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The Time Domain Reflectometer is a device generally used to find the distance to a fault in a transmission line by measuring the round trip travel time of a pulse sent down the line. In this circuit, inverting Schmitt triggers in parallel (to simulate realistic current loads per gate in something like a 74LS14) drive a fast-rising-edge pulse down the line. A XOR gate creates a high pulse equal to the round trip travel time of a pulse sent down the transmission line. One input of the gate connects directly to the pulse generator, the other is connected to the transmission line via a resistor divider, which keeps the input at a logical zero until the pulse returns, at which time the voltage rises high enough to transition to a logical one, driving the XOR output low. The XOR remains low until the next rising edge from the generator.
(XOR high time)/2 x (phase velocity in the line*) = distance to open circuit
*Available in the line's datasheet
Via a low pass filter at the XOR output, over many pulsing cycles, the output will converge on a DC voltage which is proportional to the line length. This is a better output method for slower MCUs which might not be capable of tracking nanosecond pulses, but are equipped with ADCs.
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