Setting the Max Master on the Data Bus Master (Building Controller) to the Highest MAC Address Used on the BACnet MS/TP Data Bus

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 permitted 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 parameter 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-BOSAX (Building Controller) to the highest Master device’s MAC Address number to optimize the efficiency of the data bus.
 


Setting the Max Master and Max Info Frames

The Max Master and Max Info Frames are parameters used to optimize a BACnet MS/TP Data Bus. This is set in the Data Bus Master (EC-BOSAX Building Controller) for the MS/TP port of the Data Bus Master and for each BACnet MS/TP device connected on that port.
A Configure MS/TP Devices tool is available to automate this task. This tool:
 Can only be used with a live database during commissioning. If you add more devices to the data bus, you must run this tool again.
 Automatically calculates the value for the Max Master by finding the highest Master device MAC Address on the connected BACnet MS/TP data bus plus 1.
 Sets the Max Master for all master devices including the Data Bus Master (EC-BOSAX).
 Sets the Max Info Frames for all master devices excluding the Data Bus Master (EC-BOSAX).
For the Data Bus Master (EC-BOSAX), set the Max Info Frames to 20 in the screen shown in Figure 2-22 as this is a device that will make more requests for service from other devices on the network. In general, according to the way a device is programmed, the Max Info Frames may have to be set to a higher value than for other devices. For example, when Roof Top Unit Controllers are used with VAV controllers that use gfxApplications code, they should also have their Max Info Frames set to a higher value such as 5, as Roof Top Unit Controllers will poll many VAV controllers for information.
 

1:\ Set the Max Master and Max Info Frames as follows.
Right-click the Link node in the Nav tree and select Ms/Tp Configuration.
 
Figure 2-20: Launching the Configure MS/TP Devices tool
 
2:\ The network must be live; the Status shown in Configure Ms/Tp devices window must be {ok}.
Figure 2-21: Setting the Max Master on the Data Bus Master (EC-BOSAX) to the Highest MAC Address Used on the BACnet MS/TP Data Bus
 
3:\A confirmation message is shown.
4:\ Set the Max Info Frames to 20 for the Data Bus Master (EC-BOSAX) as shown in the screen below.
Figure 2-22: Setting the Max Info Frames on the Data Bus Master (EC-BOSAX)

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