6400 Reference Manual
18
Prox Negative (P-) - This mode accommodates a negative supply proximity probe that is looking at a hole or
pin target to obtain a TDC (Common with Bentley panels).
The threshold adjustment operates the same
as the MP mode except that It has a wider
range (0-40). Unlike magnetic pickups, prox
probe signal levels do not change with speed.
However, unlike magnetic pickups, which
have no DC component, prox probes can
have the AC signal appear over a wide DC
component range, making it necessary to
identify the upper and lower bounds for best
threshold selection.
The Prox waveform below provides an example for setting the best threshold level. It is essentially the same
approach as described for the MP mode.
a. Start with the max gauge value (40) which will usually result in no RPM reading and a gauge value in
RED.
b. Lower the value until a stable RPM reading shows and stable GREEN color for the gauge value which
for this example is 34 and an RPM of 250.
c. Continue lowering the level until the RPM changes and/or the gauge value turns RED. For this
example, a value of 9 detects another pulse from the uneven flywheel surface resulting in erratic RPM
and the gauge value color turns RED so 10 would be the last GREEN value.
d. Adjust the gauge level to the midpoint of 22 (34+10 / 2) which yields maximum separation from both the
uneven flywheel surfaces and the peak of the TDC signal.
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