About The Mac Address; Bacnet Ms/Tp Data Bus Token-Passing Overview - Acuity Controls nLight ECLYPSE User Manual

About the MAC Address

The MAC Address is a number from 0 to 255; however, we recommend reserving some MAC Addresses for common com-
missioning and maintenance tasks. For example, when a portable adapter is set to use one of these reserved MAC Ad-
dresses, it can be temporarily connected with certainty to any BACnet MS/TP data bus of any site without conflicting with
other devices already connected to the BACnet MS/TP data bus. We strongly recommend that the MAC address of
ECLYPSE Controller's MS/TP port be always set to 0.
MAC Addresses should be used as shown in the following table.
MAC Address Value / Usage
Range
0 Data Bus Master (ECLYPSE Controller)
1 Temporary commissioning connection
2 Reserved
3-127 Master Range
128-254 Slave Range
255 Broadcast
Table 8: Recommended BACnet MS/TP Bus MAC Address Values / Ranges for BACnet MS/TP Data Bus Devices

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 pass-
ing a token between Master devices on the data bus that authorizes the device that is holding the token to initiate commu-
nications on the data bus. Once the device has completed its request(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.
nLight ECLYPSE
BACnet MS/TP Communication Data Bus Fundamentals
Devices
This address is invalid for other BACnet devices
This address is invalid for other BACnet devices
Other
Master devices: All master devices should be in this MAC Address
range
Slave devices and network sensors
Do not apply address 255 to any device
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