Pacing/Triggering of DMA
A data acquisition DMA process, whether input or output, requires an event to start transfers, an event to stop transfers, and a series of events to indicate when each successive transfer is to occur. The start and stop events are referred to as triggers, while the series of events are referred to as pacers. Typically, a DMA process will be either started or stopped by a software command, with the other event coming from software, some signal on the data acquisition boards, or the completion of a given number of transfers. DMA pacer events usually come from on-board timing sources, such as the rate generator on a PCI-20041C board or the burst generator on a PCI-20098C board.
DMA triggers can come from a number of sources. As mentioned above, the most common is from software commands. The MasterLink DMAStart and DMAStop functions are forms of DMA trigger events (though they perform other necessary tasks, as well). Hardware trigger signals can come from the external interrupt digital inputs on the PCI-20428W, PCI-20041C, and PCI-20098C boards, from the PCI-20020M (analog trigger) module, and from other sources. Each data acquisition board which supports DMA supports a different set of legal trigger sources. Different boards may also support different combinations of starting and stopping on triggers (that is, some boards may not support starting on triggers, while others may not support stopping on hardware triggers, but may support stopping on a given transfer count or under software control).
DMA pacers can also come from a number of sources, but software is typically not used for this function. The pacer function causes analog input (A/D) conversions to occur and the data from the various DMA channels to be transferred between the DMA buffer and the data acquisition board. The use of a hardware pacer allows regularly-spaced samples to be taken from each channel and is one of the advantages of DMA over CPU-controlled transfers. Since the pacing source can be running at high speed (compared to the rate at which CPU-controlled sampling can occur), hardware pacing of DMA allows higher-speed sampling. Each data acquisition board supports a different set of DMA pacer sources, depending upon its capabilities. The PCI-20428W boards provide separate rate generator channels for pacing analog input DMA and analog output DMA, while the PCI-20098C board provides a burst generator for the purpose of pacing analog input DMA.
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