The solid orange wire of the twisted pair takes the 17mA from the current source of RPUno's FT/Pulse section and runs it to the LS (loop source input) pin on CL8 (J1.1). An HT sensor is connected to LS0 (loop source output zero, J2.1) and the LT node on J2 (LT is a common node, so all sensors can return there current to the loop termination). The returned current flows on the orange-white wire from the LT connection on CL8 (J1.2) to the 100 Ohm resistor on RPUno's PL pin. When LS0 is selected its current flow will pass through the 100 Ohm resistor and bias an open collector NPN transistor that pulls down ICP1 when the current flow is over about 6.5 mA.
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