Hardware Configuration and Installation
Chapter 2
VXI-AT2000/NI-VXI MS-DOS Getting Started
2-8
© National Instruments Corporation
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.
Although the master-mode DMA jumpers are set at the factory to use DMA channel 6, the
software statically disables the master-mode DMA channel with a setting of
NONE
and cannot be
changed. While the AT-MXI itself is capable of using DMA to perform high-speed block-mode
transfers to or from external MXIbus devices, the processor is able to transfer data at
significantly faster rates by using the
movs
(move string) instruction. DMA transfers offer
superior performance only in asynchronous function calls, which begin an operation and return
immediately, freeing the processor to do other useful work. However, because DMA transfers
would be applicable only for the
VXImove
function, which is not asynchronous, the software is
configured to ignore this option, making the hardware setting irrelevant. The field remains in the
VXIEDIT
software configuration program for backward compatibility only.
The slave-mode DMA jumpers are set at the factory to use DMA channel 3. Although the
default software configuration enables this setting, several factors effectively disable the slave-
mode DMA channel. As a result, DMA channel 3 can be used by other devices if you do not
need to use slave-mode DMA with your AT-MXI.
The slave-mode DMA channel is necessary only for setting up shared access to PC AT resource
from external MXIbus devices. If you intend to communicate with VXI devices using a shared
