This is a BACnet specification Addendum. Below is an outline of what is described.
Although EIA RS-485 practice allows for a third wire to connect transceiver common or reference points together
so the receiver common mode rejection voltage limit is not exceeded, the BACnet specification in Clause “9.2.1
Medium” only mentions “shielded, twisted-pair cable.”
Due to electrical noise issues, some hardware applications cannot successfully communicate over EIA RS-485
without a remote common or reference connection and internal isolation. This has been particularly noticed in
variable speed drive controllers that can produce local ground noise in excess of the EIA-485 common mode
voltage limit.
BACnet MS/TP also requires 1500-volt isolation when crossing buildings, but does not specify any mechanism for
doing this.
This proposed change is designed to describe wiring topologies for a reference wire in EIA RS-485 and to specify
acceptable topologies for interoperable connections for both single-building and multiple-building installations.
