National Instruments NI-VXI - Manual (Page 17)

Chapter 1 Overview of NI-VXI
NI-VXI User Manual 1-4 © National Instruments Corporation
a fashion very similar to IEEE 488 instruments. In general,
message-based devices typically contain some level of local
intelligence that uses or requires a high level of communication. In
addition, the Word Serial Protocol has messages for configuring
message-based devices and system resources.
All VXI message-based devices are required to use the Word Serial
Protocol and communicate in a standard way. The protocol is called
word serial, because if you want to communicate with a message-based
device, you do so by writing and reading 16-bit words one at a time to
and from the Data In (write Data Low) and Data Out (read Data Low)
hardware registers located on the device itself. Word serial
communication is paced by bits in the device’s response register that
indicate whether the Data In register is empty and whether the Data
Out register is full. This operation is very similar to the operation of a
Universal Asynchronous Receiver Transmitter (UART) on a serial port.
Commander/Servant Hierarchies
The VXIbus specification defines a Commander/Servant
communication protocol you can use to construct hierarchical systems
using conceptual layers of VXI devices. The resulting structure is like a
tree. A Commander is any device in the hierarchy with one or more
associated lower-level devices, or Servants. A Servant is any device in
the subtree of a Commander. A device can be both a Commander and a
Servant in a multiple-level hierarchy.
A Commander has exclusive control of its immediate Servants’ (one or
more) communication and configuration registers. Any VXI module
has one and only one Commander. Commanders use the Word Serial
Protocol to communicate with Servants through the Servants’
communication registers. Servants communicate with their
Commander, responding to the Word Serial commands and queries
from their Commander. Servants can also communicate asynchronous
status and events to their Commander through hardware interrupts, or
by writing specific messages directly to their Commander’s Signal
register.
Interrupts and Asynchronous Events
Servants can communicate asynchronous status and events to their
Commander through hardware interrupts or by writing specific
messages (signals) directly to their Commander’s hardware Signal