by RetireeJay » 2017-Jul-Tue-22-Jul
It's absolutely possible. How to do it depends on which specific model of Printrboard you have.
The original circuit, before the probe came out, used a 4.7k pullup resistor, and the end-stop switch held the pin at ground until the switch was touched. On Rev D boards, the 4.7k pullup resistor was actually built-in to the ATMEL chip. On Rev F boards, you have a transistor in between the external pin and the ATMEL Z sensing input pin, so there's a inversion. Depending on the model, there may or may not be a pullup resistor in the circuit on the input side of the transistor (some probes have "open collector" outputs; others have active pull-up and pull-down circuitry). You will have to do a bit of research.
If you want to avoid having to modify the firmware, then you'll need to emulate the probe's high and low states depending on whether or not you are "triggered". Once you do that, then simply never issue a G29 command. A G28 Z0 will suffice to lower the carriage until it triggers your switch. You will want to have a convenient way of precisely adjusting the trigger point. And it will be up to you to get the plane of the bed parallel to the plane of X-Y motion of the hot end.
Several of us old-timers don't have probes and don't want them. A good switch and a trammed bed work just fine with ultimately less hassle.
Printrbot Plus operational January 2013
Brass threaded rods (5/16" X 18) & nuts for Z axis
GT2 belts & pulleys
Cable chain to reduce probability of fatigue failure in wires
E3D V5 Hot End, 0.4mm nozzle, also 0.8 and 0.25 in use occasionally
PB fan mount + 40mm fan -- using printed mount adapter, not the E3D supplied fan
Injection molded extruder gears
Optical Z "endstop" (custom designed and built)
Have used many pounds of T-Glase filament. Now also doing some work with Ninjaflex SemiFlex
Print on glass with Scotch Craft Stick or other glue stick
"My next printer is..." Prusa i3 MK3, upgraded to MK3S