Principles and technologies of local area networks - Intelligent Buildings and Building Automation - page 65

Table 4.3 ARCnet technical specifications

Specification Characteristics

Network standard ANSI 878.1 (close to IEEE 802.4)

Protocol Standard for layers 1 and 2

Architecture Bus or star topology

Transmission speed 2.5 Mbps

Network node capacity 254 nodes

Network length 122 m (twisted pair)

305 m (coaxial)

Physical path cabling Twisted pair or coaxial

Medium Access Control (MAC) method Token passing

characteristics are well suited for control. ARCnet has proven itself to be very
robust. ARCnet also is fast, provides deterministic performance and can span
long distances, making it a suitable fieldbus technology.
The term fieldbus is used in the industrial and building automation indus-
tries to signify a network consisting of computers, particularly controllers
and devices mounted in the 'field' (various locations). ARCnet is an ideal
fieldbus. Unlike office automation networks, a fieldbus must deliver mes-
sages in a time-predictable fashion. The token-passing protocol of ARCnet
provides this timeliness. Fieldbus messages are generally short. ARCnet
packet lengths are variable from 0 to 507 bytes with little overhead and are
coupled with the high data rate of ARCnet, typically 2.5 Mbps, which is
not particularly high these days but is still high enough for field networks in
industrial and building automation applications.

4.5.3 LonTalk

LonTalk is a LAN protocol which is closely associated with communication
hardware (Neuron chips) also marketed under Echelon Corporation's name.
While the BACnet committee elected not to endorse Echelon Corporation's
definition of a standard protocol (at higher layers), they did allow for BACnet
messages to be sent via LonTalk. The technical specification of LonTalk
LANs is shown in Table 4.4.
LonWorks refers to a family of products developed by Echelon Corporation
on the basis of the LonTalk protocol. In recent years, several vendors of BA
and control equipment have incorporated Echelon technology into some of
their products.
All of these products use a proprietary chip called the Neuron chip,
which is a dedicated communications processor. Neurons use a technology
called LonWorks on the basis of the communications protocol LonTalk for