Peer Communications Protocol; Communications Media; Twisted Copper Pair; Fiber Optic - Honeywell AUTOMATIC CONTROL SI Edition Engineering Manual

BUILDING MANAGEMENT SYSTEM FUNDAMENTALS

PEER COMMUNICATIONS PROTOCOL

Peer communications protocol has the following advantages
over poll/response communications protocol:
— Communication not dependent on a single device as
the master.
— Direct communication between bus-connected devices
without going through the BMS central processor.
— Global messages transmitted to all bus-connected devices.
In peer communications a time slot is automatically passed
from one bus-connected device to another as the means of
designating when a device has access to the bus. Since the time
slot passes in an orderly sequence from one device to the next,
the communications network is sometimes termed a ring.
However, the bus is not necessarily physically looped nor are
the devices physically connected to form a ring. Any device on
the bus can be designated as the first to receive the time slot,
and any other device the next to receive it, and so on.

COMMUNICATIONS MEDIA

The most common choices for BMS transmission trunks are:
— Twisted copper pairs
— Fiber optic cable
— Common carrier telephone channels
The media best suited for a given installation depends on the
signal, cost, geographic layout, and the possibility of line
interference.

TWISTED COPPER PAIR

Twisted pair copper conductors ranging from 1.307 to 0.2051 mm
are the most commonly used and the best economic choice as the
communications media for single building applications. Bus
lengths up to 1200 meters are common without use of extenders
or repeaters. When repeaters are used, extensions up to three or
four times this distance are possible. Serial bus and star wiring
configurations (Fig. 2 and 3) permit efficient wiring layouts.
COMMUNICATIONS
SYSTEM/ZONE
LEVEL
CONTROLLER
BMS
CENTRAL
Fig. 2. Typical Serial Bus Wiring Configuration.
OPERATIONS-LEVEL
PROCESSOR
BUS
SYSTEM/ZONE SYSTEM/ZONE
LEVEL LEVEL
CONTROLLER CONTROLLER
C1863
MODEM
PHONE LINE
INTERFACE
Fig. 4. Dialup to Remote Controllers.
COMMUNICATIONS
BUS
SYSTEM/ZONE
LEVEL
CONTROLLER
SYSTEM/ZONE
LEVEL
CONTROLLER
Fig. 3. Typical Star Wiring Configuration.

FIBER OPTIC

Fiber optic transmission media is particularly suited to
installation in an environment that interferes with
communications, such as high electrical interference or
frequent electrical storms. The disadvantages of fiber optic
transmission are the cost and lack of industry standards. Fiber
optic links are most often found between buildings.

PHONE LINES

Common carrier telephone channels link distant buildings.
Telephone line transmissions require either full time dedicated
phone lines or automatic dialing through modems (Fig. 4).
2
A modem is required for signal compatibility with the phone
line. The BMS dial-up phone line interface provides
compatibility with transmission rate and protocol of the phone
line and reduces the leased line operation to costs associated
only with call frequency and connect time. As an example of
reduced costs, historical storage of alarm activity can be included
in the remote phone line interface so that the data is
transmitted during periodic dialup by the central system.
Critical alarms can be programmed at the remote to dial the
central system immediately.
MODEM
PHONE LINE
INTERFACE
ENGINEERING MANUAL OF AUTOMATIC CONTROL
188
BMS
CENTRAL
SYSTEM/ZONE
LEVEL
CONTROLLER
SYSTEM/ZONE
LEVEL
CONTROLLER
SYSTEM/ZONE
LEVEL
CONTROLLER
C1862
REESTABLISHED
TWISTED PAIR
COMMUNICATIONS
LINK
SYSTEM/ZONE
LEVEL
CONTROLLERS
C1864