BACnet bCX MSTP network loses communication when a b-Link is added or removed

Issue

BACnet bCX MSTP network loses communication when a b-Link is added or removed

Product Line

Andover Continuum

Environment

MSTP b3 bCX4040  b-Link

Cause

The bCX MSTP circuitry is slightly different to the b-Link  MSTP circuitry , thus under some circumstances, other faults on the MSTP network can cause communication to change, when the network is connected via a b-Link

Resolution

  1. If an MSTP network is installed correctly, then it will operate correctly at all speeds up to 76.8k (assuming all devices operated up to this speed)
  2. If the MSTP network is not stable up to 76.8k, or adding or removing a b-Link causing a loss of communication, then it is likely there is some fault on the network.
    (adding or removing the end of line termination resistors, is also a useful fault finding option)
  3. The bCX MSTP components are slightly different to the b-Link MSTP components, thus under some circumstances, other faults on the MSTP network can cause communication to be lost, when the network is connected via a b-Link

Example
This has been seen when third party variable speed drives (VSDs) were connected to a bCX MSTP network (these unit had switchable termination resistors that by default were in circuit)
The communication was ok at a low speed (19200) connected directly to a bCX, but when the network was put on the secondary side of a b-Link, the communication failed.

Once the termination resistors were switched out of circuit, the communication via the b-Link was fine.

In the example above the VSDs were by ABB

Related: Lessons Learned Article #2934