Contemporary Research TD960801-0MC - Manual (Page 24)

TD960801-0MC
20
3.1.1 Repeaters
The usual approach to increasing network distance is to use
repeaters. Repeaters provide signal boost to make up the loss of
signal strength on a long segment. However, the problem with
long CAN segments is usually not lack of signal strength but
excessive signal latency. This latency is due to the propagation
delay introduced by the transceivers and twisted-pair wiring. If
this latency approaches one bit time, the non-destructive bit-
wise arbitration mechanism fails. Repeaters actually introduce
more delay due to the additional electronics and are not effective
in increasing the overall length of CAN networks. Repeaters
are generally used to increase the effective length of drop cables
from CAN trunk lines. Repeaters operate on the physical layer.
3.1.2 Bridges
Bridges are defined as devices that link two similar networks,
however, bridges can mean different things to different people so
further clarification is necessary. A local bridge stands by itself
connecting adjacent wiring segments together as in the case of a
repeater. Remote bridging interconnects two physically
separated but similar networks together using a different
interconnecting medium. Therefore, a pair of bridges are
required to interconnect two networks the way two modems are
used on leased phone lines. Sometimes bridges block network
traffic by restricting data only to stations specified in the
transmission that reside on the network controlled by the bridge.
This blocking is difficult to implement in broadcast networks
such as CAN and, therefore, not recommended. Bridges are
ignorant of the higher level protocols sent over CAN since
bridges operate at the data link layer. Therefore, protocols such
as DeviceNet, Smart Distributed System and CANopen are
passed without modification.
3.2 Theory of Operation
The EXTEND-A-BUS is classified as a remote bridge and
contained in a two piece metal enclosure suitable for panel