National Instruments 6711 User Manual

Also see for DAQCard-6715: User manual User manual

Contents

National Instruments 6711 User Manual Manual pdf 63 page image

9-2 | ni.comChapter 9 Bus InterfaceData Transfer MethodsThere are three primary ways to transfer data across the PCI bus: Direct Memory Access (DMA),Interrupt Request (IRQ), and Programmed I/O.Direct Memory Access (DMA)DMA is a method to transfer data between the device and computer memory without theinvolvement of the CPU. This method makes DMA the fastest available data transfer method.National Instruments uses DMA hardware and software technology to achieve high throughputrates and to increase system utilization. DMA is the default method of data transfer for DAQdevices that support it.Interrupt Request (IRQ)IRQ transfers rely on the CPU to service data transfer requests. The device notifies the CPUwhen it is ready to transfer data. The data transfer speed is tightly coupled to the rate at whichthe CPU can service the interrupt requests. If you are using interrupts to acquire data at a ratefaster than the rate the CPU can service the interrupts, your systems may start to freeze.Programmed I/OProgrammed I/O is a data transfer mechanism where the user’s program is responsible fortransferring data. Each read or write call in the program initiates the transfer of data.Programmed I/O is typically used in software-timed (on demand) operations.Changing Data Transfer Methods between DMAand IRQThere are a limited number of DMA channels per device (refer to the specifications documentfor your device). Each operation (such as AI, DIO, and so on) that requires a DMA channel usesthat method until all of the DMA channels are used. When all of the DMA channels are used,you will get an error if you try to run another operation requesting a DMA channel. Ifappropriate, you can change one of the operations to use interrupts. For NI-DAQmx, use theData Transfer Mechanism property node. For Traditional NI-DAQ (Legacy), use the Set DAQDevice Information VI or function.