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5 Tips for BACnet Best Practices


February 22, 2019

1. Recognize the difference between BACnet options

BACnet has a multitude of protocols available, but Setra products typically only operate on one of two options.

The first option is BACnet MS/TP. MS/TP is a simple system with a low installation cost that doesn’t require IT support. However, BACnet MS/TP has its disadvantages. MS/TP devices store little data, are slow to transmit data, and have a limit on the number of devices per branch.

The second option is BACnet Ethernet/IP. The Ethernet/IP option boasts high speeds and can host a large number of devices. It is a complex data-capturing system and most are in place with IT support. Despite its advantages, the Ethernet/IP option is expensive to install and requires hubs and routers. Without a hub or switch, this option has a maximum distance of 100 meters and IT support is required.

2. Reference Drawing

A drawing should indicate the string of BACnet devices and their wiring position within the branch. On the drawing label each device with a title (ex: room ID), the device product name, the MAC address, device instance, and baud rate. If a new BACnet device is added, the drawing should be revised to show its position in the branch.

3. Proper Wiring

Check to assure all wiring is properly done. All A(-) go to all A(-), all B(+) go to all B(+), and all GRND go to GRND. GROUND wiring should be a conductor and not a shield drain.

Ideally, the wiring should be a shielded twisted pair with 3rd conductor and shield drain. Keep the wire from the daisy chain to the device to less than 1 inch. The maximum wire length allowed is 4,000 feet.

4. Startup Procedure

For proper setup, set the baud rate to 9600. Use slow speed to verify a working system and once verified, set the device to its recommended baud rate. Start with only 2 devices to prove the MS/TP is working, and add on each additional device after the system is proven to be working. If the system stops working after several devices are connected, activate the “PULL-UP & PULL-DOWN” resistors in only one of the devices. Additionally, it is good practice to activate the “TERMINATING” resistor in only 1 unit at either end of the branch.

5. Troubleshooting

Although BACnet Ethernet/IP requires IT support to troubleshoot, BACnet MS/TP does not. To troubleshoot BACnet MS/TP, follow these steps:

Step 1: Make sure all devices are powered

Step 2: Isolate branch to only 2 devices. If all the devices are from Setra, there is no need to remove the communication wires; simply pull the “power jumper” to de-energize Setra devices. De-energized Setra devices wired to the communication lines will not affect between units online.

Step 3: Verify communication wires are polarity mated and attached to units maintaining polarity.

Step 4: Reduce baud rate to 9600. Once the system is working, set baud rate to the system design.

Step 5: Verify MAC address and Device instance of each unit

Step 6: Use PULL-UP & PULL-DOWN resistors and TERMINATION resister if needed.

Step 7: If it is still not working, use a trouble shooting/system observation system such as WireShark. This will enable you to capture the transmission between units.

ASHRAE BACnet

Topics: HVAC/R, FAQ & Troubleshooting, Particle Counting