■ FIGURE 2
When the line is shorted or
loaded with less than the
characteristic impedance, a large
current occurs at the load end which
flows back to the source. Since this
current is in the reverse direction, it
produces a negative voltage at the
transmit end. If the time delay is less
than the pulse width, the return pulse
subtracts from the input and will go
negative when the input pulse ends.
A miswired cable can appear
open or shorted depending on how it
is miswired. If wires of different pairs
are paired, the characteristic
impedance will be higher than
normal (I think); so, the 100 ohm
load will be too small and a negative
return pulse will happen. If the
miswiring is such that the test does
not ground one side of the line, it will
appear open and a positive pulse will
happen. In these cases, you cannot
tell which end has the error.
