COP884BC Datasheet, PDF (33/57 Pages) National Semiconductor (TI) – 8-Bit CMOS ROM Based Microcontrollers with 2k Memory, Comparators, and CAN Interface

Frame Formats (Continued)
module 1 = error active transmitter detects bit error at t2
module 2 = error active receiver with a local fault at t1
module 3 = error active receiver detects stuff error at t2
DS012067-62
FIGURE 27. Error Frame — Error Active Transmitter
• Within one system each message must be assigned a
unique identifier. This is to prevent bit errors, as one mod-
ule may transmit a “dominant” data bit while the other is
transmitting a “recessive” data bit. This could happen if
two or more modules start transmission of a frame at the
same time and all win arbitration.
• Data frames with a given identifier and a non-zero data
length code may be initiated by one node only. Other-
wise, in worst case, two nodes would count up to the bus-
off state, due to bit errors, if they always start transmitting
the same ID with different data.
• Every remote frame should have a system-wide data
length code (DLC). Otherwise two modules starting
transmission of a remote frame at the same time will
overwrite each other’s DLC which result in bit errors.
ACCEPTANCE FILTERING
Every node may perform acceptance filtering on the identi-
fier of a data or a remote frame to filter out the messages
which are not required by the node. In they way only the data
of frames which match the acceptance filter is stored in the
corresponding data buffers. However, every node which is
not in the bus-off state and has received a correct CRC-
sequence acknowledges each frame.
ERROR MANAGEMENT AND DETECTION
There are multiple mechanisms in the CAN protocol, to de-
tect errors and to inhibit erroneous modules from disabling
all bus activities.
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