GPIB TUTORIAL
Contents
1. keywords in their long form MEASure or their short form shown in capital letters MEAS SCPI offers numerous advantages to the test engineer One of these is that SCPI provides a comprehensive set of programming functions covering all the major functions of an instrument This standard command set ensures a higher degree of instrument interchangeability and minimizes the effort involved in designing new test systems The SCPI command set is hierarchical so adding commands for more specific or newer functionality is easily accommodated The SCPI Instrument Model As a means of achieving compatibility and categorizing command groups SCPI defined a model of a programmable instrument This model shown in Figure 8 applies to all the different types of instrumentation NWeasurement Function Signal Ge nre ration The SCPI Instrument Model All of the functional components of the instrument model may not apply to every instrument For example an oscilloscope does not have the functionality defined by the signal generation block in the SCPI model SCPI defines hierarchical command sets to control specific functionality within each of these functional components The signal routing component controls the connection of a signal to the instrument s internal functions the measurement component converts the signal into a preprocessed form and the signal generation component converts internal data to real world signals The memory com2. IEEE 488 standards IEEE 488 HANDSHAKE The standard IEEE 488 1 3 wire handshake shown in Figure 9 requires the Listener to unassert Not Ready for Data NREFD the Talker to assert the Data Valid DAV signal to indicate to the Listener that a data byte is available and for the Listener to unassert the Not Data Accepted NDAC signal when it has accepted that byte A byte cannot transfer in less than the time it takes for the following events to occur NRED to propagate to the Talker DAV signal to propagate to all Listeners the Listeners to accept the byte and assert NDAC the NDAC signal to propagate back to the Talker and the Talker to allow a settling time T1 before asserting DAV again Frstbyte tarefered Second bye Third byte Normal IEEE 488 1 Handshake HS488 HANDSHAKE HS488 increases system throughput by removing propogation delays associated with the 3 wire handshake To enable the HS488 handshake the Talker pulses the NRFD signal line after the Controller addresses all Listeners If the Listener is HS488 capable then the transfer occurs using the HS488 handshake shown in Figure 10 Once HS488 is enabled the Talker places a byte on the GPIB DIO lines waits for a preprogrammed settling time asserts DAV waits for a preprogrammed hold time unasserts DAV and drives the next data byte on the DIO lines The Listener keeps NDAC unasserted and must accept the byte within the specified hold time A byte
3. Talker and a Listener before the Talker can send its message to the Listener After the message is transmitted the Controller may address other Talkers and Listeners Some GPIB configurations do not require a Controller For example a device that is always a Talker called a talk only device is connected to one or more listen only devices A Controller is necessary when the active or addressed Talker or Listener must be changed The Controller function is usually handled by a computer A computer with the appropriate hardware and software could perform the roles of Talker Listener and Controller THE CONTROLLER IN CHARGE AND SYSTEM CONTROLLER Although there can be multiple Controllers on the GPIB at any time only one Controller is the Controller In Charge CIC Active control can be passed from the current CIC to an idle Controller Only the System Controller can make itself the CIC GPIB SIGNALS AND LINES The GPIB interface system consists of 16 signal lines and eight ground return or shield drain lines The 16 signal lines discussed below are grouped into data lines eight handshake lines three and interface management lines five see Figure 2 Ys Daa Lines _ Signal Lines Ye Handsheking Lines S bherfece Meanagemert Lines ae Grounds GPIB Signals and Lines Data Lines The eight data lines DIO1 through DIO8 carry both data and command messages The state of the Attention ATN line de
4. es IEEE 488 2 Status Report Model SCPI On April 23 1990 a group of instrument manufacturers announced the SCPI specification which defines a common command set for programming instruments Before SCPI each instrument manufacturer developed its own command sets for its programmable instruments This lack of standardization forced test system developers to learn a number of different command sets and instrument specific parameters for the various instruments used in an application leading to programming complexities and resulting in unpredictable schedule delays and development costs By defining a standard programming command set SCPI decreases development time and increases the readability of test programs and the ability to interchange instruments SCPI is a complete yet extendable standard that unifies the software programming commands for instruments The first version of the standard was released in mid 1990 Today the SCPI Consortium continues to add commands and functionality to the SCPI standard SCPI has its own set of required common commands in addition to the mandatory IEEE 488 2 common commands and queries Although IEEE 488 2 is used as its basis SCPI defines programming commands that you can use with any type of hardware or communication link SCPI specifies standard rules for abbreviating command keywords and uses the IEEE 488 2 message exchange protocol rules to format commands and parameters You may use command
5. must transfer in the time set by the settling time and hold time without waiting for any signals to propagate along the GPIB cable Frstbyte tenstared using nom d handshake Ti Tii TZ irie HS488 Handshake HS488 Data Transfer Flow Control The Listener may assert NDAC to temporarily prevent more bytes from being transmitted or assert NRFD to force the Talker to use the 3 wire handshake Through these methods the Listener can limit the average transfer rate However the Listener must have an input buffer that can accept short bursts of data at the maximum rate because by the time NDAC or NRFD propagates back to the Talker the Talker may have already sent another byte The required settling and hold times are user configurable depending on the total length of cable and number of devices in the system Between two devices and 2 m of cable HS488 can transfer data up to 8 Mbytes s For a fully loaded system with 15 devices and 15 m of cable HS488 transfer rates can reach 1 5 Mbytes s HS488 Controllers always use the standard IEEE 488 1 3 wire handshake to transfer GPIB commands bytes with Attention ATN asserted Reference Documents For more information on the GPIB standards refer to the following documents ANSI IEEE 488 1 1987 IEEE Standard Digital Interface for Programmable Instrumentation ANSI IEEE 488 2 1992 IEEE Codes Formats Protocols and Common Commands and Standard Commands for Programmable Instru
6. GPIB TUTORIAL BACKGROUND Instrumentation has always leveraged off widely used electronics technology to drive its innovation The jeweled movement of the clock was first used to build analog meters The variable capacitor the variable resistor and the vacuum tube from radios were used to pioneer the first electronic instruments Display technology was leveraged off the television for use in oscilloscopes and analyzers Today cost effective and powerful desktop and notebook computers are paving the way for new types of instruments virtual instruments Virtual instruments are designed and built by the user to match specific needs by leveraging off the power and low cost of PCs and workstations Software is the key to virtual instruments Application software empowers the user with the tools necessary to build virtual instruments and expand their functionality by providing connectivity to the enormous capabilities of PCs workstations and their assortment of applications boosting performance flexibility reusability and reconfigurability while diminishing at the same time development and maintenance costs Function specific stand alone with Application oriented system with connectivity to limited connectivity networks peripherals and applications Closed fixed functionality Open flexible functionality leveraging off familiar computer technology ee DRS an ology Aonlt Yeabille Fast turn on technology 1 2 year life cycle Minimal eco
7. GURATION REQUIREMENTS To achieve the high data transfer rate for which the GPIB was designed the physical distance between devices and the number of devices on the bus are limited The following restrictions are typical for normal operation A maximum separation of 4 m between any two devices and an average separation of 2 m over the entire bus A maximum total cable length of 20 m No more than 15 device loads connected to each bus with no less than two thirds powered on For higher speed systems using the 3 wire IEEE 488 1 handshake T1 delay 350 ns and HS488 systems the following restrictions apply A maximum total cable length of 15 m with a device load per 1 m cable All devices should be powered on All devices should use 48 mA three state drivers Device capacitance on each GPIB signal should be less than 50 pF per device IEEE 488 2 AND SCPI The SCPI and IEEE 488 2 standards addressed the limitations and ambiguities of the original IEEE 488 standard IEEE 488 2 makes it possible to design more compatible and productive test systems SCPI simplifies the programming task by defining a single comprehensive command set for pro grammable instrumentation regardless of type or manufacturer The scope of each of the IEEE 488 IEEE 488 2 and SCPI standards is shown in Figure 6 Common Command and Queres Syntax and Data Structures Evolution of GPIB Instrumentation Standards The ANSI IEEE Standard 488 1975
8. Interface messages manage the bus Usually called commands or command messages interface messages perform such functions as initializing the bus addressing and unaddressing devices and setting device modes for remote or local programming The term command as used here should not be confused with some device instructions that are also called commands Such device specific commands are actually data messages as far as the GPIB interface system itself is concerned TALKERS LISTENERS AND CONTROLLERS GPIB Devices can be Talkers Listeners and or Controllers A Talker sends data messages to one or more Listeners which receive the data The Controller manages the flow of information on the GPIB by sending commands to all devices A digital voltmeter for example is a Talker and is also a Listener The GPIB is like an ordinary computer bus except that a computer has its circuit cards interconnected via a backplane the GPIB has stand alone devices interconnected by standard cables The role of the GPIB Controller is comparable to the role of a computer CPU but a better analogy is to compare the Controller to the switching center of a city telephone system The switching center Controller monitors the communications network GPIB When the center Controller notices that a party device wants to make a call send a data message it connects the caller Talker to the receiver Listener The Controller usually addresses or enables a
9. become CIC REN remote enable The System Controller drives the REN line which is used to place devices in remote or local program mode SRQ service request Any device can drive the SRQ line to asynchronously request service from the Controller EOI end or identify The EOI line has two purposes The Talker uses the EOI line to mark the end of a message string and the Controller uses the EOI line to tell devices to identify their response in a parallel poll PHYSICAL AND ELECTRICAL CHARACTERISTICS Devices are usually connected with a shielded 24 conductor cable with both a plug and receptacle connector at each end see Figure 3 You can link devices in either a linear configuration see Figure 4 a star configuration see Figure 5 or a combination of the two Linear Configuration Star Configuration The standard connector is the Amphenol or Cinch Series 57 MICRORIBBON or AMP CHAMP type For special interconnect applications an adapter cable with non standard cable and or connectors is used MANAGEMENT LINES Pin No DATA LINES EC EA DIO1 E DIO2 EA DIO3 DIO4 DIO5 DIO6 DIO7 DIO8 N E O ov fe ES ssafSrokiRE amp GPIB Connector and Signal Assignment The GPIB uses negative logic with standard TTL levels When DAV is true for example it is a TTL low level lt 0 8 V and when DAV is false it is a TTL high level gt 2 0 V CONFI
10. emove the ambiguity of the possible bus conditions so instruments and Controllers are much more compatible By exactly defining the state of the bus and how devices should respond to specific messages IEEE 488 2 removes such system development problems IEEE 488 2 Protocols Protocols are high level routines that combine a number of control sequences to perform common test system operations IEEE 488 2 defines two required protocols and six optional protocols as shown in Table 2 a RESET __ FINDROS ALLSPOLL Passe REQUESTCTL_ FINDLSTN SETADD TesTsYs IEEE 488 2 Controller Protocols These protocols reduce development time because they combine several commands to execute the most com mon operations required by any test system The RESET protocol ensures that the GPIB has been initialized and all devices have been cleared and set to a known state The ALLSPOLL protocol serial polls each device and returns the status byte of each device The PASSCTL and REQUESTCTL protocols pass control of the bus between a number of different devices The TESTSYS protocol instructs each device to run its own self tests and report back to the Controller whether it has a problem or is ready for operation Perhaps the two most important protocols are FINDLSTN and FINDRQS The FINDLSTN protocol takes advantage of the IEEE 488 2 Controller capability of monitoring bus lines to locate listening devices on the bus The Controller implements
11. esign more compatible and efficient instruments The benefits of this standardization for the test system developer are reduced development time and cost because it solves the problems caused by instrument incompatibilities varying command structures and data formats Requirements of IEEE 488 2 Controllers IEEE 488 2 defined a number of requirements for a Controller including an exact set of IEEE 488 1 interface capabilities such as pulsing the interface clear line for 100 us setting and detecting EOI setting asserting the REN line sensing the state and transition of the SRQ line sensing the state of NDAC and timing out on any I O transaction Other key requirements for Controllers are bus control sequences and bus protocols IEEE 488 2 Control Sequences The IEEE 488 2 standard defined control sequences that specify the exact IEEE 488 1 messages that are sent from the Controller as well as the ordering of multiple messages IEEE 488 2 defined 15 required control sequences and four optional control sequences as shown in Table 1 Eend apoge mesigt a address to receive data D nr a a SETUP ence Reccharspome mesg ATN false data RECENE oo MESSAGE Mandate a devices in remote with local lockout SET RWLS IEEE 488 2 Required and Optional Control Sequences The IEEE 488 2 control sequences describe the exact states of the GPIB and the ordering of command messages for each of the defined operations IEEE 488 2 control sequences r
12. ments The latest SCPI Standards are published by the SCPI Consortium Project specific references PCL S48A B Multifunction IEEE 488 Interface Card User s Manual Advantech Co Ltd Apr 1989 Philips PM5192 manuals Fluke Philips PM3365A manuals AVCOM PSA 65A manuals Fluke Philips PM3585 manuals Other information related to Automation Instrumentation Protocols could be found in 1 GPIB related materials O Hewlett Packard Tutorial Description of the Hewlett Packard Interface Bus Hewlett Packard Nov 1987 2 Fieldbus related materials O FURNESS Harry Fieldbus Series Part 1 Digital Communication Provides Control Engineering 41 1 pp 23 25 O FURNESS Harry Fieldbus Series Part 2 The Differences Start from the Bottom Up Control Engineering 41 3 pp 75 77 O CHATHA Andrew Fieldbus Series Part 3 The Foundation for Field Control Systems Control Engineering 41 6 pp 77 80 O LASHER Richard J Fieldbus Series Part 4 Fieldbus Advancements and Their Implications Control Engineering 41 8 pp 55 59 O PIERSON Lynda L Fieldbus Series Part 5 Broader Fieldbus Standards Will Improve System Functionality Control Engineering 41 12 pp 58 59 O JOHNSON Dick Fieldbus Series Part 6 The Future of Fieldbus at Milestone 1995 Control Engineering 41 13 pp 49 52 3 CAN Controller Area Network 4 MAP TOP Manufacturing Automation Protocol Office Protocol
13. nomics of scale Maximum economics of scale High development and maintenance a Mie Baus Software minimizes development and maintenance costs INTRODUCTION In 1965 Hewlett Packard designed the F Pa Hewlett Packard Interface Bus HP IB to a Pd ES gs siete connect their line of programmable instruments ee sf Be S as P p oS to their computers Because of its high transfer a os s ra ota y P S i 5 rates nominally 1 Mbytes s this interface bus S amp s Sy ae g Ori quickly gained popularity It was later accepted as IEEE Standard 488 1975 and has evolved to 1965 1975 1987 1999 1992 1993 ANSI IEEE Standard 488 1 1987 Today the name General Purpose Interface Bus GPIB is more widely used than HP IB ANSI IEEE 488 2 1987 strengthened the original standard by defining precisely how controllers and instruments communicate Standard Commands for Programmable Instruments SCPI took the command structures defined in IEEE 488 2 and created a single comprehensive programming command set that is used with any SCPI instrument Figure 1 summarizes GPIB history TYPES OF GPIB MESSAGES GPIB devices communicate with other GPIB devices by sending device dependent messages and interface messages through the interface system Device dependent messages often called data or data messages contain device specific information such as programming instructions measurement results machine status and data files
14. now called IEEE 488 1 greatly simplified the interconnection of programmable instrumentation by clearly defining mechanical electrical and hardware protocol specifications For the first time instruments from different manufacturers were interconnected by a standard cable Although this standard went a long way towards improving the productivity of test engineers the standard did have a number of shortcomings Specifically IEEE 488 1 did not address data formats status reporting message exchange protocol common configuration commands or device specific commands As a result each manufacturer implemented these items differently leaving the test system developer with a formidable task IEEE 488 2 enhanced and strengthened IEEE 488 1 by standardizing data formats status reporting error handling Controller functionality and common commands to which all instruments must respond in a defined manner By standardizing these issues IEEE 488 2 systems are much more compatible and reliable The IEEE 488 2 standard focuses mainly on the software protocol issues and thus maintains compatibility with the hardware oriented IEEE 488 1 standard SCPI built on the IEEE 488 2 standard and defined device specific commands that standardize programming instruments SCPI systems are much easier to program and maintain In many cases you can interchange or upgrade instruments without having to change the test program The combination of SCPI and IEEE 488 2 offer
15. ponent stores data inside the instrument The format component converts the instrument data to a form that you can transmit across a standard bus The trigger component synchronizes instrument actions with internal functions external events or other instruments The measurement function gives the highest level of compatibility between instruments because a measurement is specified by signal parameters not instrument functionality In most cases you can exchange an instrument that makes a particular measurement with another instrument capable of making the same measurement without changing the SCPI command The MEASurement component is subdivided into three distinct parts INPut SENSe and CALCulate The INPut component conditions the incoming signal before it is converted into data by the SENSe block INPut functions include filtering biasing and attenuation The SENSe component converts signals into internal data that you can manipulate SENSe functions control such parameters as range resolution gate time and normal mode rejection The CALCulate component converts the acquired data into a more useful format for a particular application CALCulation functions include converting units rise time fall time and frequency parameters The signal generation component converts data into output as physical signals SCPI subdivides the signal generation block into three function blocks OUTPut SOURce and CALCulate The OUTPut block condition
16. s significant productivity gains and finally delivers as sound a software standard as IEEE 488 1 did a hardware standard IEEE 488 2 TEEE 488 2 1987 encouraged a new level of growth and acceptance of the IEEE 488 bus or GPIB by addressing problems that had arisen from the original IEEE 488 standard IEEE 488 2 was drafted on the premise that it stay compatible with the existing IEEE 488 1 standard The overriding concept used in the IEEE 488 2 specification for the communication between Controllers and instruments is that of precise talking and forgiving listening In other words IEEE 488 2 exactly defined how both IEEE 488 2 Controllers and IEEE 488 2 instruments talk so that a completely IEEE 488 2 compatible system can be highly reliable and efficient The standard also required that IEEE 488 2 devices be able to work with existing IEEE 488 1 devices by accepting a wide range of commands and data formats as a Listener You obtain the true benefits of IEEE 488 2 when you have a completely IEEE 488 2 compatible system CONTROLLERS Although IEEE 488 2 had less impact on Controllers than it did on instruments there are several requirements and optional improvements for Controllers that made an IEEE 488 2 Controller a necessary component of test systems IEEE 488 2 precisely defined the way IEEE 488 2 Controllers send commands and data and added functionality Because of these IEEE 488 2 Controller requirements instrument manufacturers can d
17. s the outgoing signal after it is generated OUTPut block functions include filtering biasing and attenuation The SOURce block generates a signal based on specified characteristics and internal data SOURce block functions specify such signal parameters as amplitude modulation power current voltage and frequency The CALCulate block converts application data to account for signal generation anomalies such as correcting for external effects converting units and changing domains Example SCPI Command The following command programs a digital multimeter DMM to configure itself to make an AC voltage measurement on a signal of 20 V with a 0 001 V resolution MEASure VOLTage AC 20 0 001 The leading colon indicates a new command is coming The keywords MEASure VOLTage AC instruct the DMM to take an AC voltage measurement The instructs the DMM to return its measurement to the computer controller The 20 0 001 specifies the range 20 V and resolution 001 V of the measurement THE HIGH SPEED GPIB HANDSHAKE PROTOCOL HS488 National Instruments has developed the patented high speed GPIB handshake protocol called HS488 to increase the data transfer rate of a GPIB system All devices involved in a data transfer must be HS488 compliant to use the HS488 protocol but when non HS488 devices are involved the HS488 devices automatically use the standard IEEE 488 1 handshake to ensure compatibility HS488 is a superset of the
18. tem programming even easier IEEE 488 2 defines a minimum set of IEEE 488 1 interface capabilities that an instrument must have All devices must be able to send and receive data request service and respond to a device clear message IEEE 488 2 defines precisely the format of commands sent to instruments and the format and coding of responses sent by instruments All instruments must perform certain operations to communicate on the bus and report status Because these operations are common to all instruments IEEE 488 2 defined the programming commands used to execute these operations and the queries used to receive common status information These common commands and queries are shown in Table 3 Mnemonic Group RST Internal Operations TST Internal Operations OPC Synchronization Operation complete Synchronization Opearation complete query IEEE 488 2 Mandatory Common Commands ARARIRE sislsialalalelIsIS Wi MI MIAMI E a Z O o ECR ESS AG Because IEFE 488 2 standardizes status reporting the Controller knows exactly how to obtain status information from every instrument in the system This status reporting model builds upon the IEEE 488 1 status byte to provide more detailed status information The status reporting model is shown in Figure 7 5 T Bidi Bue m GIS U Regier BR a Feed b beral Pall Soui Eye Fler s Fei b STE aij Benue Reget Enable Fe hier ac 4R ESIR ARE
19. termines whether the information is data or commands All commands and most data use the 7 bit ASCII or ISO code set in which case the eighth bit DIO8 is either unused or used for parity Handshake Lines Three lines asynchronously control the transfer of message bytes between devices The process is called a 3 wire interlocked handshake It guarantees that message bytes on the data lines are sent and received without transmission error NRED not ready for data Indicates when a device is ready or not ready to receive a message byte The line is driven by all devices when receiving commands by Listeners when receiving data messages and by the Talker when enabling the HS488 protocol NDAC not data accepted Indicates when a device has or has not accepted a message byte The line is driven by all devices when receiving commands and by Listeners when receiving data messages DAV data valid Tells when the signals on the data lines are stable valid and can be accepted safely by devices The Controller drives DAV when sending commands and the Talker drives DAV when sending data messages Interface Management Lines Five lines manage the flow of information across the interface ATN attention The Controller drives ATN true when it uses the data lines to send commands and drives ATN false when a Talker can send data messages IFC interface clear The System Controller drives the IFC line to initialize the bus and
20. the FINDLSTN protocol by issuing a particular listen address and then monitoring the NDAC handshake line to determine if a device exists at that address The result of the FINDLSTN protocol is a list of addresses for all the located devices FINDLSTN is used at the start of an application program to ensure proper system configuration and to provide a valid list of GPIB devices that can be used as the input parameter to all other IEEE 488 2 protocols The bus line monitoring capability of an IEEE 488 2 Controller is also useful to detect and diagnose problems within a test system The FINDRQS protocol is an efficient mechanism for locating and polling devices that are requesting service It uses the IEEE 488 2 Controller capability of sensing the FALSE to TRUE transition of the SRQ line You prioritize the input list of devices so that the more critical devices receive service first If the application program can jump to this protocol immediately upon the assertion of the SRQ line you increase program efficiency and throughput IEEE 488 2 INSTRUMENTS The IEEE 488 2 specifications for instruments can require major changes in the firmware and possibly the hardware However IEEE 488 2 instruments are easier to program because they respond to common commands and queries in a well defined manner using standard message exchange protocols and data formats The IEEE 488 2 message exchange protocol is the foundation for the SCPI standard that makes test sys
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