How do you find out what voltage is being used on a data bus for the high signal?

On a small electronics I guess you can assume it will be 3.3v or 5v but what about other circuits, say in a car or on the side of a TV or some old random electrical equipment without knowing the spec of a port.

If I wanted to connect say a logic analyser to some digital bus, not knowing anything about it, how would I first find out the voltage to make sure I don't blow up my logic analyser and then, once I know what the peek voltage will be, so I'll not blow up my logic analyser by connecting it, how do I bring the voltage down to the right level (Some sort of Attenuator?) so that I can see the data on the bus?

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    \$\begingroup\$ Oscilloscope, datasheet, circuit diagram, user and service manual. \$\endgroup\$ – Anonymous Oct 22 '18 at 11:03

If you have a data bus then you'll probably have a clock signal and that clock signal will be about 50% duty cycle. So, using a DC voltmeter you could measure the clock and ascertain the average DC voltage then multiply that by 2 to get the logic 1 voltage (approximately).

If you don't have a clock signal then try measuring a data signal with the voltmeter and usually you will see about 50% of the logic voltage if the data signal is actively sending data.

  • \$\begingroup\$ Thanks Andy, that answers the first part of the question, do you have any suggestions for how to bring the logic voltage down to a level so that I would be able to attache a low voltage logic analyser to it? \$\endgroup\$ – Mark Fidell Oct 23 '18 at 10:11
  • \$\begingroup\$ @MarkFidell you may not need to attenuate and if you do you may get-away with simple series resistors of 1 kohm that act as current limiters. If your logic analyser has a data sheet please post a link. \$\endgroup\$ – Andy aka Oct 23 '18 at 11:36

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