National Instruments NI-VXI - Manual (Page 81)

Chapter 3 Software Overview
© National Instruments Corporation 3-43 NI-VXI User Manual
VXI Interrupt Functions
VXI/VME interrupts are a basic form of asynchronous communication
used by devices with interrupter support. In VME, a device asserts
a VME interrupt line and the VME interrupt handler device
acknowledges the interrupt. During the VME interrupt acknowledge
cycle, an 8-bit status/ID value is returned. Most 680x0-based VME
CPUs use this 8-bit value as a local interrupt vector value routed
directly to the 680x0 processor. This value specifies which interrupt
service routine to invoke.
In VXI systems, however, the VXI interrupt acknowledge cycle returns
(at a minimum) a 16-bit status/ID value. This 16-bit status/ID value is
data, not a vector base location. The definition of the 16-bit value is
specified by the VXIbus specification and is the same as for the VXI
signal. The lower 8 bits of the status/ID value form the VXI logical
address of the interrupting device, while the upper 8 bits specify the
reason for interrupting.
VXI status/ID values from message-based devices can be one of two
types: Response status/IDs and Event status/IDs (bit 15 distinguishes
between the two). Response status/IDs are used to report changes in
Word Serial communication status between a Servant and its
Commander. Event status/IDs are used to inform another device of
other asynchronous changes. The four Event status/IDs currently
defined by the VXIbus specification (other than Shared Memory
Events) are No Cause Given, Request for Service True (REQT),
Request for Service False (REQF), and Unrecognized Command.
REQT and REQF are used to manipulate the SRQ condition (RSV bit
assertion in the IEEE 488/488.2 status byte), while Unrecognized
Command is used to report unsupported Word Serial commands (only
in VXIbus specification, Revision 1.2). If the VXI interrupt status/ID
value is from a register-based device, the upper 8 bits are device
dependent. Consult your device manual for definitions of these values.
Because the VXI interrupt status/ID has the same format as the VXI
signal, your application can handle VXI interrupts as VXI signals.
However, because VME interrupters may be present in a VXI system,
the VXI/VME interrupt handler functions are included with the
NI-VXI software. The
RouteVXIint function specifies whether
the status/ID value should be handled as a signal or handled by a
VXI/VME interrupt handler. Two methods are available to handle VXI
signals. Signals can be handled either by calling a signal handler, or by
queueing on a global signal queue. The
RouteSignal function