SOFTMARK. USB to GPIB Interface. SOFTMARK PO Box 1609 Hornsby NSW 2077 Australia

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1 SOFTMARK USB to GPIB Interface SOFTMARK PO Box 1609 Hornsby NSW 2077 Australia Copyright 2004 Softmark The GPIB bus (formerly called IEEE-488 bus) was developed to connect and control programmable instruments. It has become an industry standard due to its versatility. The interfacing technique was originally developed by Hewlett-Packard. This Interface controls GPIB instruments using the latest USB technology. No other internal card is required. The GPIB bus was invented by Hewlett-Packard to simplify the interconnection of test instruments with the PC. The USB bus is present in all modern personal computers. In the future, the USB bus will be the only interface to the PC. Software for this Interface works on Windows 98/W2K/XP. SOFTMARK: USB to GPIB Interface 1

2 GPIB overview The signals on the GPIB bus instruments are defined as a standard (IEEE-488). The USB to GPIB Interface can be used on any instruments which support the GPIB bus. The Interface s software will control the GPIB instruments. There are 3 types of devices that can be connected to the GPIB bus - Listeners, Talkers and Controllers. The standard allows to control a maximum of 15 GPIB instruments. The GPIB bus concept is shown in Figure 1 below. The Interface has the ability to send commands and data to the bus and receive data from selected instruments. GPIB instruments have the capability to talk and listen. Figure 1. GPIB bus concept. GPIB Interface signals The GPIB bus consists of 16 signal lines and 8 ground lines. Signal lines are made up of 8 data lines, 3 handshake lines and 5 management lines. Data lines (D0 to D7) are used to transfer instrument addresses and data. SOFTMARK: USB to GPIB Interface 2

3 Figure 2. GPIB signal lines. The three handshake lines (NRFD, NDAC, and DAV) control the transfer of message bytes between GPIB instruments and the Interface. NRFD (Not Ready For Data), NDAC (Not Data Accepted), DAV (Data Valid) are described in specifications for the GPIB bus (IEEE-488) and will not be discussed in this document. The five interface management lines (ATN, EOI, IFC, REN, and SRQ) manage the flow of data. ATN (Attention) is asserted by the Interface to indicate that the commands or data are set on the GPIB bus. IFC (Interface Clear) is set by the Interface in order to initialize all GPIB instruments. REN (Remote Enable) will place the instruments in remote mode. Note that not all lines are in use. Device address The GPIB standard will allow you connect up to 15 GPIB instruments. Each instrument is assigned a primary address (0 to 30). It can be set by switches on the back of the instrument or, in modern GPIB instruments, it can be programmed and stored. This value can be set manually on the instrument and any instrument will have a specific address. In modern GPIB instruments, the address will be shown shortly after the instrument is switched on. Knowing the address is important as it is required to be specified during the transfer of data. SPPI commands The GPIB standard makes it easier to communicate with GPIB instruments, but each instrument has a unique command set which you can find in the technical references for the particular GPIB instrument. This issue will be explained below, where you can find an example about using SPPI commands for the HP Multimeter type A. The address can be set from the front panel. Here, it was set to 22 (see Figure 3 below). SOFTMARK: USB to GPIB Interface 3

4 Figure 3. Front of panel of HP Multimeter. GPIB bus cable Total GPIB cable length should not exceed 20 meters. For maximum data transfer speed, the cable between the Interface and the instrument should not exceed 2 meters. Figure 4 below shows a description of the connection between the GPIB plug and IDC-26. IDC- 26 is connected to the IDC socket on the USB to GPIB Interface. Figure 4. The connection between the USB to GPIB Interface and an instrument equipped with the GPIB bus. SOFTMARK: USB to GPIB Interface 4

5 USB to GPIB Interface initialization for the exchange of data The Interface sends an Interface Clear Command (IFC). The REN (Remote Enable) line is always enabled (it is done by the hardware, by setting it to Low using Resistor R5). After the Multimeter receives its own address, this addressed meter will be ready to be controlled by the USB to GPIB Interface. If you have more than one instrument connected to the GPIB bus, only the one with a specific address will respond to you. For example, after sending the address 22 to the HP Multimeter, the meter will be ready to exchange data. Note, the Multimeter is an example only and the address can be changed to any value. This means that we have to send the address before the commands. The GPIB instrument will acknowledge the call and will respond by sending data. Figure 5 below shows the front panel of the HP Multimeter, which was set to remote mode by specifying 22 as the instrument address. After you see this screen, the instrument will be able to communicate with your USB to GPIB Interface. Figure 5. HP Multimeter set to remote mode and ready to receive commands or send data. Sending SPPI commands to the HP Multimeter Data or commands are sent to an instrument as a string of ASCII characters. Example 1 how to change the function and range of the multimeter to AC Voltmeter MEAS:VOLT:AC? 100 The above command will set the HP Multimeter for AC Voltage measurement and the maximum voltage will be 100V (see Figure 6 below). SOFTMARK: USB to GPIB Interface 5

6 Figure 6. Changing the range to AC Voltage measurement. Press these buttons and do not forget to save the address and command before execution. Pressing these buttons consecutively: Send Adr L Send Command will transfer SPPI commands to the instrument (with the address 22 ) which will change the HP Multimeter to AC Volt measurement and the maximum voltage will be 100V. Make sure you check that the GPIB instrument has the same address as you used in the command. Figure 7. After sending the above command, your multimeter will change the function. Example 2 how to change the function and range of the multimeter to Ohmmeter MEAS:RES?: The above command will set the multimeter for resistance measurement and the maximum resistance will be 10 MOhms (see Figure 8 below). Figure 8. Changing the range to Ohm measurement. Press these buttons and do not forget to save the address and command before execution. Pressing these buttons consecutively: SOFTMARK: USB to GPIB Interface 6

7 Send Adr L Send Command will transfer SPPI commands to the instrument (with the address 22 ) which will change the HP Multimeter to Ohm measurement and the maximum resistance which you can measure will be 10 MOhms. Make sure you check that the GPIB instrument has the same address as you used in the command. Figure 9. After sending the above command, your multimeter will change the function. Example 3 how to identify particular instruments Sending the command *IDN? will send an identification request to the GPIB bus. The request will be connected with a particular addressed instrument. After sending this command, you have to send the command READ? and the response from the multimeter will identify the instrument. See Figure 10 below. For the purposes of this example, the HEWLETT-PACKART 34401A is used. Data length set to 27 Figure 10. Identification strings from the HP Multimeter. The above examples only show a few SPPI commands used to illustrate the communication concept. More commands can be used and they are described in manuals for particular GPIB instruments. SOFTMARK: USB to GPIB Interface 7

8 Circuit descriptions The circuit diagram is provided in the kit. PCB description This LED is on if the Interface is connected to the USB bus (VCC +5V) Connected via USB cable from your PC NDAC line will change status when the GPIB instrument responds Connected via ribbon cable to the GPIB instrument This LED will flicker when communication takes place Figure 11. USB to GPIB Interface PCB description. Figure 12. The Interface is connected via cable to a GPIB instrument. How a GPIB Interface is connected to a GPIB instrument The IDC-26 connector on the USB to GPIB Interface connects the Interface with the GPIB instrument via a ribbon cable. The supplied software allows you to check the connection, set a particular GPIB instrument to remote mode and establish communication. SOFTMARK: USB to GPIB Interface 8

9 Figure 13. An HP instrument controlled via the GPIB bus using the USB to GPIB Interface. Software virtual panel description On the main virtual panel there are a few sections. Section A will allow you to send a remote address and send/receive commands and data. After you write the addresses and commands, pressing the SAVE button will save that and the next time you switch the program on, the latest saved data will come up. Note that you have to save your data before you press any other button. By pressing, for example, the button SendAdr L or Send Command the software will send the content of the text boxes to a GPIB Instrument. Optionally, there is a second set of data which can be sent to a second GPIB instrument if required. Section B will allow you to read data from GPIB Instrument 1 by pressing the button READ. Section C will be used to automatically read data. Data can be read manually by pressing the button READ DATA and continuously checking the Manual text box. This section will be used if you have already configured your instrument and a connection with your USB to GPIB Interface was established. Section D will allow you to set the length of the data read from the instrument. The length can be set from 1 to 46 and faster acquisition takes place if the length of data is shorter. Section E deals with logging. You can simultaneously log data to a text file and display it on the grid while the data is read. In the logging frame there is only one variable - Interval, which can be set from 1 sec to 360 sec. This logging interval is saved on your PC and you do not need to introduce this figure whenever you call the program. Logged data is saved to a text file. Data can be converted to Microsoft Excel to graphically interpret the logged data. The name of the file has the format day,day/month,month/year,year for example, txt for 5 December This file can be found on your main C:\ drive. SOFTMARK: USB to GPIB Interface 9

10 Figure 14. An example of a text file. Data is appended to this file. The next day, the software will generate a new file and will append the new day s data to the new file. Section A Section B Section C Section D Section E Figure 15. The Interface s virtual panel. This picture also shows how to set the software and read data from two GPIB instruments. Note Sections A for Instrument 1 and Instrument 2 need to have an address and command before the reading of data takes place. The length of data can be set, for example, to 8 for Instrument 1 and Instrument 2 (in Section D). Do not forget to save data to your PC before you start reading. SOFTMARK: USB to GPIB Interface 10

11 How to connect the Interface to two sample instruments equipped with the GPIB bus Communicating with a second sample instrument (HP33120A Arbitrary Generator) The below examples only show a few SPPI commands used to illustrate the communication concept. More commands can be used and they are described in manuals for a particular GPIB instrument. Example 1 how to change output frequency in the HP 33120A Arbitrary Generator FREQ The above command will set the HP Arbitrary Generator and will set the output frequency to 10kHz (see Figure 16 below). Figure 16. Changing the output frequency. Press these buttons and do not forget to save the address and command before execution. SOFTMARK: USB to GPIB Interface 11

12 Pressing these buttons consecutively: Send Adr L Send Command will transfer SPPI commands to the instrument (with the address 14 ) which will change output frequency to 10,000Hz. Make sure you check that the GPIB instrument has the same address as you used in the command. Figure 17. Reading frequency from the HP Arbitrary Generator. Pressing the following buttons consecutively: Send Adr L Send Command Send Adr T Second Command Read Data from Instrument 1 will obtain frequency strings from the GPIB instrument which uses the address 14 as per our example. After you send the commands, sending the READ? command will return a value which is displayed in the grid and also logged to a text file (as Instrument 2 data). The logged data can be used for graphical presentations and converted to Microsoft Excel. Figure 18. After sending the above command, the HP Arbitrary Generator will change the function. SOFTMARK: USB to GPIB Interface 12

13 Example 2 how to change the output level of the HP33120A Arbitrary Generator FREQ 10000;VOLT 5 The above command will set the HP Arbitrary Generator for 10kHz and output voltage will be 5V RMS (see Figure 19 below). Figure 19. Setting frequency and amplitude. Press these buttons and do not forget to save the address and command before execution. Pressing these buttons consecutively: Send Adr L Send Command will transfer SPPI commands to the instrument (with the address 14 ) which will change/set output frequency to 10kHz and amplitude to 5V RMS on the HP Arbitrary Generator. Make sure you check that the GPIB instrument has the same address as you used in the command. Figure 20. After sending the command, your HP Arbitrary Generator will change the output level. Using the Interface card and installation Connect the Interface card with the USB port in your PC. The Windows operating system will try to find and communicate with the Interface. After a few seconds (usually 1-2 seconds), you will get a message on your screen that a new device was found. Following this, Windows will start installing the Interface. This will only take a minute and does not have to be done again. SOFTMARK: USB to GPIB Interface 13

14 After the Interface is installed, the next time you use the card, Windows will automatically find the card. No other setup will be required. Power supply not required The Interface card has its own voltage supply (+5V and GND line). This voltage is taken from the USB bus. Note: no power is required to operate this card. However, supplying voltage to the PCB may damage your card or your PC. Copyright 2004 Softmark All rights reserved. The information presented in this document is believed to be accurate and reliable and may be changed without notice. No liability will be accepted for any consequences of its use. SOFTMARK: USB to GPIB Interface 14