I'm going to use some proximity sensors for limit switches, they're NPN. Can I wire those directly to my Pokeys57CNC pins? I'm guessing I can't because it'll only provide a sinking signal to the pin and not act like a relay and have the board COM pass onto that pin?

So I'm guessing I have to wire the proximity sensor to a relay and then use that to bridge the COM to the pin?

If the input normally takes 5v as a signal, you have to pull up the pin and set the input to active low.
I have a Stepper3 Ether mach CS controller and, after modifying it by removing the input led's, use npn proximity sensors at 12v feeding the inputs through a 390 ohm resistor. The sensors have a 10K internal pull up. This gives about 3.7v at rest, enough to take the pin high, and it goes to 0v when proximity occurs.
I should note that the controller has 4.7K pull down resistors on the inputs. I removed the leds because they were also pulling down the pin with a 39 ohm resistor making them unusable. Long story.

Hi,
most of the commonly available proximity sensors have a voltage input requirement of 6-36V.

The limit/home switch inputs of your 57CNC are 5V and PoKeys specify a maximum of 5.5V.

I would want to be 100% that the proximity switches didn't blow the 57CNC. At the very least I would have a current limiting resistor in series with the 57CNC input pin and the proximity sensor and a 4.7V Zener diode to earth
and thereby limit the voltage presented to the input.

Another alternative would be to use a transistor, either a BJT or a MOSFET, either would be a few cents each and would provide some measure of isolation between the proximity sensor and the input pin.

Craig

Uh, thanks. I'm not really following all the technical jargon, but I get the gist. I messaged Pokeys tech support and linked the sensor I have and they said I can use it directly with the Pokeys57CNC board. Just hook both COM on the board to the VDC negative on the power supply and so forth. The sensor is using 12VDC, but the board has an on board pull up resistor. So I messaged them back to clarify whether i can use it without doing anything else. Haven't heard back but will when I do.

Hi,
section 3.2 page 18 of the manual shows the representative circuit diagram of a DI5P input pin.

My concern is that if your proximity sensor assumes a voltage greater than 5.5V it will be transferred via the LPF to the MCU of the controller. DON"T let that happen or you'll fry it!

Presumably the proximity senor is just an open collector outut in which case any supply voltage of the sensor cannot leak back into the 57CNC. If however there is anything resembling
a pull-up resistor in the proximity sensor that voltage will get back into the 57CNC with unpredictable results. I'm imagine you'd be rather annoyed if it transpired that a proximity sensor
had damaged your controller. I would recommend a current limit resistor and Zener diode to be safe.

Craig

Craig, totally appreciate your help. I understand in theory, but the terminology is beyond me. I'm quite capable with electronics, but all the stuff you're talking about dealing with pull up, resistors, etc... confused me and always has. I should learn.

Here's the sensor I'm using https://www.amazon.com/gp/product/B073VFRCCX/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1 will this work alone on my board or do I have to use some sort of resistor or combination of things?

I'll look up what an open collector is and the other terms and see if I can wrap my head around it.

Hi,
that link nor any that I googled really explain for sure what the output circuit of the sensor looks like.

If it just the collector of a transistor with its base controlled by the sensor there is no oppurtuiny for it to source any current and would be safe to hook direct to th 57CNC.
If for whatever reason the manufacturer put a pull up resistor or even a very low current leakage path from the 12V supply to the prox sensor to the output that voltage could
reverse bias the input of the 57CNC MCU and blow it up.

Do you want to risk it?
If you don't you'll need a resistor and a Zener diode.

Craig

What kind of verbiage am I looking for when looking the specs of a sensor?

http://www.ekt2.com/pdf/14_PROXIMITY_INDUCTIVE_18EX.pdf
This is the unit I believe.
Do you have two or three wires?

Note;  Correction to my above post. It was the BOB that had the led's on it, not the Stepper3 motion controller.

here is a diagram for the three wire type npn proximity sensor with a 10k internal pull up resistor.  When measured at rest there will be V+ on the black wire, so you shouldn't hook it directly to an input with a pull up. (will have Vcc on the input pin at rest)

Hi,
that diagram that Roaster has come up with is exactly what I mean. There is a path from the supply of the prox sensor (12V) through a diode, a 10K resistor through the 2 ohm resistor to the
57CNC. You don't want to apply 12V to the 57CNC input, it will handle 5.5V max.

In the first pic you have the sensor represented as a plain open collector transistor, no problems, the series resistor and Zener diode are not required.
The second pic the sensor is depicted as an open collector transistor with a pull-up resistor R2. This is close to what Roaster has posted. The series resistor R1 and the Zener diode are now
required otherwise the supply of the proximity sensor has a path to the 57CNC. The Zener will ensure that the pin is not subjected to greater than 4.7V.
The last picture is an alternative. It requires a diode only, it will prevent the supply voltage via R2 getting into the 57NC.

Crai