A single installation of weeWX can be used to collect data from multiple devices on one computer. But how can you tell which device is which?

How to distinguish devices?

If the devices are connected directly to serial ports, then the serial port identifies them. For example, /dev/ttyS0 and /dev/ttyS1 or COM1 and COM2.

USB devices are a bit more complicated.

Each USB device has a VendorID and ProductID. Most software uses these identifiers to distinguish one USB device from another. However, if there are two USB devices that have the same VendorID and ProductID, you must find some other attribute to distinguish between them.

If the devices are pure USB, then this distinction must be made in the software. For example, the Lacrosse WS28xx stations use USB transceivers that have a unique serial number burned into them. The serial parameter can be specified in the WS28xx driver.

If the devices are USB-to-serial converters, the software cannot distinguish between devices. The software sees each converter as a serial port. So the software can refer to a serial port such as /dev/ttyUSB0, but when the USB-to-serial converter is unplugged then replugged, it might show up as /dev/ttyUSB1 instead of /dev/ttyUSB0.

How do you ensure that the weeWX instance for device A only ever talks to device A?

Use udev rules to ensure a unique, consistent name for each device.

Example: two USB-to-serial converters

For example, if you have 2 FTDI USB-serial converters, call them A and B, they will show up as /dev/ttyUSB0 and /dev/ttyUSB1. But when you unplug A then plug it back in, it might show up as /dev/ttyUSB2.

Start with the weeWX configuration for each device. In the weeWX configuration file for device A, use port = /dev/deviceA. Similarly, for device B use port = /dev/deviceB.

Create a udev rule file /etc/udev/rules.d/weewx.rules that looks like this:

# FTDI usb-serial converter for device A
ACTION=="add", ATTRS{idVendor}=="0403", ATTRS{idProduct}=="6001", ATTRS{serial}=="XXXXXXXX", SYMLINK+="deviceA"
# FTDI usb-serial converter for device B
ACTION=="add", ATTRS{idVendor}=="0403", ATTRS{idProduct}=="6001", ATTRS{serial}=="YYYYYYYY", SYMLINK+="deviceB"

where XXXXXXXX is the serial number for the USB-to-serial converter connected to device A and YYYYYYYY is the serial number for the USB-to-serial converter connected to device B.

Find the serial numbers by looking at the detailed USB attributes of each device:

sudo lsusb -v

Tell the operating system to reload the rules with this command:

sudo udevadm control --reload-rules

Finally, unplug then re-plug the USB cable to each device to trigger udev to make the links.

You will now see /dev/deviceA and /dev/deviceB, each of which is a symlink to the actual device. If you unplug and replug devices, weeWX will detect the missing device, retry its connection, then every should continue as normal. No need to restart weeWX or make changes to configuration files.

How to determine the uniqueness of each device

The lsusb -v command will enumerate the attributes of each USB device. It is usually a good idea to run it with administrative privileges, e.g., sudo lsusb -v, since not all of the attributes are visible without administrative privileges.

To see how those attributes are seen by udev, use the udevadm commands. For example:

sudo udevadm info -a -n /dev/ttyUSB0

will list all of the attributes of the device /dev/ttyUSB0, using the labels that udev will recognize.

You must use the udev labels in the rules file; udev will not recognize many of the labels that you see in lsusb output, even though they refer to the same thing.