Clifton Laboratories 7236 Clifton Road  Clifton VA 20124 tel: (703) 830 0368 fax: (703) 830 0711

E-mail: Jack.Smith@cliftonlaboratories.com
 

Revision History
Original  24 October 2008
25 Oct 2008 - cleaned up formatting problems
 

Almost all Hewlett Packard (now Agilent) test equipment designed in the last 30 years includes a remote digital control interface, purely analog equipment excepted, of course. Originally known as the "HPIB" or Hewlett Packard Instrument Bus, it became an IEEE standard in the mid 1970's, the "IEEE-488" interface. It's also known as the GPIB or general purpose interface bus. Although developed by HP, it's widely used by other manufacturers, including Tektronix, Racal and virtually all other test equipment producers.

Instruments equipped with GPIB have the 24 position female connector pictured above. Multiple instruments may be parallel connected using GPIB connecting cables, such as the one shown at the right. Each end of the cable has a male and female GPIB connector making it easy to connect multiple instruments. Purchased new, GPIB cables are quite expensive, but they are readily available surplus. I prefer genuine HP cables as they are better mechanically and electrically constructed than some "no name" cables I've acquired.

In addition to the electrical specifications of the bus, there's also commonality in command and response syntax amongst manufacturers.

• How to connect a computer to GPIB equipped test equipment ; and
• Programs to communicate with the computer.

Prologix's GPIB-LAN Controller has a standard RJ-45 Ethernet connector and plugs into your router, or wireless bridge or directly into your computer. (If plugged directly into the computer, you may or may not need a crossover cable, depending on the age of your PC. See the GPIB-LAN operating manual for more information.) It also has a small wall-wart type power supply. (The GPIB-USB controller takes its DC power fom either the USB interface or the GPIB interface.) The GPIB-LAN Controller then appears as a device on your network, with a unique IP address. For example, in my home network, I plug the Controller into a wireless access point in the basement. I can then access the controller from any computer on my network, whether linked by wireless or by wired connection to my Linksys 802.11N router.
 

Plots and Printed Output

Before considering computer control of a GPIB equipped instrument, it's necessary to distinguish between "controller" and "device" mode.

Many instruments, such as oscilloscopes, spectrum analyzers and network analyzers have cathode ray tube displays (or LCD) and are equipped to make a printed image of the display over the GPIB port. Traditionally, the device connected to the GPIB port has  been a mechanical plotter, although newer instruments are also capable of sending a hard copy image to ink-jet or laser printers over the GPIB port.

There is software available that allows a computer connected via a Prologix Controller to emulate a plotter or a printer and receive the transmitted data. The data is then  rendered into a plot on the computer screen and may be saved in various formats or printed. When operated in this mode, the test instrument is the "network controller" and the Prologix card is a "device."

I use two programs for this purpose:

• PrintCapture—is available at http://www.printcapture.com/. A free time-limited demonstration version can be downloaded, with the licensed version costing $97.00.
• John Miles' GPIB Toolkit—is available at http://www.ke5fx.com/gpib/readme.htm. This is freeware, developed by John Miles, KE5FX, and includes a plotter emulation program, 7470.exe, as well as a configuration utility to simplify modifying configuration files needed by 7470 to communicate with the particular GPIB adapter you are using.

Both of these are excellent programs, although with some important differences.

• PrintCapture emulates not only an HP plotter, but also HP inkjet and laser printers using "PCL" (page control language) and Epson dot matrix printers. This provides more inter-operability options than 7470.exe, which only emulates the Hewlett Packard 7470 plotter.
• From my experience, every instrument that can output to a hard copy device will work with an HP plotter but the resulting image quality and features may be subtlety different, with output to a PCL printer providing better quality on some of my test gear.
• There are some quirks in how HP and other companies implemented their plotter output functions, so that some gear I own works with 7470.exe but not with PrintCapture. For example, an Advantest R3463 spectrum analyzer works with 7470.exe but will not produce a usable image with PrintCapture. On the other hand, my HP 87510A network analyzer will not successfully write a plot to 7470.exe but will to PrintCapture.
• 7470.exe supports Prologix's GPIB-LAN controller, but PrintCapture does not. (Both support Prologix's GPIB-USB controller.)
• PrintCapture is a more polished, commercial program that's easier to setup and to use. 7470.exe has a certain "tekkie" quality to it and can require more tinkering with files and options than PrintCapture. To take one simple example. To change pen color in 7470.exe  requires editing the programs INI file with a text editor. PrintCapture has a drop down menu and dialog box that lets you set the color associated with each particular pen. The dialog box displays the color options associated with each pen color and lets you define a new color association using drop down lists. (You can save the configuration, of course.)
• 7470 allows  the user to annotate the image directly, whilst PrintCapture does not. (It's possible to save a PrintCapture image and then add an annotation with a program such as Irfanview, of course.)

I find myself using both programs about equally. I should also mention that both PrintCapture and 7470.exe support GPIB adapters other than Prologix.

I've had exchanges of E-mail with both developers and found them responsive to my questions and suggestions. Mike Shride, PrintCapture's developer, for example, modified PrintCapture to work with my HP87510A vector network analyzer, which has some relatively uncommon responses expected from  the connected plotter. John, KE5FX, has implemented several of my suggestions in recent versions of 7470.exe.

The images below illustrate how PrintCapture and 7470.exe render plots.
 

The image below, for example, was generated using John's GPIB Toolkit program SSM, demonstrating the waterfall spectrum display in the top image and the normal spectrum display below.

Examples of what one might do with custom programming of computer controlled test gear can be seen in the following pages at my site:

• Lamp current versus applied voltage plots at
  http://www.cliftonlaboratories.com/Bill%20Hewlett%20and%20his%20Magic%20Lamp.htm
• Softrock receiver responses (audio out versus applied frequency) at http://www.cliftonlaboratories.com/softrock_lite_6_2.htm
• Receiver AGC response  curves at http://www.cliftonlaboratories.com/receiver_agc_curves.htm
• Diode forward voltage versus current at http://www.cliftonlaboratories.com/1n400x_diode_family_forward_voltage.htm
• Audio transformer linear modeling at http://www.cliftonlaboratories.com/audio_transformer_data_and_modeling.htm
• Diode output voltage versus applied RF voltage at http://www.cliftonlaboratories.com/diodes_for_rf_probes.htm
• Elecraft K3 transceiver AGC characteristics at http://www.cliftonlaboratories.com/elecraft_k3_agc_and_s-meter.htm
• Headphone impedance at http://www.cliftonlaboratories.com/headphone_impedance.htm
• Non-linear audio transformer behavior at http://www.cliftonlaboratories.com/non-linear_transformer_behavior.htm

Until the last week, I've written all the control programs in Liberty Basic, an inexpensive Windows-based BASIC compiler. (Well, it's probably more accurately described as a BASIC interpreter, rather than a pure compiler.) I find the editor provided with Liberty Basic not quite as good as it might be, so I've also purchased an optional editor program, Liberty Basic Workshop. The total outlay for both programs is under $100.

For more information:

• Liberty BASIC: http://www.libertybasic.com/
• Liberty BASIC Workshop: http://alycesrestaurant.com/workshop.htm

To use Liberty Basic with a Prologix GPIB-USB adapter, treat it as a virtual serial port, COM device.

However, I've decided that the freeware program EZGPIB, by Ulrich Bangert, DF6JB, is better in many instances. You can download EZGPIB and the associated manual at http://www.ulrich-bangert.de/html/downloads.html

EZGPIB is an extended version of Pascal, with built-in support for Prologix (and some other) GPIB controllers. In particular, Ulrich has written EZGPIB so that the user can concentrate on program flow and data analysis, rather than having to worry about the nuts and bolts of handling communications flow between the program and the instrument. He has accomplished this through a library of special purpose functions and procedures (each prefixed with EZGPIB) such as:

 EZGPIB_buswritedata(address, string);

The procedure EZGPIB_buswritedata sends a string to the instrument at a particular address connected to the GPIB adapter. Similar library routines read data from the instrument and handle disk file read and write operations, and user output.

I particularly like EZGPIB's automatic detection feature—it scans your computer and detects any Prologix (or other supported GPIB adapters) and connects to them. This includes the new Prologix GPIB-LAN Controller as well as  Prologix's GPIB-USB controller. Since the connection scan process is invisible to the user, code written for the USB-based controller runs on the LAN-based controller without change. That's a great feature.

I've written a lot of Pascal code over the last 25 years and consequently had little trouble getting up to speed with EZGPIB. Ulrich's extensions are to the run time library of Delphi, so anyone that has programmed with Borland's Pascal or Delphi will find it quite familiar.

 The image below shows the EZGPIB integrated development environment running, with a program I've written to control an HP3586B selective voltmeter. The program scans a sequence of frequencies and makes an amplitude and frequency measurement for each frequency. (In fact, it makes five fast measurements, discards the high and low measurements and averages the remaining three). The data is both displayed at the bottom of the IDE and saved to a data file for later analysis and plotting.

If you wish to view the complete code for this program, click here. (I've changed the file extension from .488, the default for EZGPIB programs, to txt so that it will be easier to view in a web browser.) Below the screen capture is a sample data plot of the 3586B's output data.

The plot below, done in Origin, shows raw data (yellow) and 5 point adjacent average smoothing (red) for the signal strength of AM broadcast station WXTR as measured on my 80-meter inverted vee antenna with the HP3586B selective voltmeter running the program discussed above. The change in detail between the left side of the plot and the right side is due to implementing a signal averaging routine. The earlier data is one signal reading every minute; the later data is captured via five fast readings once every three minutes, but is the average of the middle three fast signal strength measurements, with the high and low values discarded.