Mimicking udev rules with PySerial

My research group in grad school had an unfortunate culture of stealing equipment from other people’s experiments when they weren’t around. I participated in this culture as well, but I tried to replace the things I took before they returned. However, this did not spare me from the ire of a Postdoc¹ who pointed out that windows assigns device names by serial number, thus my replacing a power meter with an identical copy had caused him an hour of debugging time until he opened up the device manager.

Later, out of grad school and working on robots with even more hardware, I discovered the joy of writing udev rules and spared myself many hours of plugging in usb cables to see what they enumerate as.

I’m now out of robotics and back in experimental physics. Most experimental physicists aren’t linux users, so a lot of scientific equipment is sold with drivers which are only functional on Windows.

I had reverted back to plugging and unplugging cables, until the other experimental physicist at work switched out the hardware in my experimental setup overnight. After I had calmed down, decided to find another way.

Here’s my solution. PySerial has a lovely tool called list_ports which will list a bunch of handy information about the available com ports. By matching the vendor and product IDs, (or in the case of devices that both use the FTDI FT232R chip, also the serial number) I can guess the devices:

from serial import Serial
from serial.tools import list_ports

# hardware is represented as a tuple of (vid, pid, serial_number (if needed), baud_rate)
HARDWARE = {'temperature_controller': (1027, 24577, 'serial1', 115200),
            'counter': (1027, 24577, 'serial2', 19200),
            'motor_controller': (5824, 1155, 9600),
            'laser': (10682, 2, 115200)}

def map_hardware():
    com_ports = dict()
    for key, value in HARDWARE.items():
        found_port = False
        for port in list_ports.comports():
            if len(value) > 3:
                if port.vid == value[0] and port.pid == value[1] and port.serial_number == value[2]:
                   found_port = True
                   com_ports[key] = port.device
                   break
            else:
                if port.vid == value[0] and port.pid == value[1]:
                   found_port = True
                   com_ports[key] = port.device
                   break
        if not found_port:
            raise RuntimeError("device matching ids for key: ", key, " not found")
    return com_ports

ports = map_hardware()
handles = dict()

for key in ports.keys():
    handles[key] = Serial(ports[key], HARDWARE[key][-1])

This is a simplification. I import modules from InstrumentKit to do the actual communication rather than writing the commands directly to the handles.