NMEA 2000 backbone architecture rules

NMEA 2000 (aka N2K, SeaTalkNG on Raymarine, SimNet on Simrad) is a single backbone with drop cables to devices. Key rules: (1) Terminators required at both ends of the backbone, exactly one at each end, 120 ohm each. Missing or extra terminator = intermittent chaos. (2) Drop cable maximum 6 meters (20 ft) from tee to device. Longer causes reflections. (3) Total backbone length: 100 meters (330 ft) with heavy cable, 200m with medium cable. Most yachts under 60ft never approach the limit. (4) Power injection: one power connection per backbone segment, ideally in the middle. Two power injections = ground loop and message corruption. (5) Load Equivalency Number (LEN): each device consumes power. Backbone can support total LEN of 100. Chartplotters are 4-6 LEN, MFDs 3-4, wind instrument 1-2. (6) Voltage at any device tee: minimum 9V, measure with a multimeter, if a device is intermittent measure its voltage first. (7) Ground shield connects at one end only, usually the power injection tee.