alzhao
2
AR300M has some built-in driver of USB uart adapters, for example this one:
When plugged into the USB port it will create a device as /dev/ttyUSB0
Yes, you can use serial pins. But there is a lot info will be printed to the serial console by default. This may case a trouble to you.
I don’t have a Cisco routers so I cannot confirm it 100% works.
For AR300M, the serial device is /dev/ttyS0. You can just echo or cat this device.