TONEOHM 950 OPERATOR MANUAL
4-6
If a reading greater than 3.0 is displayed this indicates there may be multiple shorts. (Refer to Multiple
Shorts later in this section.)
Example #1
Consider a board on which the +5V plane is shorted to the 0V (ground) plane.
• Connect the Plane Stimulus leads to the corners of the ground plane, and the Plane
Clip to any point connected to the +5V plane.
• Use the Plane Probe to probe the ground plane (at IC pins or decoupling capacitors).
The arrows will lead the user to 40-50mm of the short.
• Use the tone and meter reading to locate the short precisely.
Care should be taken when probing decoupling capacitors to note that the lead being probed is connected
to the stimulated plane, as on some boards the +5V and ground may not be consistently connected to the
same side of these components.
Example #2
If a reading less than 5.0 cannot be obtained when probing the stimulated plane, this suggests that the
short is not exactly at the probed point, but some distance from it.
Consider a board where there is a short between the Vcc and ground planes, and assume that the short
actually occurs at the Vcc pin of an IC.
If the ground plane is stimulated, then the user will never probe the precise point where the short occurs,
so the lowest reading will be obtained when the probe is at a point on the ground plane that is close to the
Vcc pin of the IC. If this situation is suspected, try stimulating the Vcc plane instead of the ground plane.
High Resistance Shorts
When the resistance of the short is greater than 20–30 Ω then the TONEOHM 950 will not be able to
resolve its position as precisely as for a "hard short".
Resolution can be improved by moving the positions of the Drive Stimulus leads to reduce the area of the
plane being stimulated.
• Start with the leads attached as normal at the corners of the board.
• Probe the stimulated plane as normal to determine the approximate area of the short
(i.e. where the readings are lowest).
• Then move the Plane Stimulus leads in from the corners of the board to stimulate a
smaller area of the plane. This will increase the resolution of the reading in the
display.
• This process can be repeated if necessary, each time reducing the area of the plane
that is stimulated and hence increasing the resolution within that area.
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