FTMS lab: decode treadmill, bike and rower Bluetooth data

The structures it decodes against

Each table is the layout the lab compiled from the Bluetooth SIG’s GATT Specification Supplement files at commit 408f4a0 of the SIG’s public repository (read 2026-10-17), through this site’s generated copy. The machine data characteristics, then the Supported Range characteristics the lab reads once after connecting:

How to read an FTMS value

1. Flags first. Treadmill, Indoor Bike and Rower Data open with a two-octet Flags field (boolean[16]); Cross Trainer Data’s is three octets (boolean[24]). Read it as a little-endian number.
2. Walk the fields in order. For each field, its “Present if” line names one bit and the value that makes it present. Present: read its size and move on. Absent: it takes no octets. Several fields can share one bit (Total Energy, Energy Per Hour and Energy Per Minute all hang on one), and bit 0 is the odd one out: Present if bit 0 of Flags field is set to 0.
3. Scale each value. The SIG gives a Represented values: M = …, d = …, b = … line where the raw number is not the unit, and the lab multiplies by M × 10^d × 2^b. Instantaneous Speed: Represented values: M = 1, d = -2, b = 0, beside Unit is 1/100 of a kilometer per hour. Instantaneous Cadence: Represented values: M = 1, d = 0, b = -1, beside Unit is 1/2 of a revolution per minute.
4. Merge across notifications. A machine with more to say than fits in one value can set bit 0 (“More Data”), which drops Instantaneous Speed from that value. The lab’s “Latest values” panel keeps the newest reading of each field from whichever notification carried it; the treadmill replay sends such a split packet every fifth second.

Where the decoder reads past the SIG’s file

• Byte order. The YAML gives sizes, not byte order. The lab reads little-endian, as the Web Bluetooth specification’s heart-rate example does (result.heartRate = data.getUint16(index, /*littleEndian=*/true);, read 2026-10-31).
• The scale formula. The files do not spell out how M, d and b combine. The lab uses M × 10^d × 2^b, and its test suite checks that product against every “Unit is” sentence in the dataset it can parse (b = −10 beside “1/1024 second”, M = 5 and d = −3 beside “0.005 kilogram”, and so on): 60 lines, all agreeing except 8.
• Resistance Level. 6 of those 8 are FTMS resistance fields, where the SIG’s own lines disagree: Represented values: M = 1, d = 1, b = 0 (a factor of 10) sits beside Unit is 1. The lab shows the value both ways and picks neither.
• No structure, no decoding. At this commit the SIG’s repository has no gss file for Fitness Machine Feature (0x2ACC), Training Status (0x2AD3), Fitness Machine Control Point (0x2AD9), Fitness Machine Status (0x2ADA). The lab shows their bytes as hex and does not decode them from memory. That is also why it has no control panel: setting a target speed or power means writing op codes to the Control Point, and the layout of those op codes is not in the files this site reads. The lab never writes to the machine.

Which browsers can connect

Web Bluetooth is a Chromium feature. MDN's compatibility data lists it in Chrome (requestDevice since version 56), Edge, Opera and Samsung Internet, with the note that Chrome's Linux support is not enabled by default, and marks the API experimental. It lists no support in Firefox, in Safari on macOS or iOS, or in Android's WebView.

Source: MDN browser-compat-data, api/Bluetooth.json, read 2026-11-05. Elsewhere the paste box and replays still work.

What the lab cannot tell you

• Accuracy. The bytes say what the machine reports, not whether its belt speed or power reading is right.
• Which fields a model supports before it sends them. Fitness Machine Feature declares that, and it is one of the characteristics shown raw here.
• Anything about a machine that does not advertise the Fitness Machine service: the browser’s chooser only lists devices that do. For any other device, use the BLE GATT explorer.

The integration side is in what FTMS is, treadmill app integration, indoor bike trainer integration and rowing machine data; the radio question in ANT+ vs Bluetooth. Power meters and cadence sensors have their own cycling power and cadence lab.