GPIB Overview
What Is GPIB?
GPIB is a standardized interface that allows you to connect and control multiple devices from various vendors. GPIB is also referred to by its original name HP-IB, or by its IEEE® designation IEEE-488. GPIB functionality has evolved over time, and is described in several specifications:
The IEEE 488.1-1975 specification defines the electrical and mechanical characteristics of the interface and its basic functional characteristics.
The IEEE-488.2-1987 specification builds on the IEEE 488.1 specification to define an acceptable minimum configuration and a basic set of instrument commands and common data formats.
The Standard Commands for Programmable Instrumentation (SCPI) specification builds on the commands given by the IEEE 488.2 specification to define a standard instrument command set that can be used by GPIB or other interfaces.
For many GPIB applications, you can communicate with your instrument without detailed knowledge of how GPIB works. Communication is established through a VISA-GPIB object, which you create using visadev.
If your application is straightforward, or if you are already familiar with the GPIB specifications, you can begin with Get Started with GPIB Interface. If you want a high-level description of all the steps you are likely to take when communicating with your instrument, refer to Creating Instrument Objects.
Some of the GPIB functionality is required for all GPIB devices, while other GPIB functionality is optional. Additionally, many devices support only a subset of the SCPI command set, or use a different vendor-specific command set. Refer to your device documentation for a complete list of its GPIB capabilities and its command set.
Important GPIB Features
The important GPIB features are described in the following sections. For detailed information about GPIB functionality, see the appropriate references in the Bibliography.
Bus and Connector
The GPIB bus is a cable with two 24-pin connectors that allowyou to connect multiple devices to each other. The bus and connectorhave these features and limitations:
You can connect up to 15 devices to a bus.
You can connect devices in a star configuration, alinear configuration, or a combination of configurations.
To achieve maximum data transfer rates, the cablelength should not exceed 20 meters total or an average of 2 metersper device. You can eliminate these restrictions by using a bus extender.
GPIB Devices
Each GPIB device must be some combination of a Talker,a Listener, or a Controller.A Controller is typically a board that you install in your computer.Talkers and Listeners are typically instruments such as oscilloscopes,function generators, multimeters, and so on. Most modern instrumentsare both Talkers and Listeners.
Talkers — A Talker transmits data over the interface when addressed to talk by the Controller. A GPIB system can contain only one Talker at a time.
Listeners — A Listener receives data over the interface when addressed to listen by the Controller. A GPIB system can contain up to 14 Listeners at a given time. Typically, the Controller is a Talker while one or more instruments on the GPIB are Listeners.
Controllers — The Controller specifies whichdevices are Talkers or Listeners. A GPIB system can contain multipleControllers. One of them is designated the System Controller. However,only one Controller can be active at a given time. The current activecontroller is the Controller-In-Charge (CIC). The CIC can pass controlto an idle Controller, but only the System Controller can make itselfthe CIC.
When the Controller is not sending messages, then a Talker cansend messages. Typically, the CIC is a Listener while another deviceis enabled as a Talker.
Each Controller is identified by a unique board index number.Each Talker/Listener is identified by a unique primary address rangingfrom 0 to 30, and by an optional secondary address, which can be 0or can range from 96 to 126.
GPIB Data
Two types of data can be transferred over GPIB, instrument data and interface messages:
Instrument data — Instrument data consistsof vendor-specific commands that configure your instrument, returnmeasurement results, and so on. For a complete list of commands supportedby your instrument, refer to its documentation.
Interface messages — Interface messages aredefined by the GPIB standard and consist of commands that clear theGPIB bus, address devices, return self-test results, and so on.
Data transfer consists of one byte (8 bits) sent in parallel.The data transfer rate across the interface is limited to 1 megabyteper second. However, this data rate is usually not achieved in practice,and is limited by the slowest device on the bus.
GPIB Lines
GPIB consists of 24 lines, which are shared by all instrumentsconnected to the bus. 16 lines are used for signals, while eight linesare for ground. The signal lines are divided into these groups:
Eight data lines
Five interface management lines
Three handshake lines
The signal lines use a low-true (negative) logic convention with TTL levels. This convention means that a line is low (true or asserted) when it is a TTL low level, and a line is high (false or unasserted) when it is a TTL high level. The pin assignment scheme for a GPIB connector is shown in the following figure and table.
GPIB Pin and Signal Assignments
Pin | Label | Signal Name | Pin | Label | Signal Name |
|---|---|---|---|---|---|
1 | DIO1 | Data transfer | 13 | DIO5 | Data transfer |
2 | DIO2 | Data transfer | 14 | DIO6 | Data transfer |
3 | DIO3 | Data transfer | 15 | DIO7 | Data transfer |
4 | DIO4 | Data transfer | 16 | DIO8 | Data transfer |
5 | EOI | End Or Identify | 17 | REN | Remote Enable |
6 | DAV | Data Valid | 18 | GND | DAV ground |
7 | NRFD | Not Ready For Data | 19 | GND | NRFD ground |
8 | NDAC | Not Data Accepted | 20 | GND | NDAC ground |
9 | IFC | Interface Clear | 21 | GND | IFC ground |
10 | SRQ | Service Request | 22 | GND | SRQ ground |
11 | ATN | Attention | 23 | GND | ATN ground |
12 | Shield | Chassis ground | 24 | GND | Signal ground |
Data Lines
The eight data lines, DIO1 through DIO8, are used for transferringdata one byte at a time. DIO1 is the least significant bit, whileDIO8 is the most significant bit. The transferred data can be an instrumentcommand or a GPIB interface command.
Data formats are vendor-specific and can be text-based (ASCII)or binary. GPIB interface commands are defined by the IEEE 488standard.
Interface Management Lines
The interface management lines control the flow of data across the GPIB interface.
GPIB Interface Management Lines
Line | Description |
|---|---|
ATN | Used by the Controller to inform all devices on the GPIB that bytes are being sent. If the ATN line is high, the bytes are interpreted as an instrument command. If the ATN line is low, the bytes are interpreted as an interface message. |
IFC | Used by the Controller to initialize the bus. If the IFC line is low, the Talker and Listeners are unaddressed, and the System Controller becomes the Controller-In-Charge. |
REN | Used by the Controller to place instruments in remote or local program mode. If REN is low, all Listeners are placed in remote mode, and you cannot change their settings from the front panel. If REN is high, all Listeners are placed in local mode. |
SRQ | Used by Talkers to asynchronously request service from the Controller. If SRQ is low, then one or more Talkers require service (for example, an error such as invalid command was received). You issue a serial poll to determine which Talker requested service. The poll automatically sets the SRQ line high. |
EOI | If the ATN line is high, the EOI line is used by Talkers to identify the end of a byte stream such as an instrument command. If the ATN line is low, the EOI line is used by the Controller to perform a parallel poll (not supported by the toolbox). |
Handshake Lines
The three handshake lines, DAV, NRFD, and NDAC, are used to transfer bytes over the data lines from the Talker to one or more addressed Listeners.
Before data is transferred, all three lines must be in the proper state. The active Talker controls the DAV line and the Listener(s) control the NRFD and NDAC lines. The handshake process allows for error-free data transmission.
Handshake Lines
Line | Description |
|---|---|
DAV | Used by the Talker to indicate that a byte can be read by the Listeners. |
NRFD | Indicates whether the Listener is ready to receive the byte. |
NDAC | Indicates whether the Listener has accepted the byte. |
The handshaking process follows these steps:
Initially, the Talker holds the DAV line high indicating no data is available, while the Listeners hold the NRFD line high and the NDAC line low indicating they are ready for data and no data is accepted, respectively.
When the Talker puts data on the bus, it sets the DAV line low, which indicates that the data is valid.
The Listeners set the NRFD line low, which indicates that they are not ready to accept new data.
The Listeners set the NDAC line high, which indicates that the data is accepted.
When all Listeners indicate that they have accepted the data, the Talker sets the DAV line high indicating that the data is no longer valid. The next byte of data can now be transmitted.
The Listeners hold the NRFD line high indicating they are ready to receive data again, and the NDAC line is held low indicating no data is accepted.
Note
If the ATN line is high during the handshaking process, the information is considered data such as an instrument command. If the ATN line is low, the information is considered a GPIB interface message.
The handshaking steps are shown in the following figure.
Status and Event Reporting
GPIB provides a system for reporting status and event information.With this system, you can find out if your instrument has data toreturn, whether a command error occurred, and so on. For many instruments,the reporting system consists of four 8-bit registers and two queues(output and event). The four registers are grouped into these twofunctional categories:
Status Registers — The Status Byte Register(SBR) and Standard Event Status Register (SESR) contain informationabout the state of the instrument.
Enable Registers — The Event Status EnableRegister (ESER) and the Service Request Enable Register (SRER) determinewhich types of events are reported to the status registers and theevent queue. ESER enables SESR, while SRER enables SBR.
The status registers, enable registers, and output queue are shown in the following figure.
Status Byte Register
Each bit in the Status Byte Register (SBR) is associated witha specific type of event. When an event occurs, the instrument setsthe appropriate bit to1. You can enable or disablethe SBR bits with the Service Request Enable Register (SRER). Youcan determine which events occurred by reading the enabled SBR bits.
Status Byte Register Bits
Bit | Label | Description |
|---|---|---|
0-3 | – | Instrument-specific summary messages. |
4 | MAV | The Message Available bit indicates if data is availablein the Output Queue. MAV is 1 if the Output Queue contains data. MAVis 0 if the Output Queue is empty. |
5 | ESB | The Event Status bit indicates if one or more enabledevents have occurred. ESB is 1 if an enabled event occurs. ESB is0 if no enabled events occur. You enable events with the StandardEvent Status Enable Register. |
6 | MSS | The Master Summary Status summarizes the ESB and MAV bits. MSS is 1 if either MAV or ESB is 1. MSS is 0 if both MAV and ESB are 0. This bit is obtained from the |
RQS | The Request Service bit indicates that the instrumentrequests service from the GPIB controller. This bit is obtained froma serial poll. | |
7 | – | Instrument-specific summary message. |
For example, if you want to know when a specific type of instrument error occurs, enable bit 5 of the SRER. Additionally, enable the appropriate bit of the Standard Event Status Enable Register so that the error event of interest is reported by the ESB bit of the SBR.
Standard Event Status Register
Each bit in the Standard Event Status Register (SESR) is associated with a specific state of the instrument. When the state changes, the instrument sets the appropriate bits to 1. You can enable or disable the SESR bits with the Standard Event Status Enable Register (ESER). You can determine the state of the instrument by reading the enabled SESR bits. The SESR bits have the following descriptions.
SESR Bits
Bit | Label | Description |
|---|---|---|
OPC | The Operation Complete bit indicates that all commandshave completed. | |
1 | RQC | The Request Control bit is not used by most instruments. |
2 | QYE | The Query Error bit indicates that the instrument attemptedto read an empty output buffer, or that data in the output bufferwas lost. |
3 | DDE | The Device Dependent Error bit indicates that a deviceerror occurred (such as a self-test error). |
4 | EXE | The Execution Error bit indicates that an error occurredwhen the device was executing a command or query. |
5 | CME | The Command Error bit indicates that a command syntaxerror occurred. |
6 | URQ | The User Request bit is not used by most instruments. |
7 | PON | The Power On bit indicates that the device is poweredon. |
For example, if you want to know when an execution error occurs, enable bit 4 of the ESER. Additionally, enable bit 5 of the SRER so that the error event of interest is reported by the ESB bit of the SBR.
Reading and Writing Register Information
This section describes the common GPIB commands used to readand write status and event register information.
Register Commands
Register | Operation | Command | Description |
|---|---|---|---|
SESR | Read |
| Return a decimal value that corresponds to the weightedsum of all the bits set in the SESR register. |
Write | N/A | You cannot write to the SESR register. | |
ESER | Read |
| Return a decimal value that corresponds to the weightedsum of all the bits enabled by the |
Write |
| Write a decimal value that corresponds to the weightedsum of all the bits you want to enable in the SESR register. | |
SBR | Read |
| Return a decimal value that corresponds to the weightedsum of all the bits set in the SBR register. This command returnsthe same result as a serial poll except that the MSS bit is not cleared. |
Write | N/A | You cannot write to the SBR register. | |
SRER | Read |
| Return a decimal value that corresponds to the weightedsum of all the bits enabled by the |
Write |
| Write a decimal value that corresponds to the weightedsum of all the bits you want to enable in the SBR register. |
For example, to enable bit 4 of the SESR, you write the command *ESE16. To enable bit 4 and bit 5 of the SESR, you write thecommand *ESE 48. To enable bit 5 of the SBR, youwrite the command *SRE 32.
To see how to use many of these commands in the context of aninstrument control session, refer to Execute Serial Poll.
See Also
visadev
Related Topics
- Get Started with GPIB Interface
- Write and Read GPIB Data
- Send Trigger to Instrument
- Execute Serial Polls
External Websites
- GPIB Instruments and MATLAB
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