Visualizing PWM

my desk is not usually this tidy

I built a DS0150 — successfully, on my second try — and wanted to measure something. My demo MicroPython program from MicroPython on the terrible old ESP8266-12 Development Board has been running since May 2020, and the RGB LED’s red channel is conveniently broken out to a header. Since the DSO150 has precisely one channel, it let me see the how the PWM duty cycle affects the voltage (and brightness) of the LED.

I can’t think of oscilloscopes without being reminded of a scene from one of my favourite sci-fi books, The Reproductive System by John Sladek. Cal, the hapless new hire in the Wompler toy factory-turned-military research lab, is showing the boss some equipment while making up more and more extravagant names for them for the clueless owner:

At each exhibit, Grandison [Wompler] would pause while Cal named the piece of equipment. Then he would repeat the name softly, with a kind of wonder, nod sagely, and move on. Cal was strongly reminded of the way some people look at modern art exhibitions, where the labels become more important to them than the objects. He found himself making up elaborate names.
“And this, you’ll note, is the Mondriaan Modular Mnemonicon.”
“—onicon, yes.”
“And the Empyrean diffractosphere.”
“—sphere. Mn. I see.”
Nothing surprised Grandison, for he was looking at nothing. Cal became wilder. Pointing to Hita’s desk, he said, “The chiarascuro thermocouple.”
“Couple? Looks like only one, to me. Interesting, though.”
A briar pipe became a “zygotic pipette,” the glass ashtray a “Piltdown retort,” and the lamp a “phase-conditioned Aeolian.” Paperclips became “nuances.”
“Nuances, I see. Very fine. What’s that thing, now?”
He pointed to an oscilloscope. Cal took a deep breath.
“Its full name,” he said, “is the Praetorian eschatalogical morphomorphic tangram, Endymion-type, but we usually just call it a ramification.”
The old man fixed him with a stern black eye. “Are you trying to be funny or something? I mean, I may not be a smart-aleck scientist, but I sure as hell know a television when I see one.”
Cal assured him it was not a television, and proved it by switching it on. “See,” he said, pointing to a pattern of square waves, “there are the little anapests.”

— The Reproductive system, by John Sladek (text copypasta from Grey Goo in the 1960s)

So we were displaying roughly 500 anapests/s there. Not bad, not bad at all …

My Raspberry Pi talks to my Oscilloscope

Hey! This post is completely ancient. It doesn’t even use Python 3. Advice given here might be well out of date.

… it complains that the oscilloscope is always making waves.


Ahem. Anyway. I have a Rigol DS1102E 100 MHz Digital Oscilloscope. For such a cheap device, it’s remarkable that you can control it using USB Test & Measurement Class commands. I’d been wanting to use a Raspberry Pi as a headless data acquisition box with the oscilloscope for a while, but Raspbian doesn’t ship with the usbtmc kernel module. I thought I was stuck.

Alex Forencich turned up in the forum with an all-Python solution: Python USBTMC (source: alexforencich / python-usbtmc). I got this working quite nicely today on both the Raspberry Pi and my Ubuntu laptop. Here’s how I installed it:

  1. Check your device’s USB code with lsusb:
    $ lsusb
    Bus 001 Device 002: ID 0424:9512 Standard Microsystems Corp.
    Bus 001 Device 004: ID 1ab1:0588 Rigol Technologies DS1000 SERIES
  2. Ensure that libusb-1.0 is installed:
    sudo apt-get install libusb-1.0-0
  3. Create a new group, usbtmc:
    sudo groupadd usbtmc
  4. Add yourself to this group:
    sudo usermod -a -G usbtmc pi
  5. As root, create a file /etc/udev/rules.d/usbtmc.rules. You’ll need to put in your device’s ID values:
    # USBTMC instruments
    # Rigol DS1100 – ID 1ab1:0588 Rigol Technologies DS1000 SERIES
    SUBSYSTEMS==”usb”, ACTION==”add”, ATTRS{idVendor}==”1ab1″, ATTRS{idProduct}==”0588″, GROUP=”usbtmc”, MODE=”0660″
    (all of the SUBSYSTEMS to MODE= should be one one line)
  6. Download and install the latest pyusb (Raspbian version is rather old):
    git clone
    cd pyusb
    python build
    sudo python install
  7. Now get python-usbtmc:
    git clone
    cd python-usbtmc
    python build
    sudo python install
  8. For this simple demo, you’ll need to convert the USB vendor IDs to decimal:
    0x1ab1 = 6833
    0x0588 = 1416
  9. Now, start python as root (sudo python) then type:
    import usbtmc
    instr =  usbtmc.Instrument(6833, 1416)
  10. This should return something like:
    Rigol Technologies,DS1102E,DS1EB13490xxxx,

If you get the status line, congratulations! You now have a fully working usbtmc link. I haven’t had much time to play with this, but I know I can make really nice screenshots to an attached USB drive using the command: instr.write(“:HARDcopy”). Many more commands can be found in the DS1000D/E Programming Guide, available on Rigol‘s site.

I had a couple of problems, though:

  1. The library seems to need root privileges, despite the udev rule thing. After creating the udev rule, you will need to reboot. This is the simplest way of getting it to work without being root.
  2. Reading from the ‘scope’s memory  chokes on non-UTF8 characters. If I do:
    rawdata = instr.ask(“:WAV:DATA? CHAN1”)[10:]
    I get a lengthy Python error which ends:

    File “/usr/lib/python2.7/encodings/”, line 16, in decode
        return codecs.utf_8_decode(input, errors, True)
    UnicodeDecodeError: ‘utf8’ codec can’t decode byte 0x99 in position 10: invalid start byte
    I have no idea what that means, or how to fix it. Alex suggested using ask_raw instead of ask, and the data comes through with no complaints.

I’ve still got to work my way through the Rigol’s data format, but other people have done that before:

  1. Controlling a Rigol oscilloscope using Linux and Python | C i b o M a h t o . c o m
  2. Ken Shirriff’s blog: Four Rigol oscilloscope hacks with Python

I’ll post any updates here, along with the Raspberry Pi forum topic: USB Test & Measurement class (usbtmc) driver?

Incidentally, if you’re working with WFM data dumps from the Rigol ‘scopes (and you should, because they make storing data to USB drives quick), mabl/pyRigolWFM is basically magic. Not merely can it describe and decode those binary files, it can do pretty graphics with no thought required:

made by pyRigolWFMHat tip for the mention: MP3 Options & Oscilloscope Interfacing For Raspberry Pi @Raspberry_Pi #piday #raspberrypi « adafruit industries blog

Update, 2013-12-20: I’ve successfully managed to run most of Ken’s examples with Alex’s code. The major modification you have to do is use ask_raw instead of ask. Example code shown below:

# -*- coding: utf-8 -*-

Download data from a Rigol DS1102E oscilloscope and graph with matplotlib
         using  Alex Forencich's python-usbtmc pure python driver
scruss - 2013-12-20

based on
Download data from a Rigol DS1052E oscilloscope and graph with matplotlib.
By Ken Shirriff,

which in turn was
Based on
by Cibo Mahto.

import usbtmc
import time
import numpy
import matplotlib.pyplot as plot

# initialise device
instr =  usbtmc.Instrument(0x1ab1, 0x0588) # Rigol DS1102E

# read data
instr.write(":WAV:POIN:MODE RAW")
# first ten bytes are header, so skip
rawdata = instr.ask_raw(":WAV:DATA? CHAN1")[10:]
data_size = len(rawdata)

# get metadata
sample_rate = float(instr.ask_raw(':ACQ:SAMP?'))
timescale = float(instr.ask_raw(":TIM:SCAL?"))
timeoffset = float(instr.ask_raw(":TIM:OFFS?"))
voltscale = float(instr.ask_raw(':CHAN1:SCAL?'))
voltoffset = float(instr.ask_raw(":CHAN1:OFFS?"))

# show metadata
print "Data size:      ", data_size
print "Sample rate:    ", sample_rate
print "Time scale:     ", timescale
print "Time offset:    ", timeoffset
print "Voltage offset: ", voltoffset
print "Voltage scale:  ", voltscale

# convert data from (inverted) bytes to an array of scaled floats
# this magic from Matthew Mets
data = numpy.frombuffer(rawdata, 'B')
data = data * -1 + 255
data = (data - 130.0 - voltoffset/voltscale*25) / 25 * voltscale

# creat array of matching timestamps
time = numpy.linspace(timeoffset - 6 * timescale, timeoffset + 6 * timescale,

# scale time series and label accordingly
if (time[-1] < 1e-3):
    time = time * 1e6
    tUnit = "µS"
elif (time[-1] < 1):
    time = time * 1e3
    tUnit = "mS"
    tUnit = "S"

# Plot the data
plot.plot(time, data)
plot.title("Oscilloscope Channel 1")
plot.ylabel("Voltage (V)")
plot.xlabel("Time (" + tUnit + ")")
plot.xlim(time[0], time[-1])

silly scope

It is a truth universally acknowledged, that a single ham in possession of a good fortune, must be in want of a oscilloscope. At Hamvention, I bought slightly more of an oscilloscope than I needed, a Rigol DS1102E.

After calibrating the probes, I cast around for something to measure. Aha! There lay an Atari Punk Console (previously) ready to show the world what its waveforms look like.

Surprsingly clean output here.

Ahh, the notorious APC “squelch-fart” noise. The poor little speaker hasn’t a chance of reproducing this, so it collapses into spasms.

A noisier high frequency signal from the APC.

The Rigol is pretty easy to use. These images were captured via its USB screen dump feature; no need for an oscilloscope camera here!