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Chapter 9
Digital Routing and Clock Generation
Once all of the devices are using or referencing a common timebase, you can synchronize
operations across them by sending a common start trigger out across the RTSI or PFI bus and
setting their sample clock rates to the same value.
USB Devices
With the PFI bus and the routing capabilities of USB X Series devices, there are several ways to
synchronize multiple devices depending on your application.
To synchronize multiple devices to a common timebase, choose one device—the initiator—to
generate the timebase. The initiator device routes its 10 MHz reference clock to one of the
PFI <0..15> signals.
All devices (including the initiator device) receive the 10 MHz reference clock from PFI. This
signal becomes the external reference clock. A PLL on each device generates the internal
timebases synchronous to the external reference clock.
Once all of the devices are using or referencing a common timebase, you can synchronize
operations across them by sending a common start trigger out across the PFI bus and setting their
sample clock rates to the same value.
Real-Time System Integration (RTSI)
Real-Time System Integration (RTSI) is a signal bus among devices that allows you to do the
following:
•
Use a common clock (or timebase) to drive the timing engine on multiple devices
•
Share trigger signals between devices
Many National Instruments DAQ, motion, vision, and CAN devices support RTSI.
In a PCI Express system, the RTSI bus consists of the RTSI bus interface and a ribbon cable. The
bus can route timing and trigger signals between several functions on as many as five DAQ,
vision, motion, or CAN devices in the computer.
In a PXI Express system, the RTSI bus is replaced by the PXI and PXI Express trigger signals
on the PXI Express backplane. This bus can route timing and trigger signals between several
functions on as many as seven DAQ devices in the system.
USB devices do not support the RTSI bus.
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