NI-VISA用户手册：API应用编程接口指南

NI-VISA

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Chapter 1 Introduction

All VXIplug&play products are classified within a framework. The

concept of a framework was developed by the VXIplug&play Systems

Alliance to categorize operating systems, programming languages, and

I/O software libraries to bring the most useful products to the most end-

users. A framework is a logical grouping of the choices that you face

when designing a test and measurement system. You must always

choose an operating system and a programming language along with an

application development environment (ADE) when building a system.

There are tradeoffs associated with each of these decisions; many

configurations are possible. The VXIplug&play Systems Alliance

grouped the most popular operating systems, ADEs, and programming

languages into distinct frameworks and defined in-depth specifications

to guarantee interoperability of the components within each framework.

This manual and the NI-VISA Programmer Reference Manual describe

how to use NI-VISA, the National Instruments implementation of the

VISA I/O standard, in any environment using LabWindows

/CVI, any

ANSI C compiler, or Microsoft Visual Basic. NI-VISA currently

supports the frameworks and programming languages shown in

Table 1-1. For information on programming VISA from LabVIEW,

refer to the VISA documentation included with your LabVIEW

software.

Table 1-1. NI-VISA Support

Operating

System

Programming Language/

Environment

Framework

Windows 3.x

LabWindows/CVI, ANSI C,

Visual Basic

WIN

LabVIEW GWIN

Windows 95

LabWindows/CVI, ANSI C,

Visual Basic

WIN95

LabVIEW GWIN95

Windows NT

LabWindows/CVI, ANSI C,

Visual Basic

WINNT

LabVIEW GWINNT

Solaris 1.x

LabWindows/CVI, ANSI C SUN

Solaris 2.x LabVIEW GSUN

(continues)

Chapter 2 Introductory Programming Examples

Example 2-1 Discussion

We can break down Example 2-1 into the following steps.

1. Begin by initializing the VISA system. For this task you use

viOpenDefaultRM(), which opens a communication channel

with VISA itself. This channel has a purpose similar to a telephone

line. The function call is analogous to picking up the phone and

dialing the operator. From this point on, the phone line connects

you to the driver. Any communication on the line is between you

and the driver only. Chapter 3 will give you more details about the

viOpenDefaultRM() function, but for now it is sufficient for you

to understand that it initializes VISA and must be the first VISA

function called in your program.

2. Now you must open a communication channel to the device itself

using viOpen(). Notice that this function uses the handle returned

by viOpenDefaultRM() to identify the VISA driver. You then

specify the address of the device you want to talk to. Continuing

with the phone analogy, this is like asking the operator to dial a

number for you. In this case, you want to address a GPIB device at

primary address 1 on the GPIB0 bus. The value for x in the GPIBx

token (GPIB0 in this example) indicates which GPIB board you

want. This means that you can have multiple GPIB boards installed

in the computer, each controlling independent buses. For more

information on address strings, viOpen(), and

viOpenDefaultRM(), see Chapter 6, VISA Resource Manager.

The two VI_NULLs following the address string are not important

and are currently always VI_NULL. These parameters are reserved

for definition by a future version of VISA. Finally, viOpen()

returns the communication channel to the device in the parameter

instr. From now on, whenever you want to talk to this device,

you use the instr variable to identify it. Notice that you do not

use the defaultRM handle again. The main use of defaultRM is

to open channels to devices. You do not use this handle again until

you are ready to end the program.

3. At this point you need to set a timeout value for message-based

communication. A timeout value is important in message-based

communication to determine what should happen when the device

stops communicating. VISA has a common function to set values

such as these: viSetAttribute(). This function sets values

such as timeout and the termination character for the

communication channel. In this example, notice the function call to

viSetAttribute() set the timeout to be 5 s (5000 ms) for both

reading and writing strings.

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