I want to use a digital input pin of a NodeMCU board (ESP8266) to read a digital signal. The signal line idles at 24V DC. The logic levels are 24V and 16V. The signal speed is 500 Baud (each signal sample is approx. 2 ms long). How do I convert the signal for the NodeMCU board?
Unfortunately I do not have an oscilloscope. What I already did is: I connected the bus lines to the microphone port of an USB sound card using a voltage divider with 1MOhm and 1kOhm and used the software Soundcard Scope. Does this tell you something about how the bus works?
24V high, 16V low. No 24V supply available near the NodeMCU board. Not sure about the maximum transmitter load. It is the 2-wire communication bus of a door intercom system (TCS, https://www.tcsag.de/).
BTW, in TCS:BUS documentation I found the hint that bus messages are either 16 or 32 bit long.
Soundcard Scope screenshot with Windows Automatic Gain Control (AGC) turned off for the microphone input:
This diagram is the result of capturing bus voltages with
Any idea how this protocol could be working?