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
- 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)
- 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) - 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
