From firmware ver2.0, BACRouter introduced new “Max_info_frames by token occupy time” feature.
In BACnet standard of MSTP, a master device could hold token until it has sent frames up to Max_info_frames. The default value of Max_info_frames is 1. But for router, this value may be increased to improve bandwidth between networks. Mostly the suggested value for router is between 5 to 20.
MSTP works as a field bus for controllers; sensors and actuators. The data exchanging latency between devices usually should be guaranteed. We recommend devices get token at least every 1 second.
The APDUs passing router usually have size between 10~50 bytes, but could be up to 480 or 1476 (Extended frame). Larger APDU need more time to send or receive.
For APDU which need a reply from targeted device, router has to wait for reply. Usually the targeted device need more time to handle or generate larger APDU, router has to wait longer.
So the time router holding token could be varied much, which impacts latency guaranty of MSTP bus. To avoid this problem, “Max_info_frames by token occupy time” feature limits router’s token holding time.
The limitation is calculated by:
byte_time * 32 * Max_info_frames
For example, Max_info_frames is set to 10. The baud rate is 76800bps, so the byte_time is 0.13 milliseconds:
0.13 * 32 * 10 = 41.6 milliseconds.
When router founds it have held token for 41.6 milliseconds, it passes token to next station, though the frames it sent may be less than 10.
This feature could be enabled/disabled by user from WebUI.