What It Is
Max Masters is a BACnet MS/TP parameter that defines the highest master MAC address the token-passing algorithm will search for on the trunk.
This is one of the most important BACnet MS/TP configuration values because it directly affects whether some devices ever get the token at all.
Why It Matters
| Setting Quality | Effect |
|---|---|
| Too low | Devices with higher MAC addresses are silently excluded |
| Too high | The bus wastes time polling empty addresses |
| Correct | Token rotation reaches every master efficiently |
[!WARNING] If Max Masters is lower than the highest real master MAC address on the trunk, higher-addressed masters can look offline even when wiring and baud settings are correct.
Best Practice
Set Max Masters to the highest master MAC actually used on the trunk, not to a guessed default and not automatically to 127 unless the design really needs it.
| Example Trunk | Recommended Max Masters |
|---|---|
| Masters at 1, 5, 12 | 12 |
| Masters at 3, 18, 31 | 31 |
| Masters only below 8 | 8 or highest actual master |
Common Failure Modes
| Symptom | Likely Cause | First Check |
|---|---|---|
| High-MAC devices never appear | Max Masters too low | Compare configured value with highest active master MAC |
| Network works but feels slow | Max Masters too high | Check whether the trunk is polling unused address space |
| Different tools show inconsistent visibility | Masters use different assumptions | Confirm Max Masters values across all masters |