How do I generate a Star Trigger using a 6608 and sync clocks across 10 6115's?

[RGA]

I have been programming in Labview for some time but I am new to the process of synchronizing AI on multiple devices using the PXI Star Trigger.

I have a PXI chassis with a 6608 installed in slot 2 and 10 6115's installed in the remaining slots. How do I take an external trigger into the 6608 and use it to generate the Star Trigger?

Further, how do I ensure that the clocks are sync'ed on all of the 6115's? I need at least 5 MHz on each channel and timing is critical.

Thanks

[Russell]

The good news is that all the 6115's will already have their timebases synchronized due to the on board phase-locked loop circuitry. You don't have to do anything, it does this automatically on power up.

Actually, the 6608 is not cabable of driving the Star Trigger line, but you can still use one of the other trigger bus lines on the PXI backplane. Since the timebase period on the 6115 is comparitively long with respect to the propagation delay you'd see between slots, the trigger will still be received within the same timebase period on all devices and all 6115's should still begin their acquisitions on the exact same timebase edge.

The only thing you must consider is that there is a separate trigger bus for each group of six slots in the PXI-1006 chassis. You
will have to run an outside jumper between 6115's boardering trigger bus segments of the chassis through one of the PFI lines of the 6115. See the PXI-1006 User Manual on page 2-6 for more information.

I hope this helps!

Russell
Applications Engineer
National Instruments
http://www.ni.com/support

[RGA]

Thanks for the info, but I am still a bit confused about the Star Trigger.

Both the 1006 chassis manual and the PXI 2.0 spec indicate that an ST enabled device in slot 2 can send out 13 "individual" triggers to the devices in slots 3-15 and that the Star Trigger bus architecture "eliminates the need to combine multiple module functions on a single trigger line".

These documents seem to indicate that the functionality is there, the problem is that I can't find any info on how to use it. The 6608 manual only gives a single Star Trigger line (RTSI 6 or 7, can't remember) - does that mean that the 6608 is not capable of differentiating among the 13 different ST lines, it just sends a single trigger to them all at once? That doesn't seem logical to me - it seems
to defeat the purpose of the Star Trigger lines.

If the 6608 cannot generate the individual triggers, what PXI device can?

Thanks again,

RGA

[Russell]

Yes, I know it seems confusing. The PXI specification is for one PXI star trigger. A star trigger is different from a bus trigger. A bus trigger only uses one wire and each device is tapped into that wire. Unfortunately, due to propagation delay, the device nearest to the trigger source will receive the trigger before the device furthest away from the trigger source.

What makes a star trigger useful is a uniform propagation delay to all boards connected to the trigger. This is accomplished by running separate equal length wires to each device. So there are actually 13 equal length lines/wires that connect all the devices together. These lines all join at slot 2. This is why the device in slot 2 is the only one capable of driving the star t
rigger. It is the only slot where there is an equadistant path to all devices. So this is what is meant by "13 individual trigger lines."

Right now, the only NI devices that can drive the star trigger is the NI-5112 digitizer and the NI-4472 DSA. The About Your 6608 Device manual is confusing because it uses the term "PXI Star" when describing it's capability to replace the 10 MHz backplane clock. This is not referring to the PXI Star Trigger. The backplane clock line also happens to be wired in a "star" configuration to ensure synchronization across the chassis.

I hope this helps clear things up!

Russell
Applications Engineer
National Instruments