Chapter 9
Bus Interface
Analog Output Series User Manual
9-2
ni.com
Caution
Damage can result if these lines are driven by the sub-bus. NI is
not
liable for any
damage resulting from improper signal connections.
Data Transfer Methods
There 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 the involvement of the CPU. This method makes DMA
the fastest available data transfer method. National Instruments uses DMA
hardware and software technology to achieve high throughput rates and to
increase system utilization. DMA is the default method of data transfer for
DAQ devices that support it.
Interrupt Request (IRQ)
IRQ transfers rely on the CPU to service data transfer requests. The device
notifies the CPU when it is ready to transfer data. The data transfer speed
is tightly coupled to the rate at which the CPU can service the interrupt
requests. If you are using interrupts to acquire data at a rate faster than the
rate the CPU can service the interrupts, your systems may start to freeze.
Programmed I/O
Programmed I/O is a data transfer mechanism where the user’s program is
responsible for transferring 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 DMA and IRQ
There are a limited number of DMA channels per device (refer to the
specifications document for your device). Each operation (such as AI, DIO,
and so on) that requires a DMA channel uses that 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.
If appropriate, you can change one of the operations to use interrupts.
For NI-DAQmx, use the
Data Transfer Mechanism
property node. For
Traditional NI-DAQ (Legacy), use the
Set DAQ Device Information VI
or function.