使用NAT9914设计GPIB设备

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"Designing a GPIB Device Using the NAT9914.pdf" 本文档是National Instruments公司发布的一份应用笔记，主要介绍了如何使用NAT9914芯片和通用单片机来设计GPIB（General Purpose Interface Bus，通用接口总线）设备。GPIB是一种广泛用于仪器控制的标准接口，常在电子测试和测量领域中应用。 NAT9914是一款专为GPIB设计的集成电路，它集成了GPIB协议所需的许多功能，如数据收发、总线仲裁以及错误检测等。在本应用笔记中，作者Andrew Thomson详细阐述了利用NAT9914构建GPIB设备的硬件连接和软件设计方法。文档首先介绍了设计GPIB设备的基本步骤和考虑因素，包括性能需求的确定。性能要求可能受传输速度、总线竞争策略、错误处理机制等因素影响。设计师需要根据具体的应用场景来权衡这些因素，确保设备能与现有的GPIB网络兼容并高效运行。接着，文档深入讨论了实现GPIB命令的技术细节。GPIB协议包含了一系列命令和响应机制，用于设备间的通信和控制。NAT9914提供了相应的硬件支持，允许开发者通过编程控制这些命令的发送和接收。此外，文档还提供了一个灵活的结构框架，用于将GPIB接口集成到用户的仪器中。这个框架包括了初始化、数据传输、命令解析等关键模块，帮助开发者快速搭建自己的GPIB系统。附录A包含了关于GPIB的全面教程，包括历史概述，有助于读者更好地理解GPIB的背景和发展。附录B则提供了文中所描述的程序代码清单，便于实际开发参考。 GPIB的详细规范定义在IEEE 488.1和IEEE 488.2标准中，包括总线电气特性、机械特性以及控制信号的使用等。NAT9914参考手册进一步解释了该芯片的命令集和寄存器，为开发者提供了更具体的硬件和软件指导。这份"Designing a GPIB Device Using the NAT9914"是工程师设计GPIB设备的重要参考资料，涵盖了从硬件设计到软件实现的全过程，对具备一定单片机编程基础的读者来说，是构建GPIB接口的实用指南。

Remote Enable (REN)

The System Controller uses the REN line to put devices into a remote state. Each device has its own remote/local state

capabilities. The IEEE 488 standard requires a device to go into a remote programming state whenever the REN line

is asserted and addressed to listen.

End-or-Identify (EOI)

Some devices terminate their output data by using the EOI line. A Talker asserts EOI along with the last byte of data.

A Listener stops reading data when the EOI is asserted. More details of transfer termination are presented later. This

line is also used in parallel polling, which will be discussed later.

Service Request (SRQ)

A device asserts the SRQ line at any time in order to notify the CIC that it needs service. The SRQ line remains

asserted until the device is serial polled. The Controller

must monitor SRQ, poll the device, and determine the type of

service the device needs.

Handshake Lines

Three lines asynchronously control the transfer of message bytes among devices:

• Not Ready For Data (NRFD)

• Not Data Accepted (NDAC)

• Data Valid (DAV)

Remote Enable

(REN)

• Enables devices for remote programming.

• Is asserted by System Controller.

– 300.0005 V

End Or Identify

(EOI)

• Signals end of data.

• Signals the execution of a Parallel Poll.

– 300.0005 V

• Is asserted by current Talker.

Service Request

(SRQ)

• Alerts Controller that service is needed.

• Is asserted by Non-Controller.

– 300.0005 V

The GPIB uses a three-wire interlocking handshake scheme. This handshake scheme guarantees that message bytes

on the data lines are sent and received without transmission error.

Not Ready For Data (NRFD)

The NRFD line indicates whether a device is ready to receive a data byte. When a Controller is sending commands,

all devices drive NRFD. When a Talker is sending data messages, only Listeners drive NRFD.

Not Data Accepted (NDAC)

The NDAC line indicates whether a device has accepted a data byte. When a Controller is sending commands, all

devices drive NRFD. When a Talker is sending data messages, only Listeners drive NRFD.

Note:

This handshake scheme limits the transfer rate on the GPIB to that of the slowest active

Listener. The transfer rate is limited because a Talker waits until all Listeners are ready

(that is, NRFD is false) before sending data and waits for all Listeners to accept data (that

is, NDAC is false) before transferring more data. Therefore, the slowest device dictates

the maximum GPIB transfer rate.

Data Valid (DAV)

The DAV line indicates whether signals on the data lines are stable (valid) and whether devices can safely accept the

signals. When the Controller sends commands, it controls DAV, and when the Talker sends data messages, it controls

DAV.

Figure A-5 illustrates the three-wire handshake process.

Figure A-5.

Three-Wire Handshake Process

GPIB devices use the three-wire handshake process to transfer information. The three-wire handshake process is

identical for command and data transfers. During command transfers, the Controller drives the DIO and DAV lines;

all devices drive the NRFD and NDAC lines. During data transfers, the Talker drives the DIO and DAV lines; all

Listeners drive the NRFD and NDAC lines.

Devices drive the NDAC and NRFD lines with open-collector drivers, so if any device drives NDAC (or NRFD) to a

low voltage level, the signal is logically asserted (true). If no device drives NDAC (or NRFD) to a low voltage level,

the signal floats to a high voltage level; thus, the signal is logically unasserted (false).

Data

DAV

New Data is Valid

Don't Send More Yet

Ready

NRFD

NDAC

Byte

Accepted

Data

Transfer

Begins

Data

Transfer

Ends

Ready

Again

Data Not Valid Anymore

The GPIB uses negative logic with standard TTL voltage levels:

Logic Level

0 – false (unasserted)

Voltage Level

> +2.0 V (high)

Talker

{

Listener

{

1 – true (asserted)

< +0.8 V (low)

The following actions occur during the three-wire handshake process (refer to Figure A-5):

1. The Talker (or Controller) places data on the DIO lines and waits at least T1 seconds.

2. After the T1 delay, the Talker waits until the Listener unasserts NRFD. NRFD unasserted (not Not-Ready-For

Data) indicates that the Listener can receive the data byte.

3. The Talker asserts DAV to indicate that new data is valid on the DIO lines.

4. The Listener asserts NRFD to signal a Not Ready Status (Don't Send More Yet).

5. When the Listener accepts the current byte (by placing it in some internal buffer or by otherwise processing it),

the Listener unasserts NDAC.

6. The Talker unasserts DAV.

7. The Listener asserts NDAC, then the Talker executes step 1 to begin transferring the next byte.

Physical and Electrical Specifications

To achieve the GPIB’s high data transfer rate, you must limit the physical distance between devices and the number

of devices on the bus. This limitation is necessary because the GPIB is a transmission line system. Any distance

beyond the maximum allowable cable length, as well as any excess GPIB device loads, can surpass interface circuit

drive capability.

The IEEE 488 standard dictates the following limits:

• The total length of all cables is less than or equal to 2 m times the number of connected devices—up to a total

of 20 m.

• No more than 15 devices are connected to each bus, with at least two-thirds of the devices powered on.

If you must exceed these limits, you can purchase bus extenders and expanders.

Controllers, Talkers, and Listeners

All buses operate under rules that ensure that data passes reliably and that instruments do not use the bus

simultaneously. To determine which device has active control of the bus, devices are categorized as Controllers,

Talkers, or Listeners. Whenever two devices communicate, one device will be a Talker and the other will be a Listener.

In addition, one device will always be a Controller.

Controllers

Most GPIB systems consist of one computer and a variety of instruments. In this type of system, the computer is

typically the System Controller. If multiple computers are connected, several devices can have Controller capability,

but only one Controller is active, or Controller-In-Charge (CIC), at a time. Active control can pass from the current

CIC to an idle Controller.

For each GPIB system, you must define a System Controller. You usually define the System Controller through jumper

settings on the GPIB interface board, a software configuration file, or both. Only one device on the bus, the System

Controller, can make itself the CIC.

The four primary responsibilities of a Controller are the following:

• Defining the communication links.

• Responding to devices requesting service.

• Sending GPIB commands.

• Passing/receiving control.

Data and Command Messages

In a classroom, when the instructor tells the students who is the Talker and who are the Listeners, his or her information

is a command—not the actual data information that the instructor will send. On the GPIB, this distinction is not so

intuitive. The bus management line, ATN, determines what type of message you are sending on the bus. If this line

is unasserted, the information on the bus is a data message; if this line is asserted, the information is a command

message from the Controller to all devices. The devices on the GPIB monitor the ATN line, determine the data type,

and treat the data appropriately.

GPIB Addressing Protocol

In a classroom, an instructor either speaks to the entire class or to a particular student. To speak to a student, the

instructor first addresses that student by name.

Addressing on the GPIB follows the same idea. Before any communication can take place on the bus, you must address

the Talker and Listener. Before any data passes between devices, the Controller determines who talks and who listens.

In the classroom, we address people by their names. However, on the GPIB, each device (including the Controller)

has a unique primary GPIB address in the range of 0 to 30 (decimal). The Controller places a command message spec-

ifying the addresses of the Talker and Listener devices on the bus.

The Controller sends a single byte (8 bits) of information for a Talker or Listener address command message. Address

command messages have the following format:

Bits 0 through 4 contain the binary GPIB primary address of the device in communication, and either bit 5—Listener

Address (LA)—or bit 6—Talker Address (TA)—will be set if the device is a Talker or a Listener. Bit 7 is never used

and is considered a don’t care bit. For simplicity, assume bit 7 is zero.

Consider an example in which a Controller at primary GPIB address 0 talks to a device at primary GPIB address 1. To

establish the communication link, the Controller must send its GPIB talk address and the device's listen address over

the GPIB. In this example, these addresses are as follows:

Bit

Data

76543210

TALAXXXXX

Bit Patterns Sent to Set Up Talker

Bit pattern: 01000000

Talker's GPIB Address is 0

Hexadecimal value:

Hex 40 = ASCII "@"

00000

ADRTA

010

0100

0000

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