Help identify Rx Tx on NVR (all 3 pins have +3.3V)

XrayZ

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Dahua NVR `2020 41хх series
Help identify Rx Tx on NVR (all 3 pins have +3.3V), ground found
Probably left two are Rx Tx ?
 

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alastairstevenson

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Temporarily touch something like a 1K resistor to ground on each of the pins one by one and measure the voltage. The Rx input pin will usually be the one that drops
That's a good suggestion.
And if you don't have a 1k resistor, hold ground with one finger, moisten another finger and hold something conductive like a paper clip, touch it to the pins in sequence whilst measuring the voltage.
The RX pin has an internal pullup to 3.3v which can be dragged down a bit with a modest load.
 
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