Bluetooth Trainer Expert

Bluetooth Trainer Expert

This skill provides a proven workflow for developing biking fitness applications that interact with Smart Trainers (e.g., Wahoo KICKR, FTMS devices). It emphasizes a "Console First" approach to isolate connection and protocol logic before attempting UI integration.

Core Philosophy

1. Console First: Always prove the protocol logic in a simple C# Console Application (BikeFitnessConsole) before touching the main UI (WPF/​MAUI).
2. Physical Verification: Telemetry (Success/​Fail responses) is insufficient for brake logic. You MUST ask the user to "Pedal and verify resistance change" to confirm the device actually engaged the magnet/​motor.
3. Protocol Agnosticism: Support both standard FTMS (0x1826) and legacy Wahoo (0xA026...) protocols.

Phase 1: Diagnostic Console App

Create a lightweight console app to handle scanning, connection, and raw command testing.

Key Features to Implement:

• Scanner: Filter for 0x1826 (FTMS), 0xA026 (Wahoo), and 0x1818 (Power).
• Service Explorer: List all available Services and Characteristics to confirm UUIDs.
• Indication Handler: CRITICAL. You must subscribe to Indicate on the Control Point characteristic before sending any write commands. Failure to do so often results in silent failures or "Attribute Not Found" errors on writes.

Data Format (Endianness)

Bluetooth LE GATT characteristics standardly use Little Endian byte order.

• Reading: When parsing multi-byte values (e.g., Speed, Power, Time), interpret the least significant byte first.
  • Example: 0xE8 0x03 -> 0x03E8 -> 1000.
• Writing: When constructing commands (e.g., Target Watts, Grade), ensure you serialize UInt16/Int16 values to Little Endian.
• Note: While standard FTMS and common Wahoo OpCodes (0x40-0x45) follow Little Endian, always verify proprietary manufacturer-specific characteristics if behavior is unexpected.

Phase 2: Protocol Identification & Control Points

Identify the correct Control Point (CP) for the device.

1. FTMS (Fitness Machine Service) - Preferred

• Service UUID: 00001826-0000-1000-8000-00805f9b34fb
• Control Point: 00002ad9-0000-1000-8000-00805f9b34fb (Fitness Machine Control Point)
• Init Sequence:
  1. Subscribe to CP Indications.
  2. Send 0x00 (Request Control).
  3. Send 0x07 (Start/​Resume).

2. Wahoo Legacy (KICKR / SNAP)

• Service UUID: a026ee01-0a7d-4ab3-97fa-f1500f9feb8b
• Control Point: a026e005-0a7d-4ab3-97fa-f1500f9feb8b (Often hidden inside the Power Service 0x1818 on some firmware).
• Init Sequence:
  1. Subscribe to CP Indications.
  2. Send 0x00 (Init/​Unlock).

Phase 3: OpCode Discovery Strategy

Different devices support different modes. Use the Console App to "Scan" OpCodes 0x40 through 0x45 by sending [OpCode, 0x00] and observing the Indication response.

• Response Format: 01-OpCode-Result (Where Result 01 = Success, 04 = Not Supported, 40 = Fail).
• Common OpCodes:
  • 0x40 Level Mode: 0-9 discrete levels. Basic.
  • 0x41 Resistance Mode: 0-100% brake force. Most reliable.
  • 0x42 Sim Mode Enable: Activates physics engine (Grade/​Wind/​Weight). Warning: Many older or lower-end devices (e.g., KICKR SNAP) return "Success" for params but FAIL (0x40) when trying to Enable this mode.
  • 0x43 Sim Parameters: Sets Grade, Weight, Crr, Cw.

Phase 4: "Fake" Simulation Mode (Grade Mapping)

If the device fails to support Native Sim Mode (0x42), you must implement Grade-to-Resistance Mapping in software to support "Hilly" or "Mountain" workouts.

Recommended Mapping (User Calibrated): Map the visual "Grade %" to the device's "Resistance %" (OpCode 0x41).

Logic: Use a piecewise linear function:

1. Downhill (-10 to 0): Interpolate 0.0 -> 0.01.
2. Uphill (0 to 20): Interpolate 0.01 -> 0.40.

Phase 5: Telemetry

• Power: Standard 0x2A63 (Cycling Power Measurement).
• Speed/​Distance: Often missing from FTMS on older devices. Calculate locally using Wheel Revolutions from the Power Measurement characteristic (Data bytes flags & 0x10).
  • Distance = WheelRevs * TireCircumference
  • Speed = DeltaDistance / DeltaTime
• Cadence: Check 0x1816 (Speed & Cadence) or Crank Data in Power (0x1818).

Checklist for Implementation

1. Console Test: Can you connect and receive Indications?
2. OpCode Scan: Which modes return 01 (Success)?
3. Physical Test: Does sending 0x41 0x32 (50% res) make it hard to pedal?
4. Mapping: Does -10% Grade feel like zero resistance?
5. UI: Does the UI display Grade (not raw resistance) to the user?