Paragon-X: Why can't I change my 1PPS Reference Input?

The 1PPS Reference Input options available in the Setup Interface→References window are dependent on the configured Operation Mode:

If Master-Slave Emulation mode is not selected, either of the two physical inputs on the Paragon-X rear panel may be configured. Generation of the internal 1PPS reference requires the Master-Slave functionality, and so it is not available in this mode. Note also that the Start MSE Master using 1pps Ref option is also unavailable when not in Master-Slave mode:

If Master-Slave Emulation mode has been enabled, only the Internal option is valid, and the physical options are deactivated:

This is because accurate measurement by Paragon-X requires that the PTP and 1PPS timing (Top of Second or ToS) are exactly aligned, and so to ensure this only the self-generated 1PPS is available.

Start MSE Master using 1pps Ref is available in this mode. When this option is enabled, Paragon-X will start the MSE Master when an external 1PPS pulse is detected, then immediately switch to using its own internal 1PPS reference i.e. the 1PPS Ref is effectively acting only as a trigger to start the MSE Master.

If your Paragon-X in Master-Slave mode does not start generating packets when expected, check whether the Start MSE Master using 1pps Ref option has been enabled. If it has, check that your external 1PPS source is outputting the expected signal. Without an external pulse as a trigger, the Master emulation won't start.