I am attempting to use a ESP32 to make a Yamaha Receiver internet connected through the receiver's cabled connection to pass remote button presses. I have found a manual detailing the protocol and right now I'm attempting to get the ESP32 to trigger based on the signals.
Both scope and multimeter are attached at the same point in the circuit, I think using the same polarity. The receiver specifies to use a mono-audio cable which I replaced by a stereo 3.5mm audio cable. I have attached the red lead of the multimeter and the scope probe tip to the tip of the audio plug. I have attached the ground lead of the probe and the black COM lead of the multimeter to the bottom-most surface on the audio plug.
The protocol is serial and very slow (1ms minimum between edges) so it is well visible on my very rickety and cheap scope, a DSO-150. However, the measured voltages don't match with the readings of my inherited Fluke 87. In rest, the scope reports a voltage of around -2V, using 0.5V/div The Fluke measures 0V set to the tens-range. I expect a resting voltage of 0V.
I don't expect the Fluke to accurately measure the peak voltage of the signal. The scope indicates it is 4.5V peak to peak, which doesn't seem implausible to me. That, combined with the fact that the waveform of measured signal exactly corresponds to what the manufacturer's documentation states leads me to believe the scope is attached correctly.
What could explain the difference in resting voltage measurement?