6555 onboard memory

Question

Hi I am working on a project where we recently have purchased the PXI chassis with several 6555 cards.  The technical support gal, although she was very nice, did not know the gory detail of some of the questions I have, so I am hoping someone on the forums here has run into these or would know who to ask.
The specs for the NI PIXe-6555 card claim 8 MB per channel.
* Is it really 8 MB per the 24 DIO channels (so 192 MB total)?
* Is this mega BYTES or BITS?
* If I have a large dataset that is more than 8 MB, is there a way to pre-load or something so that there is not a break in the datastream?
* If we have four of the 6555 cards in one chassis, how close (what is the tolerance or slop) can I get the data streams to be synch'ed at the max speed (200 MHz)?
* If I am driving data at 5V logic levels on our DUT (device under test), can I get the full 200 MHz speed?  I am seeing conflicting info on this in the help files.
Thanks!
Ed

Answer 1

Hi Ed,
Hopefully this clears up confusion about how to write data to the generation session of an HSDIO board: How to Write Serial Data for Any Channel on a High Speed Digital I/O Board
If your data width is 1 byte, each bit corresponds to each lane. so 0000 0000 in your byte is a single sample bit for channels 0-7. To write data to independent streams at the same time, you'll have to combine the streams into the U8/16/32 format. For instance, two data lines toggling out of phase with each other on channel 0 and channel 7 looks like so:
Sample 0: 1000 0000
Sample 1: 0000 0001
Sample 2: 1000 0000
Sample 3 :0000 0001
serialized for ch 0: 0101
serialized for ch 7: 1010
If you want to drive only those two channels, while doing other independant operations on channels 1 through 6, you simply only include ch 0, 7 in your Channel List for the Generation Session through the HSDIO driver. By default, the channels not included in the channel list for generation will have their drivers disabled, and can be used for acquisition.
The memory issue here is that for generation, you still have to populate channels that are unused for generation based on the data width. Writing to two channels takes 2 bytes per sample in the above example since U16 (Data Width = 2) is the smallest for the board. This is less than if the samples were 4 bytes wide (U32, Data Width = 4), which takes double the amount of space in memory.
This KB is slightly outdated at the time of posting this, but it explains more about optimizing memory usage with HSDIO and Data Width. Ignore the 655x statements, as 655x only referred to 6551/2 boards at that time.  HSDIO Data Width and Memory Allocation
So in summary, it does bitmask based on the channel list into the generation session. It also writes one word (U8/16/32) at a time, for the channels that are enabled for generation, and this does take up memory space on the onboard memory. For your case, to write 11 outputs will require a U16 at least, which is smaller than the default U32, so I would recommend changing data width to 2 for acquisition and generation to obtain the maximum amount of memory available on your device.
Kyle A.
National Instruments
High Speed Digital I/O Product Support Engineer - R&D