Servotest

A simple diagnostic + benchmarking tool for STS3215 serial servos

Overview

Servotest is a lightweight CLI tool for testing Feetech STS3215 bus servos.

It helps you quickly answer:

• Is the servo alive?
• Is communication working?
• Does it move correctly across its full range?

It also includes a test mode, so you can validate logic without hardware.

Features

• Full automatic benchmark sequence
• Live manual control (raw + degrees input)
• Macro recording and playback
• Real-time voltage telemetry
• Test mode (no hardware required)
• Robust serial handling (stable at 1 Mbps)
• Built-in guardrails for common failures

Requirements

Hardware

• STS3215 servo (7.4V or 12V)
• Bus servo adapter (Waveshare / Wonrabai recommended)
• USB cable
• External power supply (5v for 7.4v and 12v for 12v)

⚠️ USB alone will NOT power the servo

Setup

Install:

pip install servotest
servotest

The setup is like this:

                     ┌──────────────────────┐
                     │      COMPUTER        │
                     │  (Python Script)     │
                     └─────────┬────────────┘
                               │ USB-C
                               │
                               ▼
                  ┌──────────────────────────┐
                  │  BUS SERVO CONTROLLER    │
                  │ (Waveshare / Wonrabai)   │
                  │                          │
                  │   [USB / UART Interface] │
                  │          │               │
                  │          │ TTL Serial    │
                  │          ▼               │
                  │     Servo Bus Port       │
                  └───────▲───────────┬──────┘
                          │           │
            External      │           │ Servo Cable
            Power Supply              │ (3-wire TTL)
          (7.4V / 12V)                │
                                      ▼
                 ┌────────────────────────┐
                 │       STS3215 SERVO    │
                 │                        │
                 │   VCC  ───────────────┘
                 │   GND  ───────────────┘
                 │   DATA ◄───────────────┘
                 └────────────────────────┘

Signal Breakdown

USB-C → Controller
    - Power (logic side)
    - Serial communication (via USB-UART chip)

External PSU → Controller
    - Main power for servo (IMPORTANT)
    - Typically 5V or 12V

Controller → Servo
    - VCC (power)
    - GND
    - DATA (half-duplex serial bus)

Important Notes

• ⚠️ The servo is NOT powered by USB

• ⚠️ Always connect external power to the controller

• ⚠️ Set jumper to:

  Mode B (USB Mode)
  

• ✔ Communication is:

  1 Mbps TTL serial (half-duplex)
  

Mental Model

[Your Code] → USB → [Controller] → Serial Bus → [Servo]
                          ↑
                    External Power

Modes

[1] Automatic Benchmark

Runs a full diagnostic sequence:

• Move to minimum (0)
• Move to maximum (4095)
• Rapid sweep test
• Return to neutral

Also prints voltage during operation.

[2] Live / Manual Mode

Control the servo interactively:

Raw position

2048

Degrees

180d
90.5d

Exit

exit

Displays:

• Live voltage
• Position bar

[3] Macro Replay

Replays movements recorded in Live Mode.

[4] Clear Macro

Clears recorded steps.

Telemetry

Currently supported:

Voltage

• Read directly from servo register
• Updated in real time

Example:

Voltage: 12.1V

Temperature (Removed)

Temperature reporting has been intentionally removed.

Reason:

• Some STS3215 firmware variants return constant 0
• Others do not expose temperature over serial
• Register mapping is inconsistent across batches

To avoid misleading data, it is not included.

Troubleshooting

Servo not moving

• Check external power supply
• Confirm correct voltage (7.4V / 12V)

No serial ports found

• Check USB cable
• Install CH340 / CP2102 drivers

Port busy error

Close other programs using the port:

• Arduino IDE
• Serial Monitor
• Other scripts

Permission denied (Linux)

sudo usermod -a -G dialout $USER

Then log out and back in.

Getting weird or unstable readings

• Ensure correct baud rate (1,000,000)
• Check wiring (TX/RX swapped is common)
• Verify adapter is in Mode B (USB mode)

Notes

• Protocol: Feetech STS serial protocol
• Baud rate: 1 Mbps
• Position range: 0–4095
• Telemetry: voltage only

Future Improvements

• Position feedback (readback)
• Load / current monitoring
• Multi-servo support
• Optional GUI

Why this exists

Because debugging servos shouldn’t take longer than building the robot.