Issue
Routers or devices with routing capabilities may not support BACnet Protocol 16 Extended Frames (1474 bytes) and only support BACnet MSTP Frame size (480 bytes), so devices that support the larger size need to receive a request with a smaller APDU defined or they will use the largest APDU they support, which will be dropped at the router. Each BACnet device has a Max APDU Length Accepted property and SpaceLogic/Easylogic, which supports Extended Frames, is 1474 bytes.
Product Line
TAC IA Series
Environment
- MP-X under Tridium offer
- SE branded Tridium offer
- I/A Series MNB 1000
- Building Operation EasyLogic RP-C
- Building Operation EasyLogic RP-V
- Building Operation EasyLogic MP-C
- Building Operation EasyLogic RP IO Expansion Module
- Building Operation Multi-purpose Controller
- Building Operation Multi-purpose VAV
Cause
Routers and devices, acting as routers, that support BACnet Protocol Revision lower than 16 will drop the extended frames (aka large packets). BACnet Protocol 16 introduced Extended Frames (1474 bytes) to BACnet MSTP, which increased the frame size from 480 bytes. BACnet devices have Max APDU Length Accepted, which communicates the largest frame size accepted. SpaceLogic/Easylogic have Max APDU Length Accepted = 1474 bytes.
Resolution
- Add bacnetUtil-rt module to the station, which is included in default Niagara installation, using Platform -> Software Manager.
- Open Palette -> bacnetUtil
- Select ApduSizeOverride from DeviceOverrides folder
- Drag-n-drop it on the BACnet controller
- The default is 480 for BACnet MSTP. To disable for SpaceLogic/EasyLogic controllers communicating via IP or on MSTP supporting Extended Frames, you can change to 1474 to match the CONFIG -> Device Object -> Max APDU Length Accepted.
NOTE A: Support was added in Niagara 4.10u10, 4.14u1, and 4.15.
NOTE B: This can be added to any device, so changing to 480 will indicate how much slower communication would be if using an MSTP router that does not support Extended Frames. I tried by viewing Device Report in N4.14 while capturing in Wireshark, so I could see packet timing on the wire, and it was about 3 times slower.
| ApduSizeOverride | # of Request Required | Period |
| 480 | 46 | 17.13 secs |
| 1474 | 14 | 6.76 secs |
