BACnet MSTP – Tusage_timeout, Nretry_token

Tusage_timeout
The minimum time without a DataAvailable or ReceiveError event that a node must wait for a remote node to begin using a token or replying to a Poll For Master frame: 20 milliseconds. (Implementations may use larger values for this timeout, not to exceed 100 milliseconds.)

Nretry_token
The number of retries on sending Token: 1.

Tusage_timeout variable is used when the token is sent out.
If the node that receives the token (NS) does not use the token before the previous stations (TS) Tusage_timeout the previous station (TS) will resend the token Nretry_token times before attempting to poll for a new master.

For example
If Tusage_timeout is set to 20 and Device A sends the token to Device B, device B has a total of 20 ms to use the token before Device A times out and sends the token again. This cycle will continue Nretry_token times. If Device B never responds in time then Device A will attempt to poll for a new master starting at Device B + 1 MAC.

I suggest that you increase the Tusage_timeout from 20 ms to something like 50-100 ms.
Must not exceed 100ms