A diplexer looks just like a 2-way splitter. As cdru wrote, a diplexer separates the satellite signals from the off-air signals and sends them out different ports. One port is labelled "SAT" or "950-2250", and the other one is labelled "ANT" or "5-862 (or 40-862)." The port all by itself is labelled "IN/OUT" or "TV/SAT."