ahh right,
i am wondering if i need to do some signal conditioning tho, not sure what kind of output the tone function outputs,
the tachograph needs a square wave, but i am not sure if it needs a negative one,
the pulse sender (that would attach to a gearbox, and convert rotating motion into electrical pulses) produces 2 square waves at 180 degrees to each other, they need to be exactly the same... just one is inverted,
i've found out that the 4 wires i mentiones, the first 1 is +8 volts from the tachograph, 2nd is earth, 3rd is one pulse output, 4th is the inverted pulse output,
BUT, the speed sensor specs says:
Signal shape rectangular
Frequency (max.) < 1 kHz
Output signal A1 UL < 1,9 V (1mA)
Output signal A2 inversion of A1
(the output signal A1 bit, the UL.. the L is positioned half way lower than the U, not sure what that means)