We are failing 10BASE-T Return Loss tests but not 100BASE-TX or 1000BASE-T tests on the same port+ yet 10BASE-T is the only one that the procedure for calibration and test refers to the use of jumper J241 on the test TF-GBE-BTP or TF-GBE-ATP fixture. What is the purpose of the jumper J241?

J241 is only required for 10BASE-T and not the other data rates. The issue arises from the fact that for 10BASE-T, the A differential pair is used for Tx, and the B differential pair is used for Rx. This is quite different from the other data rates. The TC1 section of these test fixtures basically has a power splitter that connects the AWG signal to all four of the differential pairs simultaneously. This is convenient when testing 100BASE-T and 1000BASE-T, but has an unintended consequence when testing 10BASE-T. The AWG signal that is connected to the B differential pair disturbs the DUT, and disrupts the test. So, by applying the jumper 241, we short out the AWG signal at the output of the power splitter. The B pair then is just a non-active 100 ohm load for the DUT. This allows the return loss test to be performed on the 10BASE-T Tx lines without disruption.