The star trigger is a high-performance trigger line available to synchronize all of the modules in a PXI chassis. Although synchronization can be done using the normal PXI trigger bus, the star trigger is a trigger signal that offers increased synchronization performance. Specifically, the trigger signal has a propagation delay of no more than 5 ns and an inter-module delay of no more than 1 ns. The star trigger is designed such that the lengths of the wires from the star trigger slot to any other slots are identical.
For PXI chassis, the star trigger must be controlled from slot 2. The trigger signal generated by the star trigger controller can be accessed by the other modules through the backplane. Alternatively, if the star trigger is not required, slot 2 can be used as a standard peripheral slot. This is the reason why slot 2 on the chassis is labeled with both a circle and a diamond around the number 2. The diamond represents the ability for a module in this slot to be a star trigger controller; the circle indicates that the module in this slot can also be a standard module with no extra functionality.
For PXIe chassis, the star trigger is controlled from system timing slot. The system timing slot is typically at slot 4 for 8-slot PXIe chassis, slot 6 for 10-slot PXIe chassis and slot 10 for 10-slot PXIe chassis. Please refer to the user manual
for respective PXIe chassis. Similarly if the star trigger is not required, system timing slot can also be used as a standard peripheral slot.
The system timing slot also has 3 dedicated differential pairs (PXIe_DSTAR) that can be used for high-speed triggering, synchronization and clocking.
See Table 2 in Synchronization Explained
for the comparison on different synchronization signals.