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Pulse per second (PPS) is the simplest form of synchronization. PPS is a signal that outputs a high logic level once a second. It does not contain information about the specific time of day or year. The pulse width is generally 100 ms. You can input a PPS signal on a PFI, PXI_Trig, or PXI_Star line.
Selecting PPS as the time reference for an NI-Sync device configures the device to interpret a rising edge on the configured input as representing a second's boundary. Since the PPS signal cannot indicate an absolute time, you can configure the device to use either a manual start time or its current board time, and use the PPS signal only to correct frequency.
If you configure the device to use a manual start time, the device will use the first pulse received on the PPS input terminal as the configured start time. The device interprets every subsequent pulse as occurring one second after the previous pulse. Using this configuration, you can synchronize multiple systems equipped with NI-Sync timing devices if absolute time is not a concern. You can use PPS as the time reference for all the systems you want to synchronize and then configure them with the same manual start time. You can then connect the PPS signal to the systems and start the PPS output. Since the systems are connected to the same signal with the same start time, they are closely synchronized.
If you configure the device to use current time instead of the manual start time, the device interprets the first pulse received as the time equal to the device's current time. The device applies no correction when the first pulse is received, so it interprets every subsequent pulse as occurring one second after the previous pulse. You can use this configuration to distribute frequency corrections to multiple systems without concern for actual time values.
For best results when using PPS as a selected time reference, ensure that the device supplying the PPS signal provides a stable, consistent 1 Hz signal. You can achieve optimal results when an OCXO, TCXO, or better oscillator drives the source signal. You can introduce error into the system if the reference signal contains significant jitter or the reference frequency strays from 1 Hz.