Operation Theory
29
3.5
Data Transfers
The PCI Express bus offers dedicated bandwidth of up to 250 MB/
s. Unlike the PCI bus, which is a parallel bus architecture and
divides bandwidth all devices on the bus, PCI Express is a peer-
to-peer architecture and provides dedicated data pipeline. The
data transfer can occur at 2.5 Gb/s, which enables theoretical 250
MB/s bandwidth per lane. With PCI Express, the data bandwidth is
dramatically improved when compare to PCI bus. Data can be
streamed to system faster and the amount of onboard memory
can be reduced to minimum number.
One of the most important features of the PCIe-9842 is its PCI
Express x4 interface. PCIe-9842 equips a 200MS/s high sampling
rate ADC, it generates 400MByte data rate per second. Therefore
it is very useful to have high bandwidth bus interface when stream-
ing data from ADC to system memory.
The actual data throughput for a PC system will depend on the
system topology, data transfers between other devices in the sys-
tem and other components in the system. For example, data trans-
fer between digitizers and host memory usually travel through a
PCIe switch before transferring to host system. All digitizers share
the bandwidth available on the link between PCIe switch and the
host system.
To provide efficient data transfer, a PCI bus-mastering DMA is
essential for continuous data streaming, as it helps to achieve full
potential PCI Express bus bandwidth. The bus-mastering control-
ler releases the burden of the host CPU since data are directly
transferred to the host memory without intervention. Once analog
input operation begins, the DMA returns control of the program.
During DMA transfer, the hardware temporarily stores acquired
data in the onboard AD Data FIFO, and then transfers the data to
a user-defined DMA buffer in the computer.
By using a high-level programming library for high speed DMA
data acquisition, users simply need to assign the sampling period
and the number of conversions into their specified counters. After
the AD trigger condition is met, the data will be transferred to the
system memory by the bus-mastering DMA.
Содержание PCIe-9842
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