Loop powered (e.g. gets power over the CAT5 pair). OneShot or Pulse Extender.
During the One-Shot, the loop current goes to 10mA. At the 100 Ohm termination 10mA will bias the base of an NPN transistor to turn it on, so its collector is pulled down and the MCU will see a low. When the One Shot ends the loop current returns to 3mA and that will not be enough voltage on the NPN transistor to turn on, thus the MCU pin will be pulled up to the MCU voltage.
WHY: idea is to send a measurement quality pulse over a ridicules distance (e.g. 5000 meters) of a CAT5 pair. This allows the movement of a displacer in a pipe to be timed in relation to a flow meter and thus help to calibrate the meter.
Shunt reference used because they are unconditionally stable (regulators need capacitors and that causes problems).
The 555 discharge pulls 6.9mA from a current source. The shunt reference is supplied from a 2.2mA current source.
Transmission line elements are in four parts, that total 100 meters of a CAT5 pair.
The 7555 is like a NE555 but much better, for example it works at 3.7V where 555 will not. CMOS logic gates help the output look more like the 7555. http://www.intersil.com/content/dam/Intersil/documents/fn28/fn2867.pdf
I've seen the 555 model have issues when it isn't directly ground.
http://epccs.org/indexes/Board/OneShot/
UPDATE: What if I have a 20mA current source... I don't see a difference.
My control board has a 20mA source for loop power.
http://epccs.org/indexes/Board/RPUno/
