Distech Controls ECB-VAV, User Manual

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BACnet MS/TP Communication Data Bus Fundamentals
Network Guide 33
Figure 19: Setting the Max Master on the Data Bus Master (Building Controller) to the Highest MAC Address Used
on the BACnet MS/TP Data Bus
About Tuning the Max Info Frames Parameter
Once a device has the token, it can make a number of information requests to other devices
on the BACnet intranetwork. The maximum number of requests is limited by the
Max Info
Frames parameter. Once the device has made the maximum number of requests it is permit-
ted to make according to the Max Info Frames parameter, the device passes the token to the
following device with the next higher MAC address. This makes the BACnet MS/TP Data Bus
more reactive for all devices by preventing a device from hanging on to the token for too long.
Ordinary BACnet MS/TP devices should have the Max Info Frames parameter set to between
2 and 4. The Data Bus Master (Building Controller) should have the
Max Info Frames param-
eter set to 20.
About Tuning the Max Master Parameter
To prevent the passing of the token to unused MAC Addresses situated after the final Master
device, the Max Master parameter must be set. By default, the Max Master for the Data Bus
Master (Building Controller) is set to 127 which allows for the theoretical maximum of 127
devices besides the Data Bus Master to be connected to the data bus.
In practice, the actual number of devices connected to a data bus is far less, resulting in a gap
between the highest MAC Address of any device connected to the data bus and the value set
for Max Master. This gap unnecessarily slows-down the data bus with Poll for Master
requests.
When commissioning a BACnet MS/TP Data Bus, it is useful to start with the Max Master set
to 127 so as to be able to discover all devices connected to the data bus. Then, once all
devices have been discovered and the MAC Addressing is finalized by eliminating any gaps in
the address range, set the Max Master (maximum MAC Address) in the EC-BOS (Building