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  • Author Author: Jan Cumps
  • Date Created: Date Created
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Raspberry Pico as USB test device - part 4: Python PyVISA test bed

Jan Cumps
Raspberry Pico as USB test device - part 4: Python PyVISA test bed

In this post, I test the USBTMC and SCPI compliant instrument I developed for the Pico.
I use a PyVISA script to check if the instrument is connected. Then the outputs are switched and validated.

image

The test scenario:

  • check if there's an instrument available
  • validate the *IDN? string
  • reset
  • test all pins

Helper functions

*IDN? validation:

def test_idn():
    isPico = False
    idn = inst.query("*idn?");
    print(idn)

    # check if the IDN string matches the expected pattern. Allow for different serial number
    if(re.search("PICO-PI,LABSWITCH,\d+,01.00\r\n", idn) != None):
        isPico = (inst.is_4882_compliant)


    return isPico

control a pin:

def set_pin(pin, on):
    cmd = "DIGI:OUTP{} {}".format(pin, on)
    inst.write(cmd)

get pin status:

def get_pin(pin):
    cmd = "DIGI:OUTP{}?".format(pin)
    return inst.query(cmd)

test a pin:

def test_pin(pin):
    print("Status of pin {} before setting: {}".format(pin, get_pin(pin)))
    set_pin(pin,1)
    print("Status of pin {} after setting: {}".format(pin, get_pin(pin)))
    time.sleep(1)
    set_pin(pin,0)
    print("Status of pin {} after resetting: {}".format(pin, get_pin(pin)))

The test program

The test code uses the functions above to validate settings, and loop over the pins:

#!/usr/bin/env python3

import pyvisa as visa
import time
import sys

# the functions from the previous section sit here

rm = visa.ResourceManager()
reslist = rm.list_resources("USB?::?*::INSTR")
print(reslist)

if (len(reslist) == 0):
    sys.exit()
   
i = 0
inst = None
while i < len(reslist):
    inst = rm.open_resource(reslist[i]);
    inst.timeout = 3000
    inst.clear()

    print("+ IDN")
    if(test_idn()):
        print("Instrument found")
        # reset
        inst.write("*RST")
        # test 3 pins
        j = 0
        while j < 3:
            test_pin(j)
            j += 1

    print("SCPI errors during test: "+ inst.query("SYST:ERR:COUNT?"))
    inst.close()
    i += 1

print("Test complete")

The skeleton of this test is taken from the TinyUSB USBTMC Python unit test.

Test output:

PS C:\Users\jancu\Documents\Pico\my_examples\scpi_switch_usbtmc> & C:/Users/jancu/AppData/Local/Microsoft/WindowsApps/python3.9.exe c:/Users/jancu/Documents/Pico/my_examples/scpi_switch_usbtmc/test/visaQuery.py
('USB0::0xCAFE::0x4000::123456::INSTR',)
+ IDN
PICO-PI,LABSWITCH,0,01.00

Instrument found
Status of pin 0 before setting: 0

Status of pin 0 after setting: 1

Status of pin 0 after resetting: 0

Status of pin 1 before setting: 0

Status of pin 1 after setting: 1

Status of pin 1 after resetting: 0

Status of pin 2 before setting: 0

Status of pin 2 after setting: 1

Status of pin 2 after resetting: 0

SCPI errors during test: 0

Test complete
PS C:\Users\jancu\Documents\Pico\my_examples\scpi_switch_usbtmc>

During the execution, each of the 3 pins will be set high for a second, then set back to low. You can test that with LEDs or a DMM.

Python code

#!/usr/bin/env python3

import pyvisa as visa
import time
import sys
import re


def test_idn():
	isPico = False
	idn = inst.query("*idn?");
	print(idn)

	# check if the IDN string matches the expected pattern. Allow for different serial number
	if(re.search("PICO-PI,LABSWITCH,\d+,01.00\r\n", idn) != None):
		isPico = (inst.is_4882_compliant)


	return isPico


def set_pin(pin, on):
	cmd = "DIGI:OUTP{} {}".format(pin, on)
	inst.write(cmd)

def get_pin(pin):
	cmd = "DIGI:OUTP{}?".format(pin)
	return inst.query(cmd)

def test_pin(pin):
	print("Status of pin {} before setting: {}".format(pin, get_pin(pin)))
	set_pin(pin,1)
	print("Status of pin {} after setting: {}".format(pin, get_pin(pin)))
	time.sleep(1)
	set_pin(pin,0)
	print("Status of pin {} after resetting: {}".format(pin, get_pin(pin)))


rm = visa.ResourceManager()
reslist = rm.list_resources("USB?::?*::INSTR")
print(reslist)

if (len(reslist) == 0):
    sys.exit()

i = 0
inst = None
while i < len(reslist):
	inst = rm.open_resource(reslist[i]);
	inst.timeout = 3000
	inst.clear()

	print("+ IDN")
	if(test_idn()):
		print("Instrument found")
		# reset
		inst.write("*RST")
		# test 3 pins
		j = 0
		while j < 3:
			test_pin(j)
			j += 1

	print("SCPI errors during test: "+ inst.query("SYST:ERR:COUNT?"))
	inst.close()
	i += 1

print("Test complete")

visaQuery_20230205.zip

link to all posts