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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?

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    \$\begingroup\$ You forgot to say what ranges you are using on each instrument. Hit the edit link ... \$\endgroup\$
    – Transistor
    Jul 18, 2020 at 9:15
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    \$\begingroup\$ Check both instruments with a voltage reference eg. 1.5V battery, and with probes short circuited. Check that the meter is set to Volts (not Amps etc.) with probes plugged into the correct sockets. Connect both instruments to the same place in the circuit with the same polarity, at the same time. \$\endgroup\$ Jul 18, 2020 at 9:31
  • \$\begingroup\$ Thanks! I have added a description of the circuit and how I measure as well as the ranges. \$\endgroup\$
    – Zsub
    Jul 18, 2020 at 11:19
  • \$\begingroup\$ Whoah. Even disconnected and short-circuited the scope reads -2V... When measuring a freshly charged battery it reads around -1.45V. I think I may have part of the answer there. Now to figure out how to bring the reading on the scope to 0 :) \$\endgroup\$
    – Zsub
    Jul 18, 2020 at 11:22
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    \$\begingroup\$ Any oscilloscope should be able to move the trace up of down the display under user control. Short your leads, then check your user manual for the scope to see how to move the trace to the correct zero position. \$\endgroup\$
    – JRE
    Jul 18, 2020 at 11:27

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The issue was a faulty voltage supply to the scope. It was running off of 5V while it is specified to run off of 9V. I discovered this by checking step 4 of the assembly manual for the scope.

After adjusting my power supply to 9V, the base measurement still was off (but now by around +2V). Using the VPos Adjustment (top right, last page of the manual) fixed this and now my measurements make sense again.

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