Distech Controls ECLYPSE APEX User Manual

128 ECLYPSE APEX
BACnet MS/TP Data Bus Token-Passing Overview
The BACnet MS/TP data bus protocol is a peer-to-peer, multiple-master protocol that shares data bus
bandwidth by passing a token between Master devices on the data bus that authorizes the device that
is holding the token to initiate communications on the data bus. Once the device has completed its re-
quest(s), it closes the communications channel, passes the token to the next Master device (making it
the current Master), and liberates the data bus.
The token is passed through a short message from device to device on the BACnet MS/TP data bus in
consecutive order starting from the lowest MAC address (MAC Address = 0) to the next MAC Address.
Gaps or pockets of unassigned device MAC Addresses should be avoided as this reduces data bus
performance. Once a master has finished making its requests, it must poll for the next master that may
exist on the Data Bus. It is the timeout for each unassigned MAC Address that slows down the data
bus.
The way MAC Addresses are assigned is not a physical requirement: Devices can be daisy-chained
on the data bus in any physical order regardless of their MAC Address sequence. The goal is to avoid
gaps in the device MAC Address range.
Slave devices cannot accept the token, and therefore can never initiate communications. A Slave can
only communicate on the data bus to respond to a data request addressed to it from a Master device.
Gaps in slave device MAC Addressing have no impact on BACnet MS/TP data bus performance.
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Figure104: Setting the Max Master on the ECY Series Controller to the Highest MAC Address Used on the BAC-
net 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 (ECY Series Controller)
should have the
Max Info Frames
parameter set to 20.
BACnet MS/TP Communication Data Bus Fundamentals

Distech Controls ECLYPSE APEX Specifications

General

Brand	Distech Controls
Model	ECLYPSE APEX
Category	Control Systems
Language	English