breakpoint latency

[Parag Parikh]

Hi all,

I think a similar question has been posed, but I wanted to check again with the community.

I have an encoder enabled breakpoint using the 7344.  Is the latency for the breakpoint the same as for a trigger input (ie , <100ns)?  I am reading the breakpoint directly off the breakpoint line for that axis (no s/w intermediaries). 

A previous post mentioned 62 ms... which I thought was rather high.  In fact, onboard programs and s/w loops seem to do that well.

-parag

[Jochen]

Parag,

the breakpoint signal is generated directly on the FPGA that reads the encoder signals. Thus the latency is defined by the switching time of some gates on the FPGA which is somewhere below the 100 ns range. I don't know where you have found the 62 ms spec but maybe this is related to the fastest PID timing which is 62,5 µs, but there is no relation between the PID rate and the breakpoint latency.

I hope this helps,

Jochen Klier
National Instruments

[Parag Parikh]

Thank you for the information.  The post I referred to is the following:  http://forums.ni.com/ni/board/message?board.id=240&message.id=169&query.id=52562#M169  I thought it was inaccurate.

One more question:  I am trying to build a real-time 'gating' application where I can pulse on when I am between 2 encoder positions that are set once in software.  With only 1 absolute breakpoint per axis in the 7344, is there a clever method to use these w/o having to rely on on-board programming and the like?  I am looking for 1 millisecond latency or less.

-Parag

[risheeb]

Hi Parag,

With the PCI-7344, you would need to do this in software.

If you are programming in LabVIEW, you'll configure the first breakpoint before your motion loop.  Inside of your motion loop, you could then use the Read per Axis Status VI.  The Status cluster from this VI has a Position Breakpoint boolean which is False if the breakpoint position has not been reached and True if it has.  You can wire this into a Case structure so that once it is True, you can use the Load Breakpoint Position VI and the Enable Breakpoint Output VI to reset the breakpoint at the new position.

Since you want the output to be high between these two positions, you would set the breakpoint configuration to Toggle so that it will toggle high on the first breakpoint and toggle low on the second.

I hope this helps.   Please let me know if you will be programming in a text-based language and I can help you find the functions which correlate to these VIs.


Rishee B.
Applications Engineer
National Instruments


Message Edited by risheeb on 01-24-2008 01:01 PM