What Token Passing Is
Token passing is the media access method used by BACnet MS/TP. A logical token circulates among master devices on the RS-485 bus. Only the device currently holding the token is allowed to initiate communication — all other devices must wait.
How It Works
- The token holder sends its messages (up to Max Info Frames)
- It passes the token to the next master by MAC address
- If the next expected master doesn’t respond, the token holder searches for the next higher MAC up to Max Masters
- The cycle repeats continuously
Slave devices (MAC 128–254) never hold the token. They respond only when polled by a master.
Token Loss and Recovery
If the token is lost (e.g., a master device goes offline while holding the token), the remaining masters detect the silence and initiate a Poll For Master process to re-establish token rotation. This causes a brief communication interruption.
[!NOTE] Frequent token loss events usually indicate wiring issues, duplicate MAC addresses, or a device that intermittently drops off the bus.
Performance Impact
Token rotation time directly affects polling latency. More masters and higher Max Masters values increase the time for the token to complete a full cycle.