Chapter 2
Theory of Operation
©
National Instruments Corporation
2-7
The DAQ-PnP provides the main interface to the ISA bus, including
support for the Plug and Play ISA Specification. Plug and Play provides a
mechanism for assigning bus resources to the card using software only.
This eliminates the need for setting jumpers or DIP switches on the board
to select the base I/O address, interrupt lines, and DMA channels. In a
Plug and Play aware system, the Plug and Play circuitry in the DAQ-PnP
can request a base I/O address, up to three DMA channels, and up to
two interrupt lines from the configuration manager. In other systems,
application software can simply assign particular resources to the card.
The DAQ-PnP stores the resource assignments in the Plug and Play
configuration registers. It also performs the base I/O address decoding
after the board is activated through software. The DAQ-PnP performs full
16-bit address decoding, where address lines SA<15..5> are used for the
comparison. This decoding allows the board to be located on any 32-byte
boundary within the 16-bit address space. The remaining address lines
SA<4..0> are used to select the onboard registers.
The register interface block within the DAQ-PnP is used to select the target
register of a particular read or write cycle. These accesses are controlled by
the base address decoding, SA<4..0> and the ISA control lines when the
CPU is trying to access the board. In the case of DMA transfers, the access
is controlled by the DMA block within the DAQ-PnP and the ISA control
lines. The register interface block provides all access to the registers in the
DAQ-PnP, DAQ-STC, and other onboard resources such as the FIFOs.
The DMA interface block within the DAQ-PnP provides for high-speed
data transfer between the board and the system memory. Up to three DMA
channels can be used simultaneously for data transfers with analog input,
analog output, and the general-purpose counters. The DMA circuitry can
only perform 16-bit transfers—DMA channels 5, 6, and 7 are available in
ISA computers, whereas channels 0, 1, 2, 3, 5, 6, and 7 are available in
EISA computers. The DMA transfers are initiated by the DAQ-STC, which
indicates to the DAQ-PnP when the data transfers should occur. The
DAQ-PnP then asserts the appropriate DMA request line and performs the
transfer when the DMA acknowledge occurs.
The DAQ-STC can generate interrupts from over 20 sources and can route
these interrupts to one or two of eight interrupt lines on the ISA bus
interface. The use of two interrupt lines can improve performance in some
environments but is usually more difficult to program. The AT E Series
boards connect the eight DAQ-STC interrupt lines (0..7) to the IRQ3, 4, 5,
7, 10, 11, 12, and 15 lines, respectively.
Содержание AT-AI-16XE-10
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