Data Rate Multiplier
The NI 656x can be configured for single data rate (SDR) or double data rate (DDR) operation by setting the data rate multiplier for your acquisition and/or generation session in NI-HSDIO. The data rate multiplier can be independently configured for the generation and acquisition sessions.
Single Data Rate (SDR)
When the data rate multiplier is configured for SDR operation, generation and acquisition sessions can run on all 16 channels of the NI 656x. Direction control is software-specified.
Double Data Rate (DDR)
When the data rate multiplier is configured for DDR operation, the digital waveform generator/analyzer trades channel count for data rate by generating or acquiring on half the number of channels but at twice the rate. This rate is achieved by generating data on both the rising and falling edge of the Sample clock. For generation sessions, the device generates data on the lower eight channels of the NI 656x (DIO <0..7>), and for acquisition sessions the device acquires data on the upper eight channels (DIO <8..15>).
DDR mode has some important implications for some NI 656x functionality.
- Memory Usage—Memory size in samples is effectively doubled since the data width and channel count are halved.
- Marker Positions—Marker positions have a quantization twice that of SDR mode. Refer to the NI 656x specifications for more information about quantization.
- Waveform Sizes—The size of the waveforms you save to the onboard memory have a quantization twice that of SDR mode. Refer to the NI 656x specifications for more information about quantization.
- Initial/Idle States—If a channel's Idle state is configured for "hold last value," the last value held is the last DDR data sample.
- Data Width—Data width is a function of your data rate multiplier. Since data width refers to how large your sample is in bytes, using DDR mode effectively halves your allowable data width. For NI 656x devices, SDR operation would have a data width of 2 bytes per sample while DDR mode would have a data width of 1 byte per sample.
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