How Serial Servos Work (and Why They’re Awesome for Robotics)

If you’ve worked with regular PWM hobby servos before, you know the drill: you feed them a repeating pulse, and they move to a position based on pulse width. Simple, cheap, and good enough for many projects.

But once you step into robotics or more advanced motion control, you’ll run into a more powerful alternative: serial servos. These devices look like normal servos, but instead of a wobbly pulse signal, they talk back and forth with your controller over a digital serial link.

In this post, we’ll explore how they work, why you’d use them, and even write a MicroPython example to get you started.

PWM vs Serial Servos

• PWM Servos

  • Control via 1–2 ms pulse repeated every ~20 ms.
  • Each servo needs its own control pin.
  • One-way communication: you send position, they (hopefully) move.
  • Cheap, great for simple projects.

• Serial Servos

  • Controlled by packets of digital data (UART, RS-485, CAN, etc.).
  • All servos share a single data line (daisy-chained).
  • Each servo has an ID, so you can address them individually.
  • Two-way communication: ask them for position, load, temperature, voltage.
  • Advanced features: torque limiting, smooth motion, velocity profiles.

How IDs Work

Each servo has a unique ID stored in EEPROM.

• New servos often default to ID = 1.
• You can change the ID via a special command.
• Unlike addressable LEDs (which rely on physical order in the chain), servo IDs are fixed and persistent.

Tip

Always set IDs one at a time when first configuring your servos. It prevents collisions when multiple devices try to reply at once.

One Wire, Two Directions: Half-Duplex

Most serial servos use half-duplex UART:

• One data wire for both TX and RX.
• Only one side talks at a time.
• The controller sends a command, then releases the line. The servo replies with feedback.

Some higher-end servos use RS-485 differential pairs, which are noise-resistant and great for long cables.

Example: Dynamixel Packet

Here’s what a “move to position” command looks like for a Dynamixel AX-12:

FF FF 01 05 03 1E 00 02 D5

• FF FF → header
• 01 → servo ID
• 05 → length (instruction + params + checksum)
• 03 → WRITE_DATA instruction
• 1E → address (goal position)
• 00 02 → value (512 = center position)
• D5 → checksum

You don’t need to memorize these — libraries exist — but it’s useful to understand what’s happening under the hood.

MicroPython Example: Moving a Serial Servo

Let’s control a LewanSoul LX-16A (a popular budget serial servo) from a MicroPython board (e.g., ESP32 or Raspberry Pi Pico).

from machine import UART, Pin
import time

# Configure UART on GPIO pins
# Note: LX-16A uses 115200 baud, half-duplex UART.
uart = UART(1, baudrate=115200, tx=Pin(4), rx=Pin(5))

def checksum(data):
    return (~sum(data) & 0xFF)

def move_servo(servo_id, position):
    """
    Move a servo to the target position (0–1000).
    """
    # LX-16A move command: 0x55 0x55 ID LEN CMD POS_L POS_H TIME_L TIME_H CHKSUM
    cmd = [0x55, 0x55, servo_id, 0x07, 0x03,
           position & 0xFF, (position >> 8) & 0xFF,
           0x20, 0x00]  # move time = 32 ms
    cmd.append(checksum(cmd[2:]))  # checksum excludes headers
    uart.write(bytearray(cmd))

# Example usage
while True:
    move_servo(1, 300)   # Move to position 300
    time.sleep(1)
    move_servo(1, 700)   # Move to position 700
    time.sleep(1)

Hints & Tips

• Start with one servo: Set its ID, confirm it moves, then add more.
• Use proper power: Serial servos often draw more current than PWM ones — a stable power supply is essential.
• Daisy-chain cleanly: Keep wires short where possible, and avoid crossing noisy power lines.
• Check feedback often: Read servo temperature or voltage to protect your hardware.
• Broadcast commands: Most protocols have a “broadcast ID” to move all servos at once (great for symmetrical motions like robot legs).

Why Use Serial Servos?

If your project is simple (like a small pan-tilt camera), PWM servos are fine.

But if you’re building:

• A robotic arm with many joints
• A walking robot
• A system where reliability and feedback matter

…then serial servos will make your life much easier. Fewer wires, smarter control, and access to diagnostics.

🔧 Bottom line: Serial servos take a little more setup, but open up possibilities that PWM servos simply can’t touch.