Typical faults and their solution

BACnet on MS/TP involves the use of a large number of components and this results in a certain degree of complexity and a corresponding vulnerability to errors.

If a problem arises in the system, it must be located in a step-by-step procedure. Always start you analysis at the lowest system. Only continue to the next higher system once you are sure that the system is 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 manager) or no subordinate proxy is set for the subordinate.
  • Check the terminating resistances and terminals or cabling.
  • The network is not or incorrectly configured. Check the settings on the BACnet routers PXC5.E003 and on the DXR2.M... devices.

New BACnet MS/TP devices are not recognized.

  • The max_manager 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.

Communication is slow.

  • A manager device normally holds the token for only a few milliseconds, with the forwarding to the next device taking up to 250 ms. In other words, a high number of devices causes a high cycle which requires a very conservative (long) timeout value. You must configure the following timeouts if MS/TP devices participate in the communication:
    • APDU timeout: 6000 ms
    • APDU segment timeout: 5000 ms
    • Max Info frames: 50
  • Operation on the Desigo CC management platform requires, where there are lots of BACnet MS/TP devices, reading the functions sequentially by discipline. This requires training course and operator instructions.

Communication no longer restarts after a power outage or communication no longer starts after a power outage on the BACnet/IP router.

  • Communication – Data traffic is too high or way too high. In other words, reduce or cease the following:
    • Room segment communication
    • No lighting or blinds application
    • Max. one Desigo CC management platform
    • Max. 5 trends per room
    • Max. 3 alarms per room
    • No central functions for gathering, i.e. for the hot water supply chain, chilled water, air

BACnet networks

  • PXC5.E003 operates on large projects as BBMD. These devices must have a fixed IP address and not DHCP.
  • Only one PXC5.E003 with BBMD functionality permitted per BACnet network.

Have you considered notes from technical documentation?

  • Desigo Ethernet, TCP/IP, MS/TP and BACnet fundamentals (CM110666)
  • Desigo room automation engineering, mounting and installation (CM111043, Section Network topology)