Image Processing Reference
In-Depth Information
is used, the net data rate is bps. A message cycle consists of the master sending a request (
in-
channel
) consisting of the -bit slave address ( bit is used for selecting between individual and group
addresses)anda-bitcommand.Aslave(ballast)maythenreturnan-bitvalue(
out-channel
), and
the cycle starts over.
For remote meter reading, M-Bus [] has gained a certain degree of importance in Europe. Its
application layer supports various metering applications and includes support for advanced function-
ality like multiple tariffs. It operates on low-cost twisted-pair (TP) cabling, with the data link layer
based on the IEC -- / -- standard for telecontrol transmission protocols. A serial master-
slave protocol with data rates between and bps is used. A segment can contain up to
devices and cover a maximum distance of m (multiple segments are possible). In the master-to-
slave direction, data is transmitted using voltage modulation, while in the reverse direction, current
modulation signaling is used.
Also in Europe, SMI (Standard Motor Interface) is emerging as a German multivendor standard
for communication with sunblind drives. However, it has remained unpublished to date.
29.5.2 BACnet
BACnet (Building Automation and Control Network) was designed specifically for building automa-
tion purposes []. It was designed with the task of interconnecting DDC equipment in mind, but
covers functions of all levels. Due to its strengths regarding management functions it is often used to
complement large LonWorks and KNX/EIB (European Installation Bus) based installations.
First published as a standard in by ASHRAE (American Society of Heating, Refrigeration
and Air Conditioning Engineers) together with ANSI, BACnet was made ISO standard - in
[] and was revised in . BACnet is maintained by ASHRAE SSPC
∗
. Implementations
are not subject to license fees. BACnet user/manufacturer organizations exist around the world, called
BACnet Interest Groups (BIG) and BACnet International (previously the BACnet Manufacturers
Association).
In general, BACnet is network agnostic, but explicitly includes a number of network layers to create
a “critical mass” for interoperability. Of these, MS-TP (Master-Slave Token-Passing, a BACnet pro-
prietary, RS- based . kbps multidrop protocol) and IP are the most commonly used ones. In
United States and Asia, ARCNET
†
is also popular. he other choices are raw Ethernet, LonTalk, and
PTP (point-to-point). PTP, a BACnet proprietary protocol, is designed to support modem dial-up
connections via EIA-. The use of the LonTalk protocol is limited to transporting BACnet mes-
sagesanddoesnotimplyanycompatibilitywithLonMarkconceptssuchasstandardnetworkvariable
types (SNVT). While the original standard already specified tunneling routers (Annex J) for the IP
medium, IP only became a first class medium in (BACnet/IP). BACnet/IP made BACnet devices
with direct IP connection a reality.
‡
UDP is used as a transport protocol. BACnet/IP specifies a vir-
tual link layer whose most important task is to handle internetwork broadcasts without resorting to
Internet group management protocol (IGMP).
BACnet
networks
consist of
segments
ofthesamemediatypecoupledbyrepeatersorbridges.
BACnet networks are connected by routers to form a BACnet
internetwork
. he topology must not
contain loops. Segments need not be connected permanently, for example, those attached via PTP. A
BACnet network address consists of a -byte BACnet network number and a local address. he local
address is specific to the link layer medium, e.g., an IP address for BACnet/IP or a medium access
∗
Standing Standard Project Committee.
†
Unlike Ethernet, ARCNET guarantees deterministic medium access. For BACnet, the cost efficient kbps TP variant
is typically preferred.
‡
Before, an Annex J router would have had to be included in every device.
