COP685BC datasheet(32/57 Pages) NSC

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Frame Formats (Continued)

tive node detecting an error, starts transmitting an active er-

ror flag consisting of six “dominant” bits. This causes the de-

struction of the actual frame on the bus. The other nodes

detect the error flag as either a violation of the rule of bit-

stuffing or the value of a fixed bit field is destroyed. As a con-

sequence all other nodes start transmission of their own er-

ror flag. This means, that the error sequence which can be

monitored on the bus as a maximum length of twelve bits. If

an error passive node detects an error it transmits six “reces-

sive” bits on the bus. This sequence does not destroy a mes-

sage sent by another node and is not detected by other

nodes. However, if the node detecting an error was the

transmitter of the frame the other modules will get an error

condition by a violation of the fixed bit or stuff rule.

Figure 26

shows how an error passive transmitter transmits a passive

error frame and when it is detected by the receivers.

After any module has transmitted its active or passive error

flag it waits for the error delimiter which consists of eight “re-

cessive” bits before continuing.

OVERLOAD FRAME

Like an error frame, an overload frame consists of two bit

fields: the overload flag and the overload delimiter. The bit

fields have the same length as the error frame field: six bits

for the overload flag and eight bits for the delimiter. The over-

load frame can only be sent after the end of frame (EOF)

field and in they way destroys the fixed form of the intermis-

sion field.

ORDER OF BIT TRANSMISSION

A frame is transmitted starting with the Start of Frame, se-

quentially followed by the remaining bit fields. In every bit

field the MSB is transmitted first.

FRAME VALIDATION

Frames have a different validation point for transmitters and

receivers. A frame is valid for the transmitter of a message, if

there is no error until the end of the last bit of the End of

Frame field. A frame is valid for a receiver, if there is no error

until and including the end of the penultimate bit of the End

of Frame.

FRAME ARBITRATION AND PRIORITY

Except for an error passive node which transmitted the last

frame, all nodes are allowed to start transmission of a frame

after the intermission, which can lead to two or more nodes

starting transmission at the same time. To prevent a node

from destroying another node’s frame, it monitors the bus

during transmission of the identifier field and the RTR-bit. As

soon as it detects a “dominant” bit while transmitting a “re-

cessive” bit it releases the bus, immediately stops transmis-

sion and starts receiving the frame. This causes no data or

remote frame to be destroyed by another. Therefore the

highest priority message with the identifier 0x000 out of

0x7EF (including the remote data request (RTR) bit) always

gets the bus. This is only valid for standard CAN frame for-

mat. Note that while the CAN specification allows valid stan-

dard identifiers only in the range 0x000 to 0x7EF, the device

will allow identifiers to 0x7FF.

There are three more items that should be taken into consid-

eration to avoid unrecoverable collisions on the bus:

DS012067-60

FIGURE 25. Interframe Space for Nodes Which Are Not

Error Passive or Have Been Receiver for the Last Frame

DS012067-61

FIGURE 26. Interframe Space for Nodes Which Are Error Passive

and Have Been Transmitter for the Last Frame

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