Contemporary Research TD960801-0MC - Manual (Page 28)

TD960801-0MC
24
Within a CAN segment, at least one device must acknowledge
the valid receipt of another device’s transmission. That
acknowledgment, however, does not extend beyond an
EXTEND-A-BUS. Even though a successful transmission
occurred on a CAN segment, that transmission must be
replicated on all other CAN segments generating additional
acknowledgments. Therefore, it is possible that a replicated
transmission on one CAN segment may fail due to a cabling
problem resulting in no acknowledgment while all other CAN
segments view the transmission successful. However, the
DeviceNet protocol does not rely upon the CAN data link
acknowledgment as sole indication of a successful transmission.
Additional error checking has been incorporated in the upper
layer DeviceNet protocol.
Single nodes can operate on an individual CAN segment with
remote bridging. Since each EXTEND-A-BUS has one internal
CAN chip, this CAN chip acknowledges the single node’s
message. Without remote bridges, a single node will fail to hear
an acknowledgment and will continuously retry.
The DeviceNet protocol supports autobauding which is possible
for the EXTEND-A-BUS to implement. One EXTEND-A-BUS
acts as a master for all other bridges on the network functioning
as slaves. The master EXTEND-A-BUS must be connected to
the CAN segment connected to the master controller. As the
master controller transmits data, the master EXTEND-A-BUS
determines the data rate and informs all other EXTEND-A-
BUSes the required data rate over the ARCNET connection.
Once the data rates are determined, traffic is sent between the
bridges functioning as one long extension cord. The EXTEND-
A-BUS data rates can be manually set by way of a switch and
there is no inherent reason why individual CAN segments
cannot be set to different data rates.
Using the same extension cord analogy, it would appear that a
remote bridging system must be powered before or at the same
time as the slave devices or master controller in order that all
devices can execute initialization routines such as duplicate