Question 3-89N2

Shielding on an NMEA 0183 data line should be grounded only at the talker (source) end and left unterminated at the listener end. This is a fundamental technique to prevent ground loops, which are a common source of radio frequency interference (RFI) and noise in communication systems. If the shield were grounded at both ends, any difference in ground potential between the talker and listener would cause a current to flow through the shield. This current effectively creates a loop antenna, which can pick up ambient electromagnetic interference (EMI) and induce noise into the data signals, or conversely, radiate noise. Grounding the shield at only one end provides a drain path for induced noise currents on the shield to flow to ground without forming a noise-generating loop. Therefore, options A, C, and D are incorrect. Leaving the shield unterminated at both ends (A) renders it ineffective. Grounding at the listener only (C) is less optimal than at the source, and grounding at both ends (D) actively creates a ground loop, introducing noise.