e phase offset between the two masters (which is a different prob- lem). In both cases, the OFM value should quickly converge toward zero. If it does not, then the PDV is very bad, and under worst-case conditions, the slave clock may never be able to lock at For the MPD, ideally it should remain constant. If it is varying, then there is PDV in the net- work. The slave will have no idea whether this is coming from the forward path or reverse path because the calculated mean is an average of the round-trip time. Some servos display a separate value for both directions, but to calculate these values, the servo assumes the local clock is correct. In the previous example, if the servo assumes the local clock is correct, then it would be able to detect that the forward path took 400 us (because assuming this would allow the OFM calculation to equal 0 us). Taking the preceding example, if the clock assumes that its time is accurate, it could see that the forward path was 400 us and the reverse path was 100 us and SO may ignore that data because it knows it to be in error. Of course, it cannot continue to treat the data as wrong because it is ba- sically not believing anything coming from the master unless it agrees with the local clock. And not to believe the signals from the master is the opposite of what the slave is supposed to do. If the slave continues to receive sets of timestamps telling it that its clock is too fast, then it prob- ably is! There is valuable information to be learned from the timestamp calculations, but just be careful to understand what these pieces of data are showing you. As has been reiterated several times, there is almost no way to determine if a standalone slave clock is truly aligned to the master clock. To en- sure that, the slave clock must be compared to another reference clock, and for that, the engineer needs to make use of probes or a local reference. 12.5.1 Probes: Active and Passive Making sure synchronization is working as designed is not a trivial task. As mentioned previously, 