The GPIB Interface. Last modified Week 4 - part 1. What we have here is a failure to communicate - Cool Hand Luke

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1 What we have here is a failure to communicate - Cool Hand Luke The GPIB Interface GPIB - General Purpose Interface Bus - is a standard interface for communications between instruments and controllers (eg. PCs). Since it s a standard, one vendor s instrument can talk with another vendor s controller! GPIB - originated in 1960 s by HP (then called HPIB) for purpose of instrument control by HP computers GPIB sometimes referred to as ANSI/IEEE Standard Standard was updated in 1992 to standardize data codes and formats, device level commands, etc. (488.2 standard) What we have here is a failure to communicate optically - Prof. Federici

2 Speed and Configuration Digital, 8-bit parallel communications interface Data transfer rates at 1MB/s or more using 3-wire handshake Can connect 1 system controller (PC) and up to 14 additional instruments (all in parallel). You can use more than one GPIB at a time Items on Bus can function as Talkers - sends data/ commands to another device Listeners - receives data from another device or controller Controllers - manages the flow of info on the bus. Defines communication links and sends GPIB commands to devices.

3 GPIB - Cont. A device can be more than one of the three. Eg. An oscilloscope can read commands from the bus or report (write) data or its status to the bus. GPIB addressing - each GPIB device/ board must have its own unique GPIB address (usually 0 to 30). GPIB controller uses this address to generate a hardware address which it using when addressing a device. The hardware address is different depending on whether something is a listener or talker. Bit Position Meaning 0 Talk Add List Add GPIB Primary address Federici suggestion #1: Use the same address in different VI s for the same instruments. Eg. the SR510 Lock-in amplifier is ALWAYS device 23.

4 GPIB addresses Setting GPIB address - Check instrument s user guide - can be set via DIP switches on back of instrument or through front panel buttons/software. Try to maintain a uniform convention. Eg. the HP Oscilloscope model XYZ is always address 10. DOCUMENT in your VI what you assume the address to be or how to change it (Global variable?) IEEE 488 versus IEEE Yes There is a way to find all talkers and listeners on the Bus Yes You can write code to figure out who is who and have your VI be super smart. WHY spend the computation time to do that?

5 Wiring of GPIB 24 pin connector - 16 signal lines and 8 grounds (shields) Data lines - DIO1 through DIO8 - carry data and commands (ASCII codes) Handshake Lines - (three) (you ready? I m ready. Send data. Got data. Data valid ) NRFD (not ready for Data) - held low by a listener who is not ready to accept data. Line goes high when listeners ready and talker puts data on bus. After data on bus, talker makes DAV (Data Valid low). At this point each listener retrieves data. Before and during the data retrieval, listener holds NDAC (No Data Accepted) low. Once every listener receives data, NDAC is high and talker makes DAV high. Listener makes NDAC low until another transfer is initiated.

6 Wiring of GPIB (cont.) Interface Management lines - (five) EOI (End or Identify) - line is used by talker to designate the end of a message. SQR (Service Request) - line used by any device to ask for service (eg. overload, unlocked Lock-in amplifier). Controller can serial poll each instrument (returns an 8-bit serial poll byte) to determine what needs attention. ATN (Attention) - makes both talkers and listeners accept info and pass control of DAV line to controller REN (remote Enable) - changes the status of an instrument from local (front panel control) to remote (GPIB) control. This is a good feature to Lockout front panel control. IFC (Interface Clear) - clears bus of all data and activity.

7 Configuration For high data transfer rate, need to limit capacitance in bus system. A maximum separation of four meters between any two devices. Any average separation of two meters for entire bus. Maximum total cable length of 20m Maximum of 15 devices connected to each bus, with at least 2/3 powered on. For high-speed : All devices powered on. Cable lengths short as possible (<15m total) 1 device per meter of cable (on average).

8 Setting Up GPIB There are several things which need to be done to communicate via GPIB Choose GPIB board (PC/ platform dependent) (skip) Install GPIB board (skip) Must modern boards are plug and play Install software (skip) Install from a CD-ROM Determine/set software properties of GPIB board and Devices Meas urement & Automation.lnk

9 Measurement & Automation Explorer MAX allows you to access (without a VI) your GPIB, DAQ, IMAQ (imaging), Motion, VISA, VXI (etc. Devices) Configure National Instruments ( Hardware and software. View Instruments and Devices connects to your computer Execute System Diagnostics Meas urement & Automation.lnk

10 Setting Up GPIB Select Devices and Interfaces: This will show you the GPIB interface as well as the serial interface on your Computer. If you open the Ports (Serial and Parallel) section you will see the serial ports (COM1, Com2) and Printer ports (LPT1). If you click on the GPIB selection, the GPIB will find the current devices on the interface bus and list their GPIB address and some of the instrument s characteristics. For each GPIB board (eg. GPIB0), you can right click and choose PROPERTIES>>SOFTWARE to change various parameters

11 This is an old Dialog box, but the same features are present in MAX Time out: This is the maximum amount of time which the GPIB will allow for an operation (typically read or write). Eg. If you read data from a device which is not attached, the GPIB read command will wait 10sec before it times out with an error.

12 GPIB termination Methods Termination Methods: data transfers are terminated either by setting GPIB EOI line with last byte of transfer or when a preconfigured end-of-string (EOS) character is transmitted. Note from dialog box on previous slide that termination methods can be different for read and write. EOS byte - This is a byte of data which can indicate the end of a message. You can make it anything you want, but standards are <NUL> 00h, <CR> 0Ah, or <LF> 0Dh. Option exists for 7 or 8 bit compare

13 GPIB Termination (cont.) The termination, GPIB address, and I/O timeout can be chosen for each Device.

14 GPIB sending/ receiving data Data is sent on GPIB bus as ASCII data (8 bit data). Each instrument has its own instruction set. (See manual) Generally, you need to append the EOS character to the data to terminate the writing of data (if EOI line not used!). (see Instrument Manual) Example from SR510 Lock-in Amplifier: Q - The Q command returns the output reading in units of volts. It returns a string such as 50.00E-6 For further calculations, need to convert string to floating point number.

15 ASCII CODES ASCII PDF file

16 GPIB - read/write logic sequence Wstring$ := Q +<EOS> IBWRT(address%, Wstring$) IBRD(address%, Rstring$) value := convert (Rstring$) C o nc a te na te S trings! Form GPIB command string! Write string to bus! Read response string from bus! Convert to real value G P IB W rite S c a n F ro m S tring G P IB R e a d

17 Testing the GPIB How do I make sure that instrument is hooked up correctly, EOS/ termination is done correctly, address correct etc.? In MAX, select an instrument. Right mouse click and choose COMMUNICATE WITH INSTRUMENT -Allows you to send and receive strings directly to/ from instrument How do I make sure that the calls to GPIB interface contain the correct commands etc.? Use COMMUNICATE WITH INSTRUMENT to write and read from the instrument or Use Interactive Control program or use I/O Instrument Assistant Express VI or Use GPIB SPY program or Place a Probe on the appropriate wire in LabVIEW

18 Testing the GPIB We will emphasize three main options to testing the GPIP In MAX, select an instrument. Right mouse click and choose COMMUNICATE WITH INSTRUMENT -Allows you to send and receive strings directly to/ from instrument Use Interactive Control program Use I/O Instrument Assistant VI

19 Interactive Control Utility Right Click on the GPIB0 board in the Tree diagram and choose Interactive Control Step 1 - open a board or device handle for use with subsequent GPIB calls : ibdev ud0: enter board index: 0 enter primary address: 6 enter secondary address: 0 enter timeout: 10 enter EOI on last byte flag: 1 enter EOS mode/byte: 0 User input in red 1 so EOI asserted with last byte 0 if device does not use EOS

20 Interactive Control Utility If you enter a command and no parameters, the software will automatically prompt you for the parameters. You can also input everything in one line. Common commands: ibclr - clear device - settings revert to defaults ibwrt - write a string to device ibrd - read a string from device ibrsp - return serial poll byte Equivalent icons are in LabVIEW

21 Interactive Control utility Federici Hint #2: With IC utility, use a return serial poll byte to verify that the device is online and working properly. If you get a timeout error, then there is a problem: device not turned on wrong GPIB address For IEEE devices, can use IDN? command so instrument sends back its identifying string. Federici Hint #3: with IBRD, you can see the bytes of data and see what extra control characters are appended to data received from an instrument.

22 Interactive Control cont. Federici Hint #4: with IBWRT and IBRD, you can work out the syntax, extra EOS bytes etc., EOI etc which are needed for GPIB communications BEFORE you use LabVIEW udo: ibwrt enter string: Q\r [0100] (cmpl) count: 2 udo: ibrd enter byte count: 20 [0100] (cmpl) ASCII Codes count: e b a 0d Last modified <cr> <lf> Week 4 - part 1 Write a string to GPIB device udo ASCII string Q plus a <CR> Transfer completed, 2 bytes transmitted 3.740E+01<cr><lf> E

23 IC - Cont. In case you missed it in the manual, the SR510 always appends a <cr> <lf> to its data. IC utility allows you to send hexidecimal bytes with the ASCII string. The two most common EOS characters are representated by \r - <cr> and \n - <lf>. NOTE: \r and \n work with IBIC program. They DO NOT work in Labview as part of a string (Eg. Q\r\n is interpreted as 5 characters of ASCII) But variations of them work if they are part of a FORMAT string within LabVIEWS string functions

24 NI Communicator Federici Hint #5: With MAX utilities, you can quickly verify that a device is online and working properly. (In a LabVIEW code, have device report a serial poll byte) If you get a timeout error, or an error message then there is a problem: device not turned on wrong GPIB address For IEEE devices, can use IDN? command so instrument sends back its identifying string. Federici Hint #4: with MAX, you can see the bytes of PRINTABLE data received from an instrument. Use Interface Control program to see non-printable characters.

25 GPIB Communication with Lockin Let s try communicating with the Lock-In amplifier and/or Function Generator. MANUALS AVAILABLE ON COURSE WEB SITE First Use Max Program, Then use Interactive Control Utility. Next use I/O Instrument Assistant Express VI

26 General Flow Chart for GPIB calls Initialize Device Set up Device to take Data Yes No Finish Getting Data? Device Acquires Data Read Data from Device

27 GPIB Programming Each of the fundamental Tasks on the previous flowchart should be written as a SubVI. Each instrument should be initialized in some way since the device parameters (sensitivity, filter settings) may depend on who last used the instrument. Many instruments have VI drivers written for them by manufacturers, National Instruments, or by LabVIEW users (Check national instruments website:) NOTE: If you use any code not written by yourself in your VIs which you turn in, you are REQUIRED to cite your source of the VI. Otherwise, its plagiarism.

28 LabVIEW GPIB Calls Read/ Write mode indicates how to terminate the GPIB Write/ Read. 0: Send EOI with the last character of the string. 1: Append CR to the string and send EOI with CR. 2: Append LF to the string and send EOI with LF. 3: Append CR LF to the string and send EOI with LF. 4: Append CR to the string but do not send EOI. 5: Append LF to the string but do not send EOI. 6: Append CR LF to the string but do not send EOI. 7: Do not send EOI. Similar Controls and Indicators for WRITE

29 GPIB Forming GPIB Commands You can add EOS by concatenating strings. G G P IB W rite You can build up GPIB write strings from controls using scan to string function G %d\r G P IB W rite

30 GPIB example VIs GPIB Example 1: Reading Voltage from Lock-in using sequence structure GPIB Example 2: Reading Voltage from Lock-in by writing Error IN/OUT together GPIB Example 3: Setting the Sensitivity of Lock-in using RING control GPIB Example 4: Setting Sensitivity of Function generator using RING control and Case statement. All of the above examples need the following GLOBAL VI: GPIBGLOBALS