I have an e-stop switch wired to PIN10 as shown in the image:
Switch connected to +5V source, 3.9K pulldown to GND and 220ohm resistor on the input pin for current limiting.
This setup is not working because when the switch is closed I am reading proper 5V on the input, but when it is open the voltage stays at ~2v instead of 0.
Is there a strong pullup on the serial port?
How should this be setup?

Hi Boris,

I doubt that there is a pull-up on the computer’s LPT parallel port (although you could possibly have a bad Pin 10). There are, however, often a pull-up’s on the inputs to the breakout board / controller board.

I suggest that you wire your e-stop so that it takes pin 10 to ground with a pull-up to +5 if necessary then configure e-stop as Active Low.

Tweakie.