I will try to make some sense out of the probe logic.
The signal the probe circuit sees is either a Hi, or Low. It was designed to work with a touch plate
for tool setting. A "Tool Setting" touch plate is insulated from the machine table, and wired to the
probe input pin. Its overall thickness is measured, and entered into the probe program set-up box.
A special tool is used to set-up the Z0 position of a job. This tool is usually an edge finder, or some
other type tool that does not get changed, or used to cut metal. All tool offsets are relative to this
first tool.
1. When the G31 signal is received by Mach3, the spindle starts moving down.
2. The tool in the spindle touches the insulated metal touch plate;
3. The probe circuit immediately changes from an active HI (+5 volts) to a Low (0 volts) because the
tool bit is at "Ground";
4. Mach3 immediately stops.
5. The program reads the Z axis position, and enters it into a parameter;
6. The touch plate thickness has previously been entered into the probe "set-up";
7. The probe program adds the touch plate thickness to the Z position parameter;
8. This new calculated number becomes the "Tool Height offset" for the tool in the spindle.
9. This value is usually put into the tool offset table, by the program, which requires a tool# before
each probing cycle.
Now, the probe is entirely different. Instead of a signal going from a Hi (+5v) to a Low (0v), by
grounding the tool bit, it must go from a closed circuit, that conducts +5v, to an open circuit 0v.
This would work OK, except for the LED in the circuit. The LED is wired across the probe circuitry, with
a 1K resistor in series. The LED can only take a tiny amount of current to light up. A full 5v would
burn it out.
So the probe circuit is seeing a solid 5v, and the LED does nothing because the voltage goes past it,
because the probe contacts are closed. The LED does not turn on, because the +5v is going right around it.
Now, the probe makes contact, and the probe circuit opens; Instead of the probe circuit going to 0v,
a tiny bit of voltage is now going through the LED circuit, and the probe circuit. Because of this small
current, there is a tiny voltage, maybe 1 or 1.5v and about 20 milliamps flowing in the circuit.
As far as the probe program and Mach3 are concerned (Mach3 does think, you know), the probe has not yet
made contact!
Since the circuit is still not 0 volts, it requires pull-up, or pull-down resistors to trigger the probe
circuit.
There are better explanations than this one, and a diagram helps a bunch.
Lots of people have wrecked their probe contact tips getting everything working OK. Certainly NOT a
"Plug and Play" operation.
While you are working out the bugs, get it working without driving the fragile probe tip up against
a solid object with the machine axis motors. Just move the probe by hand, and watch the probe activity light
on the Mach3 screen.
John