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If you are using the usb → RS232 adapter, the serial drivers may need to be installed for the device to work properly. After plugging in the device, first check the hardware messages to see the device was detected:

dmesg

Torward the end of the messages, you should see this:

usb 1-1: new full speed USB device using uhci_and address 2
usb 1-1: configuration #1 chosen from 1 choice

if you see this, then you do not need to do the rest

[ 5429.330501] moschip7720 2-1.1:1.0: Moschip 2 port adapter converter detected
[ 5429.331157] usb 2-1.1: Moschip 2 port adapter converter now attached to ttyUSB1
[ 5429.331245] usb 2-1.1: Moschip 2 port adapter converter now attached to ttyUSB2

Now, we need to figure out the device vendor number and product number to tell the usbserial driver. For this we can use lsusb. The easiest way to find the information is to first unplug the device, then run lsusb:

lsusb

You will see a summary list of your USB devices on the system, I only have one:

Bus 001 Device 001: ID 0000:0000

Now, plug the device back in, and run lsusb again:

lsusb