National Instruments VXI-1394 - Manual (Page 30)

Chapter 3 Developing Your Application
© National Instruments Corporation 3-9 VXI-1394 User Manual for Windows
Compatibility Layer Options
Although NI-VXI supports multiple VXI controllers through NI-VISA, the
NI-VXI API supports only a single controller. To specify which controller
the emulation layer should use, run MAX. Select Tools»NI-VXI»VXI
Options. Select the VXI system that will support the emulation layer.
In NI-VXI 3.0 or later, when you enable for triggers or interrupts, only the
local controller is enabled. In the NI-VXI API functions for enabling
triggers and interrupts, the controller parameter is ignored. If you need to
enable a remote controller for triggers, use the MAX frame resource to map
the trigger back to the local controller.
The interrupt and trigger routing in the NI-VXI 3.0 or later low-level
drivers is somewhat different from the default routing in previous versions
of NI-VXI. Therefore, the compatibility layer may behave differently than
the original NI-VXI API with regard to these settings. In particular, if you
are receiving triggers on an external controller, you may need to modify the
trigger configuration on your extender module using MAX. In general,
interrupts are routed automatically based on the interrupt configuration the
resource manager detects. Whether the changed routing behavior affects
your program is application dependent.
Because VISA is an instrument-centric API, certain functions from the
more controller-centric NI-VXI API do not match perfectly with a VISA
counterpart. When an application enables an event with the NI-VXI API
compatibility layer, each logical address is enabled for that event
separately. For example, if the application enables an interrupt level, VISA
will enable the interrupt on each logical address, one at a time, until all the
devices are enabled. This means that some interrupts could be lost from
devices with higher numbered logical addresses. MAX provides an
option for users to pick which logical address is enabled first. Select
Tools»NI-VXI»VXI Options. Set Prioritized Signal LA to the logical
address of the device that generates the events. This prevents possible loss
of events from that device.