Issue
Communication issues caused by improper grounding of the BACnet MS/TP shield connections to the B-LINK-AC repeater.
Environment
BACnet MS/TP installation with B-LINK-AC repeaters.
Cause
The shield connection on port 1, labeled "(Input)", of the B-LINK-AC repeater is internally wired differently than the four other ports.
Resolution
Due to the differences in how the shield terminals are wired, the following techniques must be followed when wiring the B-LINK-AC BACnet MS/TP repeater.
- Port 1, marked (Input), must have its MS/TP bus shield connected to the SHLD terminal, which is isolated from Earth Ground. The incoming MS/TP shield must be grounded at only one point, at a location other than the repeater. Typically this is at the opposite end of the bus, or where the MNB-1000, UNC, ENC or other BACnet parent controller resides.
- All four of the other repeater ports, # 2 - 5, have their SHLD terminals tied together, and tied to Earth Ground internally, through the power supply's Earth Ground connection. For each MS/TP bus cable connected these ports, the single point of ground for the shield is at the repeater and, therefore, the shield must not be grounded at any other point on the bus.
None of the five MS/TP ports on the repeater have internal termination resistors. If a MS/TP trunk ends at the repeater, a 120 ohm terminating resistor must be connected between the MSTP+ and MSTP- terminals on that port of the repeater. Likewise, the far end of each MS/TP trunk must also be terminated with a 120 ohm resistor between the MSTP+ and MSTP- terminals of that last controller.
None of the five MS/TP ports on the repeater have internal bias resistors. Standard practices, regarding the biasing of the MS/TP trunks, should be applied, as described in the I/A Series MicroNet BACnet Best Practices Guide, F-27360.
Additional information on the use of these repeaters can be found in the "Approved MS/TP Repeaters" section of Chapter 5 of the I/A Series MicroNet BACnet Smoke Control Systems manual, F-27419.
