Issue
BACnet MS/TP Bus Communication Troubleshooting Checklist
Product Line
TAC IA Series
Environment
I/A Series BACnet controllers (MNB-70, MNB-V1, MNB-V2, MNB-300, MNB-1000) on an MS/TP bus.
Cause
BACnet MS/TP Bus Communication Troubleshooting Checklist
Resolution
This checklist provides several settings/configurations to review when faced with a communication problem at the controller level. The content was compiled from different resources, summarized, and laid out in an easy-to-use manner so as to provide a quick-reference document.
Power
Each controller requires 24 VAC from a Class 2 transformer.
See the Installation Instructions for each controller, and the MicroNet BACnet Wiring, Networking, and Best Practices Guide, F-27360-7, Chapter 1, “Power Supply Wiring”.
Read-Only Flag
Use the WorkPlace Tech Commissioning Tool (WPTCT) to check “Device Properties” of the controller. If the status is “Operational, Read-Only”, the input voltage to the controller may be too low. See the MicroNet BACnet Wiring, Networking, and Best Practices Guide, F-27360-7, Chapter 1, “Power Supply Wiring”.
Due to the short duration of the low-voltage condition required to set the flag, it is unlikely that you will see it when measuring the input voltage with a digital meter, so check the “Device Properties”, and correct the problem as noted on page 28 of the guide, (reduce transformer loading, reduce wire length between transformer and controller, and /or use wire with higher current rating).
Diagnostic LEDs
The MNB controllers have onboard diagnostic LEDs that are useful in troubleshooting. See the MicroNet BACnet Wiring, Networking, and Best Practices Guide, F-27360-7, Chapter 3, “Communications Hardware Checkout”, Table: “LED Indications”, for a detailed description of the behavior of the LEDs and their diagnostic uses.
Topology
It is very helpful to draw the topology of the bus when you start troubleshooting. If there are no repeaters on the bus, the topology must be a daisy-chain, with no tees or stars. MS/TP network wiring to MN-Sx sensors must be run to the sensor, then to the next controller on the bus, in a daisy-chain; no tees or spurs are allowed. Maximum length of a wire segment is 4000 feet.
See the MicroNet BACnet Wiring, Networking, and Best Practices Guide, F-27360-7, Chapter 1, “MicroNet MS/TP Network Wiring”, for details on Communications wiring.
Repeaters
- B-LINK-AC-S, Andover Controls MS/TP Repeater
- B-LINK-AC-F-S, Andover Controls MS/TP Repeater Fiber
See the MicroNet BACnet Smoke Control Systems Manual, F-27419, for details on the use of these wire and fiber repeaters with MNB controllers. See F-27419, Chapter 5, “Approved MS/TP Repeaters”, including the wiring example.
Maximum Number of Devices
The maximum number of MNB controllers depends on the cable used, and the Baud Rate.
See the MicroNet BACnet Wiring, Networking, and Best Practices Guide, F-27360-7,Chapter 1, Table, “Approved Cable Types”.
The MNB controllers are quarter-load devices, so a maximum of 128 or less are allowed on an MS/TP trunk, depending on the cable used.
Addressing
Avoid duplicate addresses.
See the MicroNet BACnet Wiring, Networking, and Best Practices Guide, F-27360-7, Chapter 1, “Required Configuration”,
CAUTION:
“Duplicate addresses on an MS/TP network trunk will result in erratic behavior, lost tokens, and disrupted communication.
There is no software tool that will identify duplicate addresses on an MS/TP network trunk. Typically, if two controllers are set to the same address, one of the controllers will appear to be missing from the list, and the address shared by the two controllers will intermittently come and go from the list.”
Biasing
As a requirement of EIA-485 (formerly RS-485) bus topology, an MS/TP network must have a minimum of one set, and a maximum of two sets, of bias resistors in place. The MNB-300, MNB-1000, UNC-520, and ENC-520 have jumpers to enable or disable the bias resistors.
See bulletin TB06-09, “RS-485 Bias Resistors and the UNC-520-2 / ENC-520-2”, page 3, for a photo of the bias jumpers on a UNC-520-2.
Terminators
A 120-Ohm End-of -Line (EOL) resistor is required across the “MS/TP +” and “MS/TP -” terminals at each controller at the end of a wire segment. The MNB-1000 and MNB-300 are equipped with a jumper-selectable end-of-line (EOL) termination resistor. The default position for the EOL jumper is “EOL termination not present.”
See the MicroNet BACnet Smoke Control Systems Manual, F-27419, Chapter 5, “Approved MS/TP Repeaters”, Figure titled “UL 864 Listed MS/TP Repeater and Isolation”, Note 4, for an important exception, requiring external EOL for the MNB-300 and MNB-1000, when used on a bus with an approved repeater.
If a UNC-520-2, ENC-520, MNB-V1 or MNB-V2 is installed at the end of the trunk, install a discrete 120 Ohm EOL resistor, part number EMSC-546, or equivalent.
Cable
Use of the proper cable is critical to the communications performance of the networked controllers. See the MicroNet BACnet Wiring, Networking, and Best Practices Guide, F-27360-7,Chapter 1, Table, “Approved Cable Types” for a list of recommended cable types. As shown in the table, cable selection affects both the maximum number of controllers allowed on the trunk, and the maximum baud rate.
WorkPlace Tech Interface
The MNB-ROAM-IA is the recommended communications converter for connecting WorkPlaceTech, including the WPT Commissioning and Flow Balance Tools, to an MS/TP trunk. It is supported in versions 5.7.1 and later. A converter that had been recommended earlier, the B & B Electronics model USOPTL4, was found not to operate reliably at speeds above 38.4 K baud. (See Direct MS-TP Connection with B & B Electronics Converters (Revised) - TB08-02-02 for details). The MNB-ROAM-IA supports Bluetooth wireless connection between WorkPlace Tech suite and the MS/TP network, for distances up to about 40 feet. One external Bluetooth adapter has been tested and approved - the Belkin USB adapter. (See Supported models of Bluetooth adapters with WorkPlace Tech for details).
A problem we have seen with some MNB-ROAM-IAs is when trying to add Bluetooth device, the Bluetooth Wizard identifies it as "Serial Port Device, Unknown: Major(0), Minor(0)" instead of "tac-RoamIA:xxxxxxxx" where xxxxxxx is the serial number. There is no way for WPT to use the ROAM-IA if the computer can’t pair with it. Currently, there is no known solution, other than replacing the device.
Divide and Conquer
This is a strategy for dividing a bus into smaller sections in order to isolate a controller or section of bus that might be causing communication problems.
The general idea is to divide the bus in half, and see if communications are possible from one end to the other end. If not, divide each half again, and continue until the problem(s) are found.
Reference Documents
- F-27360 TAC I/A Series MicroNet BACnet Wiring, Networking, and Best Practices Guide
- F-27358 TAC I/A Series MicroNet BACnet Workplace Commissioning Tool and Flow Balance Tool User’s Gui...
- F-27456 TAC I/A Series MicroNet BACnet MNB-70 Zone Controller Installation Instructions
- F-27345 TAC I/A Series MicroNet BACnet MNB-300 Unitary Controller Installation Instructions
- F-27346 TAC I/A Series MicroNet BACnet MNB-V1, MNB-V2 VAV Controller Installation Instructions
- F-27347 TAC I/A Series MicroNet BACnet MNB-1000 Controller Installation Instructions
- F-27419 TAC I/A Series MicroNet BACnet Smoke Control Systems Manual
