DESIGN OF A CUSTOM ASIC INCORPORATING CAN™ AND 1 ...

In addition to being conducted at two different speeds, the tests are also conducted using
both 11-bit and 29-bit message identifier fields. Without hot swapping, no failures are noted
while running any of the tests but a considerable degradation in the performance is noticed in the
bandwidth when using the extended 29-bit message identifiers at the 125 kbps speeds. When
making an attempt to perform hot swapping on any individual node (regardless of any of the
eight tests being conducted or bus speeds), it is noticed that all communications on the CAN
bus come to a halt and the only way to restore communications is to perform a hard reset,
powering down the Altera DE2 board and all CAN nodes.
It is clear that more extensive testing is needed to find a solution for this problem. It is
common that adding additional components to a network most often requires shutting down the
entire network to prevent costly system errors, as was evident in this case. The ability to plug-in
or remove a CAN node from the system would be a valued asset for many CAN
applications.
At this time, it is believed that the transceivers on each CAN node are the root cause of
the failure, since hot swapping CAN nodes requires that the transceiver output remain stable
during the unpowered to power-up transition without disturbing ongoing CAN network
communications. Since many CAN transceivers on the market today have very low output
impedance when unpowered, this causes the device to sink any signal present on the bus and
would effectively shut down all bus communication. Clearly, further research and testing is
needed in this area along with CAN transceivers from different manufacturers being replaced
on individual CAN nodes.
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