Issue
All MNB devices under the UNC-520 are offline and not communicating.
Product Line
TAC IA Series
Environment
MNB controllers connected to UNC-520 RS-485 port
Cause
The MNB-300 and MNB-1000 bias resistors (jumpers label 'MS BIAS') should be disabled by default as indicated in their respective documentation. However, it appears that the MNB-1000 bias resistors are coming enabled.
As a requirement of EIA-485 network topology, an MS/TP network must have at least one set, and no more than two sets, of network bias resistors on each MS/TP network segment, preferably (but not required to be) in the middle of the segment. In MS/TP networks, this requires an MNB-300, MNB-1000, or UNC-520 with the appropriate jumper settings.
UNC-520 bias resistors on its RS-485 ports are enabled by default.
Resolution
Ensure that no more than 2 sets of bias are enabled on the MS/TP segment. The bias on the UNC-520 is enabled by default and as such, there should not be more than 1 MNB controller with its bias resistors set enabled on the BACnet MS/TP segment.
Refer to page 71 of the document ‘F-27360 IA Series Micronet BACnet Wiring, Networking and Best Practices Guide’ for further information.
Also refer to Enabling MSTP bias resistors on a MNB-1000.
