<Test equipment
<GPIB on Raspberry Pi

GPIB program examples with PyVISA

Having set up GPIB and PyVISA on a Raspberry Pi, I had to get in at least some practice with PyVISA and with Python in general. Here's some minimal utilities I wrote mainly as exercises for myself, but which I do actually use from time to time. I run these on a headless Raspberry Pi, which I can either SSH into from my primary desktop PC, or, more conveniently, I can access most of them from my desktop through a web browser. These work through old-school html and cgi-bins, no PHP, Javascript or other newfangled garbage required. No need even for a dedicated Raspberry Pi; these can all run on a desktop PC just as well—I just preferred to isolate my lab equipment from my main PC to their own separate network.

Bode plot on Siglent oscilloscope with non-Siglent signal generator

More than just a program example, this was a project unto itself... (In fact, this is why I started setting up GPIB in the first place.) See more details on its own page.

The popular SDS 1104X-E oscilloscope by Siglent has a useful "Bode plot" function, which requires a Siglent AWG (Arbitrary Waveform Generator) to work. There exist various utilities to mimic the Siglent AWG with other manufacturers' devices using a microcontroller or Raspberry Pi to translate commands and responses between the two. I did the same using my Agilent E4421B RF signal generator, which is connected via GPIB (although it does have a serial port as well). The program I made for this purpose, sds_pyvisa_bode.py, uses PyVISA to talk to the RF signal generator, and should in principle work with any RF signal generator, regardless of manufacturer, as long as it understands a few standard SCPI commands. And, thanks to PyVISA, it can be connected via GPIB, serial/RS232, USB/USBTMC or Ethernet/VXI-11! Read more about the program here!

Screenshot from my HP 54501A oscilloscope

Just for kicks, I wrote a short python script that will take a screenshot from my old HP 54501A oscilloscope and store it as a PNG file. I used PyVISA, although a program this simple could just as easily be done with just the Gpib.py library. The program is incredibly short, and is shown in its entirety below. It's not really portable to other instruments (except maybe other HP oscilloscopes of the same time period), as the output data format is probably unique to the 54501A and its kin.

Oh yuck the screenshot looks ugly in black and white—the green monochrome screen of the oscilloscope is much nicer to look at!  :)

#!/usr/bin/env python3
addr = "GPIB0::7::INSTR"   #replace the 7 with your scope's GPIB address!
import pyvisa
from PIL import Image
rm = pyvisa.ResourceManager()
instr = rm.open_resource(addr)
answer = instr.read_raw(30742)
img = Image.frombytes("1", (640, 384), answer[23:])
img = img.crop((0, 0, 595, 384))
Some comments about the code: I had to limit the read operation to 30742 bytes (determined experimentally) in order not to have the scope complain of "Query UNTERMINATED" as it seemed to do with an unlimited read. The image conversion starts at an offset of 23 bytes, as those first bytes are probably commands intended for the HP printer on the same GPIB interface, instead of actual image data. Also the right-hand edge of the image is garbage, probably printer commands again, so that has to be cropped away.

Screenshot from my Siglent SDS 1104X-E oscilloscope

The more modern Siglent SDS 1104X-E oscilloscope provides screenshots directly in a variety of standard image formats. They can easily be saved to a USB thumb drive, but it's more convenient to grab them directly over the network. There's a Python script on Siglent's website which does just that by messing with sockets and wait times and all kinds of low-level stuff.

My script below does the same thing: it requests a screenshot over the network and saves it as a PNG file, and demonstrates how simple PyVISA makes this operation. Plus, if you were to connect the scope by USB instead of Ethernet, the low-level Siglent script would absolutely not work! My script would, if you just change the address to whatever USB0::something::something::INSTR that the list_resources() command finds.

#!/usr/bin/env python3
address = "TCPIP::"  #use your scope's address!
import pyvisa
from PIL import Image
rm = pyvisa.ResourceManager()
instr = rm.open_resource(address)
answer = instr.read_raw()
img = Image.open(io.BytesIO(answer))

Screenshot from my HP 8562B spectrum analyzer

Getting a screenshot from my HP 8562B spectrum analyzer was a bit more involved. Firstly, it would not voluntarily send the screenshot without extra coaxing (it needs to believe it's talking to a plotter on the same GPIB bus, but I honestly don't know how this "coaxing" I did differs from an ordinary GPIB transaction). Secondly, when it finally gave up its jealously guarded data, it came in HPGL format! But eventually I was able to convert it to a relatively clean looking SVG output. Here is a spectrum of the FM radio band, for example, measured using nothing but a rubber duck antenna on the input port. (Here the SVG has further been converted to PNG. The script does not do that automatically.)

The script to get the screenshot and to convert it to SVG vector graphics (perhaps more common nowadays than HPGL) is shown below. It requires the python-hpgl library by Alex Forencich for the image conversion.

#!/usr/bin/env python3

GPIB_ADDR = 18   # change to suit your instrument
import pyvisa, hpgl, sys, io
bus.send_command(bytes([0x3f, 0x20, 0x5f, 0x40+GPIB_ADDR]))

# Fix the HPGL fonts, convert to SVG and fix linewidth
hpgl_data=hpgl_data[0].replace(b';SR1.67,1.78;', b';SR0.1,0.1;')
svg_data=svg_data.replace('stroke-width="14.000"', 'stroke-width="1.500"')
with open("output.svg", "w") as f: f.write(svg_data)
Some comments on this script:
Perhaps this script is needlessly messy, and what is it with all the imports I always end up needing??? They make all my Python scripts terribly slow to start! Not that my spectrum analyzer is a fantastically fast instrument either, and the data transfer also takes some significant time. I don't know whether this will work with any other instrument than the HP 8562B, but if you find that it does, by all means, let me know!  :)

Integration with web server: Remote screenshots via web browser

To conveniently get the screenshots to my desktop PC, I installed a web server on the Pi and enabled its cgi-bin functionality:
   apt-get install apache2
   a2enmod cgi
   systemctl restart apache2
I then modified the above three scripts to simply write the proper http headers followed by the image file data to stdout. You can look at or download these scripts:
I simply placed them in my cgi-bin directory with the required execute permissions. That's it—it really is that easy. Now I can point my desktop PC's web browser to my Pi via to obtain a screenshot from the HP 54501A for whatever use I like. That URL can, of course, also be used inside an <img src=...> tag. (And yes, there's absolutely no error checking anywhere, so don't use these as-is on a production web server!)

A web interface for direct SCPI commands

Finally, as a quick brush up on how cgi-bins and forms work, I wrote a web interface to run arbitrary SCPI commands on any PyVISA instrument (similar to the SCPI command interface offered on the web interface of some modern instruments e.g. Siglent's oscilloscopes). The script, direct_scpi.py, is placed in the web server's cgi-bin directory. A text file called "scpi.include" is placed in the same directory, containing PyVISA resource names and optional comments, one per line, for example:
TCPIP:: Siglent SDS 1104X-E oscilloscope
GPIB0::7::INSTR HP 54501A oscilloscope
GPIB0::19::INSTR Agilent E4421B RF signal generator
GPIB0::11::INSTR HP 34401A multimeter
These instruments can then be selected from a list, or you can enable "Scan for instruments" and hit "Refresh" to also find any unlisted GPIB or USB/USBTMC instruments, as well as all serial/RS232 ports (whether or not there's an instrument connected)—but not Ethernet/VXI-11 instruments however. You can also enter a valid resource ID manually. After selecting the instrument, enter e.g. "*IDN?" as the query and hit Submit to see what happens.
See above about installing the web server and enabling cgi-bins. Note that all devices must have read-write permissions for everyone, since the web server runs typically as "nobody", "www-data" or some similar unprivileged user. Note also, that for some reason PyVISA will hang when executing list_resources() if none of the instruments on the GPIB bus are switched on. I initially had the bus scanned for new instruments automatically every time, but eventually had to add a checkbox to enable scanning only when needed.

Some obvious notes:

I wrote these simple scripts for my own enjoyment, my own use, and for a bit of exercise in Python, which I'm a complete newbie in. Please do not assume these to be secure, sensible, safe, or even functional pieces of code! Please do not use these in any mission critical application without thoroughly checking what they do and adding necessary fail safes! I use these in my home lab, and do not do anywhere near enough error checking in them. Please do not place the cgi-bin versions on any publicly accessible Internet-facing server! These most likely contain every single kind of code injection vulnerability ever documented, and then some. All the code on this page is in the public domain, free to use as you wish, but whatever you do, don't blame me for what happens!

And sorry if my Python looks like I'm coding with my left foot—I said I'm an absolute, complete, helpless newbie in Python!  :)

Antti J. Niskanen <uuki@iki.fi>