28 Network Guide
Using Repeaters to Extend the Data Bus
A BACnet MS/TP data bus segment can be up to 1220 meters (4000 feet) long with up to a
maximum of 50 devices. When a greater length is required, a solution is to use a repeater. A
repeater increases the maximum total length of the data bus.
Using an EC-BOS to Increase the Number of Connected Devices
If your BACnet MS/TP network has an EC-BOS, it may be more cost effective to add one or
two MS/TP network cards to the EC-BOS in place of an MS/TP Repeater. A standard EC-BOS
can support up to four MS/TP networks when equipped with two optional MS/TP network
cards. Furthermore, network performance is increased as each MS/TP network will have lower
communication latency since each MS/TP network has its own token to pass between
devices. See BACnet MS/TP Data Bus Token-Passing Overview on page 32.
With up to 50 devices per MS/TP network, this allows up to a total of 200 devices to be con-
nected to an EC-BOS. See Figure 22.
Using a Repeater to Extend the Length of the BACnet MS/TP Data Bus
Repeaters can be used to extend a BACnet MS/TP data bus up to 3660 meters (12 000 feet)
maximum total length. Do not use more than two repeaters on a BACnet MS/TP LAN.
A BACnet MS/TP repeater is a bi-directional device that regenerates and strengthens the
electrical signals that pass through it. It creates two electrically-isolated BACnet MS/TP data
bus segments that transparently enable devices on one side of the repeater to communicate
with any device on the other side. The two BACnet MS/TP data bus segments have the same
requirements of an ordinary BACnet MS/TP data bus segment; that is, each BACnet MS/TP
data bus segment:
• Can be up to 1220 meters (4000 feet) long.
• Must have End-of-Line (EOL) termination resistors connected across the two data lines/
wires of the twisted pair at the first and last device on the data bus.
• Must respect the maximum limit for Device Loading on page 15.
• Will have the same network number as they remain part of the same network or LAN.
Distech Controls recommends that you connect no more than 50 of our ⅛ or ½-load devices
on all BACnet MS/TP Data Bus repeater segments when a baud rate of 19 200 or higher is
used (preferably 38 400 baud). This is to ensure that the BACnet MS/TP Data Bus has enough
bandwidth to efficiently communicate network variables between controllers.
A repeater can be added anywhere to a data bus segment including the end of the segment as
shown below.
Do not use more than two repeaters on a BACnet MS/TP data bus.
A repeater can only connect two BACnet MS/TP data bus segments even if it has
ports to support more than two BACnet MS/TP data bus segments.
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