Dma Channel Selection; Master Mode Versus Slave Mode - National Instruments VXI-AT2023 Getting Started

Hardware Configuration and Installation

DMA Channel Selection

The AT-MXI interface board can use one, two, or none of the seven DMA channels of the PC AT
I/O channel. Setting up a DMA channel for operation involves two steps. First you select the DMA
channel by arranging the jumpers on an array of pins. Next you enable the DMA channel in the
system software.
Note: You must enable the DMA channels in the system software before they can function. If
you do not enable the DMA channel, that channel will not be driven and can be used by
other devices, regardless of the positions of the jumpers.
Select the DMA channels on the two 3 by 7 arrays of pins labeled W1 and W2, located above the
I/O card-edge connector on the AT-MXI (refer to Figure 2-1). Use the W1 array to select the DMA
request line(s), and use the W2 array to select the DMA acknowledge line(s). You must position
two jumpers to select a single DMA channel. The DMA ACKnowledge (DACKn) and DMA
ReQuest (DRQn) lines selected must have the same numeric suffix for proper operation. Therefore,
make sure that the jumper positions on the W1 array are identical to the jumper positions on the W2
array.

Master Mode Versus Slave Mode

The AT-MXI can function as both a MXIbus master and a MXIbus slave. As a MXIbus master, the
AT-MXI circuitry determines whether a PC AT cycle is to be mapped into a MXIbus cycle intended
for some external MXIbus device such as a VXIbus mainframe. As a MXIbus slave, the AT-MXI
circuitry determines whether an external device is attempting to access PC AT memory or I/O
resources.
Note: When allocating DMA channels for use by the AT-MXI, keep in mind that master-mode
and slave-mode operation are two distinct asynchronous functions and require different
DMA channels.
You must enable the slave-mode DMA channel to set up shared access to PC AT resources from
external MXIbus devices such as a VXI bus master in an external mainframe. If you intend to use a
VXI shared memory protocol that uses the PC AT memory, you must select and enable a slave-
mode DMA channel.
You can use the master-mode DMA channel to perform high-speed block-mode transfers to or from
external MXIbus devices. If you selected a master-mode DMA channel and enabled it in software,
any block-mode transfer that maps to or from the MXIbus will use the DMA Controller to perform
the block-mode move via that channel.
If you do not enable the master-mode DMA channel in software, the jumper-selected DMA channel
is not used by the AT-MXI and can be used by other devices. You can still perform block-mode
transfers by using the processor's movs (move string) command or by writing a move string
function. This does not necessarily mean that it will take any longer for the data to be transferred.
In fact, most PC ATs can transfer data faster using the movs instruction than they can using the
DMA controller. However, you may prefer to use the DMA controller if you have other useful
work to do during a block-mode operation (such as when using a multitasking operation system).
VXI-AT2023/NI-VXI ISC 386/ix Getting Started 2-8 © National Instruments Corporation
Chapter 2