Four Zone Heart Rate Limits (0x2B4C): byte layout
GATT characteristic · org.bluetooth.characteristic.four_zone_heart_rate_limits · read from the Bluetooth SIG’s repository on 2026-10-17 · commit 408f4a0 (2026-10-14)
Four Zone Heart Rate Limits is Bluetooth GATT characteristic 0x2B4C, org.bluetooth.characteristic.four_zone_heart_rate_limits in the Bluetooth SIG's assigned numbers. The SIG's GATT Specification Supplement file lays its value out as 3 fields: Four Zone Heart Rate Limits - Light / Moderate Limit, Four Zone Heart Rate Limits - Moderate / Hard Limit and Four Zone Heart Rate Limits - Hard / Maximum Limit. Every field has a fixed size, 3 octets in all. The SIG's User Data permitted-characteristics list includes it.
- UUID
- 0x2B4C
- Identifier
org.bluetooth.characteristic.four_zone_heart_rate_limits- The SIG’s description
- The Four Zone Heart Rate Limits characteristic is used to represent the limits between the heart rate zones for the four-zone heart rate definition (Maximum, Hard, Moderate, and Light) of a user.The structure of this characteristic is defined below.
- Service the SIG’s files tie it to
- User Data (0x181C) — listed in uds_permitted_characteristics.yaml
- Source file
- gss/org.bluetooth.characteristic.four_zone_heart_rate_limits.yaml
Byte layout: 3 fields
In the order the SIG’s structure lists them. Type and size are as published; sizes count octets except where a struct is marked as sized in bits. The offset is printed only where every earlier field has a fixed size; where an earlier field can be absent or vary, the cell names what it depends on. “Present when” is read from the field’s own “Present if” line, shown in full in the field notes below.
| Offset | Field | Type | Size | Present when | Unit |
|---|---|---|---|---|---|
| 0 | Four Zone Heart Rate Limits - Light / Moderate Limit | uint8 | 1 | Always (fixed size) | period (beats per minute)org.bluetooth.unit.period.beats_per_minute |
| 1 | Four Zone Heart Rate Limits - Moderate / Hard Limit | uint8 | 1 | Always (fixed size) | period (beats per minute)org.bluetooth.unit.period.beats_per_minute |
| 2 | Four Zone Heart Rate Limits - Hard / Maximum Limit | uint8 | 1 | Always (fixed size) | period (beats per minute)org.bluetooth.unit.period.beats_per_minute |
Field notes
Each field’s text in the SIG’s file, line by line.
- Four Zone Heart Rate Limits - Light / Moderate Limit uint8 · 1
- Unit: org.bluetooth.unit.period.beats_per_minute
- Four Zone Heart Rate Limits - Moderate / Hard Limit uint8 · 1
- Unit: org.bluetooth.unit.period.beats_per_minute
- Four Zone Heart Rate Limits - Hard / Maximum Limit uint8 · 1
- Unit: org.bluetooth.unit.period.beats_per_minute
Guides on this site
The metric Four Zone Heart Rate Limits carries, on every platform this site covers — each type quoted in its platform's own words:
No guide on this site names 0x2B4C yet. The connected-devices guides cover pairing, subscribing and testing for the device types they do name.
Questions
- What is the UUID of the Four Zone Heart Rate Limits characteristic?
- 0x2B4C. The Bluetooth SIG's characteristic_uuids.yaml lists Four Zone Heart Rate Limits as 0x2B4C with the identifier org.bluetooth.characteristic.four_zone_heart_rate_limits. The SIG's User Data (0x181C) permitted-characteristics list includes it.
- Are any Four Zone Heart Rate Limits fields optional?
- No. Every field in the SIG's structure has a size that cannot be zero: Four Zone Heart Rate Limits - Light / Moderate Limit (uint8, 1), Four Zone Heart Rate Limits - Moderate / Hard Limit (uint8, 1) and Four Zone Heart Rate Limits - Hard / Maximum Limit (uint8, 1).
- What units does Four Zone Heart Rate Limits use?
- The SIG's text names a unit for 3 of the 3 fields: Four Zone Heart Rate Limits - Light / Moderate Limit: period (beats per minute) (org.bluetooth.unit.period.beats_per_minute); Four Zone Heart Rate Limits - Moderate / Hard Limit: period (beats per minute) (org.bluetooth.unit.period.beats_per_minute); Four Zone Heart Rate Limits - Hard / Maximum Limit: period (beats per minute) (org.bluetooth.unit.period.beats_per_minute).
- How many bytes is a Four Zone Heart Rate Limits value?
- 3 octets. Every field has a fixed size in the SIG's structure: Four Zone Heart Rate Limits - Light / Moderate Limit at offset 0 (uint8, 1), Four Zone Heart Rate Limits - Moderate / Hard Limit at offset 1 (uint8, 1) and Four Zone Heart Rate Limits - Hard / Maximum Limit at offset 2 (uint8, 1).
Source
Read on 2026-10-17 from gss/org.bluetooth.characteristic.four_zone_heart_rate_limits.yaml and assigned_numbers/uuids/characteristic_uuids.yaml in the Bluetooth SIG’s public repository (branch main, commit 408f4a0, committed 2026-10-14), by scripts/fetch-bluetooth-gatt.mjs. The file’s notice says it is proprietary to Bluetooth SIG, that furnishing it grants no licence, and that it is subject to change without notice; it is © Bluetooth SIG, Inc., and its text is quoted here for identification. Generated by AIFitnessAPI, which is not affiliated with Bluetooth SIG; the SIG’s file remains the authority. The full notice and the conventions used on these pages are on the reference hub.