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I have two modules that communicate over a single wire. Where would I even start about monitoring the data without introducing any kind of interference to both modules?

One of the modules has to be deleted/bypassed and my own controller will be simulating that module. However, the second module does not work unless it communicates with first module.

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  • \$\begingroup\$ Have you tried putting a probe on the signal line (oscilloscope or logic analyzer...depending on what you're looking for)? If it doesn't load down or influence the signal much, you should be OK. \$\endgroup\$
    – dext0rb
    Dec 8, 2013 at 19:20
  • \$\begingroup\$ I don't have a traditional oscilloscope or logic analyzer, but could I use one of the usb logic analyzers? \$\endgroup\$
    – jM2.me
    Dec 8, 2013 at 20:24
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    \$\begingroup\$ buspirate supports 1-Wire protocol, but I have no idea if this is the same protocol as your single wire communications. \$\endgroup\$
    – jippie
    Dec 8, 2013 at 20:26

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You should be able to use a USB-based logic analyzer just fine. They are designed to 'snoop' in on signals without influencing them.

If you look at the specifications of, for example, the Saleae Logic, you can see the input impedance is noted as: 1Mohm || 10pF

This is a fairly high impedance, and that's good. A low input impedance would load down the signal you are trying to examine. The resistive component of the impedance will not change with frequency, but the capacitive component is frequency dependent. It probably doesn't matter in your case, but it's something to keep in mind. As the frequency of the input signal goes up, the logic analyzer will load the signal more and more.

Another way to describe this behavior is provided here, which comes from an Agilent app note.

The logic analyzer probe has a high input impedance. The probe-tip circuitry consists of a tip resistor on the order of 20 kΩ. At low frequencies, the probe impedance will look like this resistance. As the frequency rises, parasitic capacitance in the probe will start to lower its impedance. The impedance will roll off following a standard RC response. This could present problems for the target system; as the probe impedance begins to approach the system impedance, the voltage divider formed with the probe becomes substantial. A low impedance will absorb the majority of the signal and cause system failure.

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  • \$\begingroup\$ And to make something like Saleae Logic work with 12VDC, could I use optoisolator or voltage divider? I made a mistake in original post, first module sends data and second module reads it (one way communication). The data sent over the wire is supposedly just bytes but I have no idea what kind of protocol is used. \$\endgroup\$
    – jM2.me
    Dec 9, 2013 at 0:23
  • \$\begingroup\$ Voltage divider should be fine. \$\endgroup\$
    – dext0rb
    Dec 9, 2013 at 4:13

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