However, Schmidt-triggers are specifically designed for distinctly detecting "legal" high and low levels without oscillating. The low impedance to GND would minimize most noise on the input cable unless it originates on the power supply for the circuit at either end and is imposed on GND. If he uses the resistors previously mentioned, it's unlikely ambient EM noise or RFI would be a factor. If the two wires in his cable are connected to +12 volts and GND as shown, the 2k-ohms to GND should make that noise irrelevant at the point (5/12 of the total swing) of input to the Schmidt-trigger gate, particularly if there's a properly chose capacitor there as well. It's unlikely any noise on the +12 will raise the input to the Schmidt-trigger gate above the positive threshold while it's low, nor will it likely drive it below the negative threshold when the input is high.

RE