National Instruments 370551A-01 用户手册
Chapter 6
NI-VXI Configuration Utility
6-16
ni.com
MXI Transfer Limit
Use this feature to control how many data transfers the PCI-MXI-2 will
perform on the MXIbus before releasing it to another master device that is
requesting use of the bus. The default setting holds the MXIbus for an
unlimited period of time.
perform on the MXIbus before releasing it to another master device that is
requesting use of the bus. The default setting holds the MXIbus for an
unlimited period of time.
The other options you can choose from are 256, 64, and 16 transfers. If you
do not want the PCI-MXI-2 to hold the MXIbus for an unlimited period of
time, you can use this control to select one of these values.
do not want the PCI-MXI-2 to hold the MXIbus for an unlimited period of
time, you can use this control to select one of these values.
Synchronous MXI
The MXIbus has a special high-speed block protocol for transferring large
blocks of data. This protocol, SYNC-MXI, locks the MXIbus during the
transfer, which prevents anyone else from using the bus.
blocks of data. This protocol, SYNC-MXI, locks the MXIbus during the
transfer, which prevents anyone else from using the bus.
VXImove()
uses
this protocol to transfer data.
If a VXI interrupt or signal comes in while a synchronous MXI move is
underway, there can be a problem. Using the SYNC-MXI protocol will
prohibit access to the MXI bus, which will prevent you from responding to
an interrupt, and prevent bus monitoring devices from accessing the bus at
regular intervals.
underway, there can be a problem. Using the SYNC-MXI protocol will
prohibit access to the MXI bus, which will prevent you from responding to
an interrupt, and prevent bus monitoring devices from accessing the bus at
regular intervals.
By default, this option is enabled—the VXImove uses Synchronous MXI
checkbox is checked. You can also disable SYNC-MXI programmatically
in
checkbox is checked. You can also disable SYNC-MXI programmatically
in
VXImove()
. However, if you have an older NI-VXI application that does
not disable SYNC-MXI programmatically, you can deselect the checkbox
to force your application to not use SYNC-MXI, if necessary.
to force your application to not use SYNC-MXI, if necessary.
MXI-2 Auto Retry
The PCI-MXI-2 has an automatic retry feature for cycles that map from the
MXI bus to the PCI bus. By default this option is enabled—the Enable
MXI-2 Auto Retry checkbox is checked.
MXI bus to the PCI bus. By default this option is enabled—the Enable
MXI-2 Auto Retry checkbox is checked.
Normally, when a cycle maps from the MXI bus to the PCI bus, any retry
response received on the PCI bus is passed to the MXI bus. When the
MXI-2 auto retry feature is enabled, the PCI-MXI-2 automatically retries
any PCI cycle when the PCI host responds to a cycle with a retry. The
PCI-MXI-2 automatically continues to retry the PCI cycle until it receives
either a DTACK or BERR response, which it then passes to the MXI bus.
This is the default situation because many external masters do not support
VXI/MXI retries. If the external master does support retries, it may be
response received on the PCI bus is passed to the MXI bus. When the
MXI-2 auto retry feature is enabled, the PCI-MXI-2 automatically retries
any PCI cycle when the PCI host responds to a cycle with a retry. The
PCI-MXI-2 automatically continues to retry the PCI cycle until it receives
either a DTACK or BERR response, which it then passes to the MXI bus.
This is the default situation because many external masters do not support
VXI/MXI retries. If the external master does support retries, it may be