BACnet MS/TP Communication Data Bus Fundamentals
ECLYPSE User Guide 127
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 an ECY
Series Controller or a Soft EC-BOS
AX
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 ECY Series
Controller and in the Soft EC-BOS
AX
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 ECY Series Controller and separately with the Soft EC-BOS
AX
for
each connected BACnet MS/TP device.
For the ECY Series Controller, set the Max Info Frames to 20 in the screen shown in
BACnet Settings of the Network Port MS/TP on page 82 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.
To set the Max Master and Max Info Frames for BACnet MS/TP devices (for example, an
ECB series controller), use a Soft EC-BOS
AX
to do so. See the Network User Guide for more
information.
Default Device Instance Number Numbering System for Distech Controls’ controllers
By default, controllers from Distech Controls automatically self-assign a Device Instance
number generated from the unique MAC Address assigned to the controller during
installation. The Device Instance number is calculated as follows:
Device Instance number = 364 X 1000 + MAC Address
Where 364 is Distech Controls unique BACnet Manufacturer ID.
This Numbering system is sufficient for a BACnet network that has only one ECY Series
Controller. For larger BACnet networks that have more than one ECY Series Controller (to
form a BACnet intranetwork), set the MAC Addresses, Device Instance Numbers and
Network Numbers according to the numbering scheme below.
Adopting a Numbering System for MAC Addresses, Device Instance Numbers, and Network
Numbers
Good network planning requires a well thought-out numbering scheme for device MAC
Addresses, Device Instance Numbers (DI), and Network Numbers. We recommend the
following scheme, as it reuses the MAC Address and Network Number in the Device
Instance number to make it easier for a network administrator to know where a device is
located in the network. This is shown below.
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