Connecting embedded Linux development boards, routers, or industrial equipment requires a USB-to-RS232 or USB-to-UART serial converter (FTDI, Prolific PL2303, or CH340). When serial communication fails, verifying whether the USB adapter hardware and Linux kernel drivers are functioning properly is the first diagnostic step.

Step 1: Detect USB Serial Hardware in Kernel (dmesg)

Plug in your USB serial adapter and run dmesg to inspect kernel initialization log messages:

Kernel Log Inspectionbash
# Inspect kernel message buffer for new USB serial device binding
dmesg | grep -i -E "tty|usb"
 
# Example Expected Output:
# usb 1-2: FTDI USB Serial Device converter now attached to ttyUSB0

Step 2: Physical Loopback Test (TX to RX Short)

A hardware loopback test verifies that data transmitted from your computer loops directly back into the receiver:

  • DB9 RS-232 Adapter: Short Pin 2 (RX) and Pin 3 (TX) together using a wire jumper or paperclip.

  • TTL UART Board: Connect the TXD pin directly to the RXD pin.

Picocom Terminal Loopback Sessionbash
# Grant user dialout group access (if permission denied)
sudo usermod -aG dialout $USER
 
# Launch picocom terminal at 115200 baud rate
picocom -b 115200 /dev/ttyUSB0
 
# TYPE CHARACTERS: In a loopback state, every typed key echoes immediately onto your screen!