Basic Documentation

BACnet networks with MS/TP trunks
Diagnostics and troubleshooting in BACnet networks with MS/TP trunks
5
66 | 71 A6V11159798_en--_03
operating correctly. A sketch of the topology is extremely helpful when attempting to locate problems.
Create a sketch if none exists.
BACnet MS/TP devices are introduced to the Desigo system over the BACnet MS/TP IP router to the
Ethernet. In other words, you consider topics from the BACnet/IP world and include it in your analysis when
troubleshooting problems.
The following questions can help to troubleshoot communication problems.
Faults Possible causes & troubleshooting
Check cabling prior to commissioning ● 3-wire cabling wherever possible. Is the common reference signal wiring only
grounded on one end with the PTC thermistor (TRF250-120)?
● Are the wires + and - for the EIA-485 bus correctly terminated with 120 Ohm
resistance at both physical ends of the wires?
● Note that some MS/TO devices have integrated resistance. Make sure they
are disabled (unless the corresponding device is in fact at the end of the EIA-
485 bus).
● Make sure in a mixed operation of 2 and 3-wire environment, that the cable
shield is not used as a common reference signal wire for 3-wire devices.
● MS/TP does not support a star topology Split up the cabling and eventually
network it over multiple trunks and routers.
● Check in a mixed operation with 2 and 3-wire environment that the cable
shield is not connected.
No communication to the BACnet MS/TP
devices.
● Is the entered baud rate incorrect? The same transmission rate must be
configured on all MS/TP devices on the same EIA-485 bus, with the slowest
of the devices specifying the value.
● Max. 115 kBaud, but the baud rate must be set back to 76.8 kBaud at cable
lengths in excess of ~750 m.
● The devices are incorrectly configured (no master) or no slave proxy is set for
the slave.
● Check the terminating resistances and terminals or cabling.
● The network is not or incorrectly configured. Check the settings on the
BACnet routers PXG3.M and on the DXR2.M... devices.
New BACnet MS/TP devices are not recognized. ● The MAX_MASTER value must be set to the highest available MS/TP address
(max. number of devices).
● Max. number of 64 MS/TP Desigo room automation devices is exceeded.
The network does not recognize the BACnet
MS/TP devices at the end of the trunk.
● Check whether the wiring between the last visible and next device is OK.
Communication is not reliable. ● Baud rates are not optimally set and should not be less than 76,800 baud.
● Check the cable shielding and terminating resistances.
● Terminating resistances are missing or installed in the wrong location.
● Wrong cable, incorrect polarity or the cable is grounded.
● A MS/TP third-party device on the trunk.
● Cable length exceeded. Install a repeater in the middle of the trunk.