National Instruments NI 6711 User Manual

Also see for DAQCard-6715: User manual User manual

Contents

National Instruments NI 6711 User Manual Manual pdf 76 page image

Chapter 9 Bus InterfaceAnalog Output Series User Manual 9-2 ni.comCaution Damage can result if these lines are driven by the sub-bus. NI is not liable for anydamage resulting from improper signal connections.Data Transfer MethodsThere are three primary ways to transfer data across the PCI bus: DirectMemory Access (DMA), Interrupt Request (IRQ), and Programmed I/O.Direct Memory Access (DMA)DMA is a method to transfer data between the device and computermemory without the involvement of the CPU. This method makes DMAthe fastest available data transfer method. National Instruments uses DMAhardware and software technology to achieve high throughput rates and toincrease system utilization. DMA is the default method of data transfer forDAQ devices that support it.Interrupt Request (IRQ)IRQ transfers rely on the CPU to service data transfer requests. The devicenotifies the CPU when it is ready to transfer data. The data transfer speedis tightly coupled to the rate at which the CPU can service the interruptrequests. If you are using interrupts to acquire data at a rate faster than therate 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 isresponsible for transferring data. Each read or write call in the programinitiates the transfer of data. Programmed I/O is typically used insoftware-timed (on demand) operations.Changing Data Transfer Methods between DMA and IRQThere are a limited number of DMA channels per device (refer to thespecifications document for your device). Each operation (such as AI, DIO,and so on) that requires a DMA channel uses that method until all of theDMA channels are used. When all of the DMA channels are used, you willget an error if you try to run another operation requesting a DMA channel.If appropriate, you can change one of the operations to use interrupts.For NI-DAQmx, use the Data Transfer Mechanism property node. ForTraditional NI-DAQ (Legacy), use the Set DAQ Device Information VIor function.