the data and bring it to the voltage of the pic. 
Thanks again everyone for the great ideas.

Mapo

[shantanu@india]

Mapo,
Quite likely it would work even if you directly connect the 27V source to an IOC pin of the PIC simply through a 100K resistance. The clamping diode of the PIC input clamps it to Vdd and the 100K restricts the current.
Valid only for a common ground system