How do you connect PyVisa to an Arduino Uno?

Question

I'm trying to connect my Arduino Uno to my computer and writing code in python using PyVisa. I have installed PyVisa correctly, since it has worked with other devices. For some reason the Arduino Uno never returns a response. I'm using the NI Visa package, because I was hoping the pyvisa-py was the problem. Is there something I'm missing?

The code I am using to connect is:

import pyvisa_py as pv

port = "ASRL4::INSTR"
rm = pv.ResourceManager()
device = rm.open_resource(port)

The ResourceManager finds the correct port, and I don't get an error making the device. When I try to use a query however, like

print(device.query("*IBN?"))

It gives the following error:

pyvisa.errors.VisaIOError: VI_ERROR_TMO (-1073807339): Timeout expired before operation completed.

Answer 1

I haven't tried to use an Arduino with PyVisa in a long time. I always use the PySerial library instead. Here is a personal note I wrote about four years ago (in 2017) for a Python module interfacing with an Arduino.

This driver does not use the VISA layer to communicate with the device. Instead, it uses the more low-level and less general PySerial library. The reason for that is that the Arduino reboots ("resets itself") whenever the serial port is opened through VISA. This is a feature, not a bug, so that you don't have to manually reset it every time you flash a new firmware version from the Arduino IDE. As a consequence, though, the controller will not respond to requests for about two seconds after opening its serial port. The only way to avoid the reboot is making sure that the serial port's DTR line is not toggled when opening the resource. VISA, however, does so by default, and there seems to be no way to disable this disruptive behavior. In fact, VISA attributes cannot be set until after the resource has been opened. The PySerial library, on the other hand, does not have this limitation.

As the note is four years old, take it with a grain of salt. Things may have changed. But using PySerial instead of PyVisa is still your best bet. See my answer here for a few more details. And maybe do a web search with the keywords "Arduino" and "DTR" to see if there have been any recent developments.