Writing BLE to Cycling Control Point - Adding Resistance - dart

Question

I have bee working with BLE for a while now, but primarily for reading and notifying characteristics.
The devices specifically are Virtual cycle trainers that support GATTS Cycling Power Service - 0x1818 link
I know that it's possible to increase resistance on this trainer, but I have read the documentation on Cycling Power Control Point - 0x2A66 [link][2] which is the only one with Mandatory write functions, but non of the documentation seem to be make sense.
Trainer: Cycleops Magnus
Reading and writing characteristic
// Reads all characteristics
var characteristics = service.characteristics;
for(BluetoothCharacteristic c in characteristics) {
List<int> value = await device.readCharacteristic(c);
print(value);
}
// Writes to a characteristic
await device.writeCharacteristic(c, [0x12, 0x34])
Reading and writing descriptors
// Reads all descriptors
var descriptors = characteristic.descriptors;
for(BluetoothDescriptor d in descriptors) {
List<int> value = await device.readDescriptor(d);
print(value);
}
// Writes to a descriptor
await device.writeDescriptor(d, [0x12, 0x34])
The closest I can see is setting the crank length, or chain weight but at this stage I am only guessing and am looking for some guidance.
The questions is this..
What characteristic or descriptor should I use to adjust Virtual Power
trainer resistance and what is the best way to do this?
Any coding Language is fine, I can transpose it.
Screenshot of services available for device
[2]: https://www.bluetooth.com/specifications/gatt/viewer?attributeXmlFile=org.bluetooth.characteristic.cycling_power_control_point.xml

Answer 1

I think you're using the wrong Bluetooth service for this. The Cycling Power Service is for collecting data from cycling power meters like this one: https://www.cyclist.co.uk/reviews/6705/long-term-review-fsa-powerbox-carbon-power-cranks
For your requirements, I believe you should be using the Fitness Machine Service (0x1826) which includes the Indoor Bike Data characteristic (0x2AD2) and most importantly for you, the Fitness Machine Control Point characteristic. Take a look at section 4.16.1 of the Fitness Machine Service specification and you'll see details of operations which the control point supports, including a reference to 4.16.2.5 Set Target Resistance Level Procedure. I think this is what you need.

Answer 2

You cannot use cycling control point(CPP) for adding resistance. CPP can only be used to copy data like wheel Revolution from old peripheral to new one or if you want to reset data on peripheral you can use cpp.
If you want to add resistance you need to check for fitness machine i am using elite and elite have Fitness Machine Control point you can write resistance and other things like inclination, elevation etc using FTCP.
Few of the vendor support fitness machine and other have given their api or source code you can use that to add resistance and other stuff like that.

Answer 3

Indoor trainers have a few services:
Cycling Power Service (ANT+ or BT also have)
ANT+FEC (ANT only)
BTLE Fitness control (FTMS)
TACX ANT+ FEC. over Bluetooth (https://blog.lazerwalker.com/2019/02/15/bike-game-part-2.html)
Wahoo's Extension to the Cycling Power Service (to be able to set Target power for instance)
To Add Resistance to the trainer to #1, you need to check if it also has the #5 service as well. (this is the UUID used - A026E005-0A7D-4AB3-97FA-F1500F9FEB8B)
#4 is actually a protocol which Tacx came up w/ before FTMS was a standard and some trainers still use this.