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Table of Contents
Questions and Answers
Here are a few interesting Q&A gathered from the BACnet mailing list.
Broadcasts
When sending a Who-Is broadcast, I have some devices that won't answer I-Am like I would expect them to.
- Christian Fortin
(…) broadcasts can and will get lost, especially when there are a lot of them. If you broadcast a Who-Is with no parameters, ALL devices have to answer, so you will get a flurry of I-Am, some of which may get dropped.
- John Hartman
See the related blog post
New version of the Tridium Jace bacnet module, if configured to act as a bacnet/ip - MSTP router, will respond to “local” broadcast message (ex. 192.168.1.255)
But not to Global broadcast messages (ex. 255.255.255.255)
- Christian Tremblay
B/IP messages sent to 255.255.255.255 should be handled as normal BACnet broadcasts, as that address is reserved for broadcasting on the local subnet. So, these messages should be handled identically to messages sent to the broadcast address of the subnet.
- Coleman
max_info_frames
Would it matter if the devices do NOT support read_property_multiple?
- Bob Kline
Adjusting max_info_frames can improve performance – or make it worse.
If the link has a “system controller” who initiates most requests (read a bunch, write a bunch, repeat), then increasing that device’s max_info_frames is usually a good idea, to avoid needing to wait for a token pass that no one else will use anyway.
Similarly, a router from IP to MS/TP should usually have a reasonably large max_info_frames, since the router often represents multiple devices on some other network.
BUT: if you go TOO large, then one device can dominate the link, slowing the token rotation time, so that everyone else will have to wait longer to send their own requests. And a gigabit IP device can ALWAYS swamp a 76800 baud MS/TP link
- John Hartman