Viconics VBZS Application Guide - Manual (Page 29)

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4) BACnet Communication Overview
The Viconics VZ7260X5x00B and VZ7656X1000B controllers use a local BACnet RS485 MS-TP
communication bus between all devices to insure proper communication and smooth data exchange of all
required information for proper system operation.
BACnet is "a data communication protocol for Building Automation and Control networks." A data
communication protocol is a set of rules governing the exchange of data over a computer network. The
rules take the form of a written specification that spells out what is required to conform to the protocol.
BACnet is a registered trademark of ASHRAE North America. For more information on BACnet please visit
www.bacnet.org .
What does this mean for us ?: It simply means that BACnet is the communication protocol or language the
controllers use to exchange information or data on a peer to peer network ( controller to controller ) in order
for the system to operate correctly.
RS485 (officially known as EIA-485) specifies the electrical characteristics of the serial half-duplex
transceiver and receiver user for communication between each device.
MS-TP is the physical layer used for communication between all the devices. It stands for Master Slave –
Token Passing where each of the devices on the network gets to passed a token. When a device has the
token, it is then the master of the communication system on the network and it gets permission to read and
write values to other devices. At this point, all the other devices are considered slaves to the current master
and will answer or acknowledge any request from the current master.
When the current controller is done with the token, it will pass it to the next controller in the network and so
on.
Baud-Rate. All controllers in the network need to communicate at the same speed in order to properly
exchange information. The speed rate is called baud rate. Each individual controller can be assigned a
specific baud-rate. But all controllers on the same network need to use the same baud rate.
The Baud rate is a configuration parameter and its default value is set to (4 = Auto Bauding). The actual
range of communication speeds available are:
- 0 = 9600 KBps
- 1 = 19200 KBps
- 2 = 38400 KBps
- 3 = 76800 KBps
- 4 = Auto Bauding ( Baud Rate will match detected Baud Rate
Baud-Rate Notes:
At least one of the controller needs to have a pre-selected baud rate for the network to operate
properly when all other controllers are left with their default value of 4 = Auto Bauding.
In stand-alone application for small and large systems is recommended to use 38400 KBps.
The lower speeds (9600, and 19200 KBps) are typically used then other existing BACnet devices are
integrated and need to co-exist on the same network.
The highest speed (76800 KBps) is typically used on large networks (80+ devices) tied in with a
supervision device where rapid turnaround is expected to refresh graphical views.
If speed is required to be changed on the whole network; first modify the baud rate configuration property
on all controllers. Then perform a full power down of all the controllers before re-applying power. This will
insure that none of the controllers will not remain in token pass mode between each other and stay locked
to the previous baud rate selected.