Contemporary-control-systems BASrouter Application Guide Manuale Utente Scaricare il pdf (Pagina 9)

AG-BASRTB00-BD0 Page 9

Application Guide — BASrouters

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CONTEMPORARY

Application #7 — Connecting Field Devices on Multiple Floors

In a typical HVAC installation, many Fan Coil Units (FCU)

or Variable-Air-Volume (VAV) terminal units will be

installed in the ceiling on each oor of a multistory ofce

building. Frequently, these units contain BACnet MS/TP

controllers all sharing one EIA-485 bus. Depending on

the rating of the EIA-485 transceivers, 32, 64 or 128

devices are allowed on the same bus. Having a large

number of devices on the same bus over several oors

with concealed wiring makes troubleshooting much

more difcult. Adding EIA-485 repeaters compounds

the problem.

A better approach has all the terminal control units on

each oor bused together with one end connected to

the MS/TP port on the BASrouter and the far-end

terminated with a 120 ohm resistor. The BASrouter is

shipped with EIA-485 bias and termination applied so

there is no need for an external terminator on the near

side. The BASrouter Ethernet port is then tied to a

10/100 Mbps Ethernet Switch and then a cable connection

is brought up through risers to the centrally-located

building or automation controllers that operate over

BACnet/IP. On each oor will be installed a BASrouter

dedicated to the eld devices on that oor. If the building

supports structured wiring, the BASrouter Ethernet

wiring can share this system. With this approach, there

is no need to carry EIA-485 wiring between oors. By

having a fewer number of eld devices on one MS/TP

segment, network trafc on that segment is reduced.

Many MS/TP eld controllers support auto-bauding so it

is only necessary to set the baud rate on the BASrouter

to the desired setting and all the eld controllers will

follow suit. The maximum number of MS/TP devices

on each segment is often arbitrary. Assuming that

half-load EIA-485 transceivers exist on the eld

controllers, a reasonable limit would be about 40. Each

MS/TP segment is treated as a separate BACnet

network so each segment must be assigned a unique

network number including the BACnet/IP network.

Network 1

Network 2004

Network 2003

Network 2002

Network 2001

1 2 3 4 5 6 7 8 9 10 11 12 13 14 ... 19 20

BASautomation 1
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Contemporary-control-systems BASrouter Application Guide Manuale Utente Pagina 9

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