UART

Overview

UART (Universal Asynchronous Receiver/Transmitter) is the most basic point-to-point serial interface. Only TX, RX, and GND are needed for bidirectional transfer. It is commonly used for debugging, sensors, servos, and similar scenarios.

Key Features

Full-duplex asynchronous: Independent transmit and receive; no shared clock—both sides agree on baud rate.

Baud rate: Common values include 9600 and 115200; both sides must match.

Frame format: 1 start bit + 5–9 data bits (usually 8) + optional parity + 1/2 stop bits.

Logic levels: Chip output is TTL/CMOS; level shifters can convert to RS-232, RS-485, etc. for longer distances.

On-board resources: RDK 40-pin header provides multiple UARTs, e.g. /dev/ttyS0, /dev/ttyAMA0.

Usage

Use pyserial for UART in Python. Install with pip, same as for USB serial:

pip install pyserial

Typical workflow: list available serial ports → open port → send/receive data → close port

# Required imports
import serial
import serial.tools.list_ports

# List available serial ports
ports = serial.tools.list_ports.comports()
for p in ports:
    print(p.device, p.description)
    
# Open on-board UART, e.g. /dev/ttyACM0; choose based on wiring
ser = serial.Serial(
    port='/dev/ttyACM0', # Adjust to your device node
    baudrate=115200, # Baud rate
    timeout=1 # Read timeout (seconds); None blocks
    )
    
# Send data
ser.write(b'Hello UART\r\n')
# Read data
data = ser.read(10) # Read 10 bytes; may return fewer on timeout
ser.close()

Example Application

Bus Servo Example

Using the STS3215 bus servo as an example.

The servo bus requires RS-485 levels. Use a suitable controller/adapter to convert USB/TTL from the board to RS-485.

The servo uses the FT-SCS custom protocol. A command frame looks like:

Header	ID	Length	Instruction	Parameters	Checksum
0xFF 0xFF	ID	Length	Instruction	Parameter1 … Parameter N	Check Sum

Consult the servo manual for length, write instruction, parameters, etc. To control angle, length is 0x04, write instruction is 0x03, with 3 parameters: target position address 0x2A, target position low 8 bits, target position high 8 bits. Target position range: -32767 ~ 32767; BIT15 is the sign bit.

Example: To set servo ID 1 to position 10000, send the following frame in one transaction, then stop:

Byte0	Byte1	Byte2	Byte3	Byte4	Byte5	Byte6	Byte7	Byte8
0xFF	0xFF	0x01	0x05	0x03	0x2A	0x27	0x10	0x6A

The program below uses S600 /dev/ttyS7 to swing bus servo ID 1 between 2000±200.

import serial
import time

ser = serial.Serial('/dev/ttyS7', 1000000, timeout=0.1)

def write(addr, data):
    cmd = bytearray([0xff, 0xff, 1, len(data) + 3, 0x03, addr]) + data
    s = sum(cmd[2:]) & 0xff
    cmd.append((~s) & 0xff)
    ser.write(cmd)
    time.sleep(0.001)

while True:
    # 1800
    write(0x2a, bytearray([0x08, 0x07, 0, 0, 0xe8, 0x03]))
    time.sleep(1)
    # 2200
    write(0x2a, bytearray([0x98, 0x08, 0, 0, 0xe8, 0x03]))
    time.sleep(1)