COP685BC datasheet(33/57 Pages) NSC

33 / 57 page

Frame Formats (Continued)

•

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.

DS012067-62

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

FIGURE 27. Error Frame — Error Active Transmitter

COP685BC Datasheet(PDF) 33 Page - National Semiconductor (TI)