NI-VISA用户手册：技术支持与资源指南

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"《NI-VISA用户手册》是National Instruments公司发布的一份技术文档，旨在帮助用户理解和使用NI-VISA软件。这份手册适用于1996年2月的版本，编号为321074A-01。手册包含了对NI-VISA的详细解释，以及如何与各种接口如GPIB、DAQ、VXI、LabVIEW、LabWindows、HiQ等进行通信的指导。此外，手册还提供了多渠道的技术支持，包括电子邮件、FTP站点、网页地址以及电话和传真支持。用户可以通过这些联系方式获取到相关的帮助服务。" NI-VISA是一种虚拟仪器标准应用编程接口（API），它允许开发者通过编程控制各种仪器设备，如数据采集系统、GPIB、VXI、PXI、USB和以太网设备等。这个接口在各种操作系统上都有支持，包括Microsoft Windows、Linux和Mac OS。NI-VISA提供了一套丰富的函数库，使得开发者可以使用C、C++、VB、Python等语言来编写应用程序，实现与硬件设备的通信。在《NI-VISA用户手册》中，用户可以学习到如何安装和配置NI-VISA，理解基本的VISA概念，如资源名称、会话管理和错误处理。手册通常包含以下关键部分： 1. **入门指南**：介绍NI-VISA的基本概念和安装步骤，帮助初学者快速上手。 2. **VISA函数库**：详细列出所有可用的VISA函数，包括其功能、参数和返回值。 3. **示例代码**：提供实际编程示例，展示如何使用VISA函数进行设备通信。 4. **资源管理**：讲解如何识别和配置仪器设备，包括资源名称的解析和创建。 5. **事件处理**：说明如何设置和响应设备事件，以实现异步通信。 6. **错误处理**：详细阐述VISA错误代码和如何处理运行时错误。 7. **高级主题**：涵盖更复杂的应用场景，如多线程、网络通信和性能优化。手册还可能包含关于特定硬件接口的详细信息，如GPIB（通用接口总线）的使用方法，以及如何通过DAQ（数据采集）设备进行数据采集和处理。同时，National Instruments提供了多种联系方式，包括电子邮箱、电话和传真，以提供全球范围内的技术支持，确保用户在遇到问题时能够得到及时的帮助。《NI-VISA用户手册》是开发者和工程师们掌握NI-VISA的关键参考资料，它提供了全面的指南，帮助用户高效地开发和调试仪器控制程序。通过深入学习这份手册，用户不仅可以理解VISA的工作原理，还能熟练运用VISA进行实际的仪器控制和数据采集任务。

Chapter 1 Introduction

NI-VISA User Manual 1-4 © National Instruments Corporation

Table 1-1. NI-VISA Support (Continued)

Operating

System

Programming Language/

Environment

Framework

HP-UX

ANSI C HPUX

LabVIEW GHPUX

Mac 68K

ANSI C *

Mac PPC LabVIEW *

* This framework is not defined, but is still supported by NI-VISA.

You may find that programming with NI-VISA is not significantly

different from programming with the I/O software products that are

currently available. However, the programming concepts, model, and

paradigm NI-VISA uses create a solid foundation for taking advantage

of VISA’s more powerful features in the future.

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.

Chapter 2 Introductory Programming Examples

NI-VISA User Manual 2-4 © National Instruments Corporation

4. Now that you have the communication channel set up, you can

perform string I/O using the viWrite() and viRead() functions.

Notice that this is the section of the programming code that is

unique for message-based communication. Opening

communication channels, as described in steps 1 and 2, and closing

the channels, as described in step 5 are the same for all VISA

programs. The parameters that these calls use are relatively

straightforward.

a. First you identify which device you are talking to with instr.

b. Next you give the string to send, or what buffer to put the

response in.

c. Finally, specify the number of characters you are interested in.

For more information on these functions, see Chapter 4,

Message-Based Communication. Also refer to the NI-VISA

Programmer Reference Manual.

5. When you are finished with your device I/O, you can close the

communication channel to the device with the viClose()

function.

Notice that the program shows a second call to viClose(). When

you are ready to shut down the program, or at least close down the

VISA driver, you use viClose() to close the communication

channel returned by viOpenDefaultRM().

Example of Register-Based Communication

Note: You can skip over this section if you are exclusively using GPIB or serial

communication. Register-based programming applies only to VXI or

GPIB-VXI.

VXI has two standard methods for accessing registers. The first method

uses High-Level Access functions. You can use these functions to

specify the address to access; the functions then take care of the

necessary details to perform the access, from mapping an I/O window

to checking for failures. The drawback to using these functions is the

amount of software overhead associated with them.

To reduce the overhead, VISA also has Low-Level Access functions.

These functions break down the tasks done by the High-Level Access

functions and let the program perform each task itself. The advantage is

that you can optimize the sequence of calls based on the style of

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