Wiring 24v NPN Proximity Switch to 5v BOB - Why does'nt it work

[Neale]

People seem to be over complicating this.
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Given the BOB in question requires switched to 0V to activate the input, all you need to do is wire 24V to BN, 0V to BU, then connect the BK to the BOB input. You may need to also connect the BOB 0V to the sensor, however I'd assume this BOB doesn't have fully isolated inputs, and the 24V and 5V supplies already have their 0V connected at some point.
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An NPN sensor simply switches the output wire to 0V, and should go essentially open circuit when unactivated. You should not need any kind of pull up resistor on the sensor output.
However as has been mentioned already, the sensor may not switch perfectly to 0V, and may struggle to reliably switch the BOB input. In that case, adding a small relay into the circuit would be needed.
Alternatively, there's these simple sensors called switches... ;-)

Reason why I over-complicated things was that I had assumed that this BOB had opto-isolated inputs (I've been working with a CSMIO card recently which does things that way and I made a false assumption). I have been trying to test my own ZPA5 with Mach3 but after a couple of hours, remembered that the parallel port version of Mach3 doesn't work on 64-bit Windows. However, I did take a look at the card itself and it looks like the inputs go into a set of buffers, no opto-isolation, but with a pull-up resistor. So as m_c accurately says, just take the black wire to the BOB input. Make sure that the 0V of the 24V power supply (blue wire to prox switch) goes to the ground connection on the BOB. In fact, just as in your first diagram but the resistor is redundant. What is probably worth doing is checking that with the prox switch disconnected, just shorting the BOB input pin to ground does register with Mach3 (stick a small screwdriver blade in the two-pin socket next to the screw terminals). If that works and you see the Mach3 diagnostic "LED" go on and off, then you can connect the proper switch and see if that works.

For those who mentioned the possible 10K resistor in the switch - the BOB appears to have a current-limiting resistor in series with the input to the buffer inverter which also includes clamp diodes to restrict input voltage at the input pin. So the 10K resistor, if it does exist, is just going to act as another pull-up when the switch is open and can be ignored when the switch is closed. If it doesn't exist, it doesn't matter as the BOB has an onboard pull-up anyway.

See - some of us have been working this evening instead of drinking