HSDIO Data Width and Memory Allocation Hardware: Digital I/O (DIO)>>High-Speed
Problem: What is the definition of data width with regards to my HSDIO device, and how does it affect the memory allocation per channel? Solution: Data width determines the size of each sample, and can be 1, 2, or 4 bytes. The sample size will determine the number of usable channels per device. For example, if the user configures the acquisition for a data width of 1 byte, data would be acquired on 8 channels (channels 0-7). Along the same lines, if a data width of 2 is selected, data would be acquired on 16 channels (channels 0-15). The following figure illustrates this concept.
One key point to take away is only the acquisition data width can be set. The generation data width is fixed for each device. By default, the data width is always set to include all the channels on the device. For example, the NI-6541 is a 32 channel device and has a default data width of 4 bytes. The following table shows the default and valid data widths for the 654x, 655x, and 656x series. (Note that the double data rate for the 656x series only allows a data width of 1, as there are 8 channels dedicated to acquisition and 8 channels dedicated to generation).
Acquisition
For HSDIO acquisition, the sample storage per channel is determined by the amount of available physical memory (Mb/ch) and the configured data width. Using this logic, the user may increase the number of samples that can be stored on onboard memory by reducing the data width to only include the channels needed.
For example, suppose a user is acquiring digital data on channels 0-5 of a NI-6552 which has the 8 Mb/ch memory option, and wants to increase the onboard memory allocated to these channels. The user would want to change the data width from the default of 4 bytes to 1 byte (channels 0-7), using the HSDIO property node shown below (Property >> Advanced >> Data Width), thus increasing the memory from 8 Mb/ch to 32 Mb/ch.
The following table shows the resulting memory size per channel by reducing the data width.
Generation
Likewise, for HSDIO generation, the sample storage per channel is also determined by the amount of physical memory and data width. However, the data width is set by the device, and is not available to be modified. The following table shows the memory size per channel for each device.
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Report Date: 01/05/2007 Last Updated: 01/18/2007 Document ID: 454AKQJQ |
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