National CP3BT26 manual Error Frame Overload Frame

CP3BT26

Error Frame

As shown in Figure 40, the Error Frame consists of the error flag and the error delimiter bit fields. The error flag field is built up from the various error flags of the different nodes. Therefore, its length may vary from a minimum of six bits up to a maximum of twelve bits depending on when a module has detected the error. Whenever a bit error, stuff error, form error, or acknowledgment error is detected by a node, the node starts transmission of an error flag at the next bit. If a CRC error is detected, transmission of the error flag starts

at the bit following the acknowledge delimiter, unless an er- ror flag for a previous error condition has already been start- ed.

If a device is in the error active state, it can send a “domi- nant” error flag, while a error passive device is only allowed to transmit “recessive” error flags. This is done to prevent the CAN bus from getting stuck due to a local defect. For the various CAN device states, please refer to Error Types on page 115.

	ERROR FRAME
6	< 6	8
ERROR	ECHO	ERROR
FLAG	ERROR FLAG	DELIMITER
DATA FRAME OR		INTER-FRAME OR
REMOVE FRAME		OVERLOAD FRAME
d d d d d d d	d d r r r r r r r r d

Note:
d = dominant
r = recessive	An error frame can start anywhere within a frame	DS024

Figure 40. Error Frame

Overload Frame

As shown in Figure 41, an overload frame consists of the overload flag and the overload delimiter bit fields. 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 overload frame can only be sent after the end of frame (EOF) field and in this way destroys the fixed form of the in- termission field. As a result, all other nodes also detect an

overload condition and start the transmission of an overload flag. After an overload flag has been transmitted, the over- load frame is closed by the overload delimiter.

Note: The CAN module never initiates an overload frame due to its inability to process an incoming message. Howev- er, it is able to recognize and respond to overload frames ini- tiated by other devices.

OVERLOAD FRAME

6

8

END OF FRAME OR

OVERLOAD

OVERLOAD

FLAG

DELIMITER

INTER-FRAME SPACE

ERROR DELIMITER OR

OVERLOAD DELIMITER

OR ERROR FRAME

Note:

d

d

d

d

d

d

d

r

r

r

r

r

r

r

r

d = dominant

r = recessive

An overload frame can only start at the end of a frame

DS025

Figure 41. Overload Frame

Interframe Space

Data and remote frames are separated from every preced- ing frame (data, remote, error and overload frames) by the interframe space (see Figure 42). Error and overload frames are not preceded by an interframe space; they can be transmitted as soon as the condition occurs. The inter- frame space consists of a minimum of three bit fields de- pending on the error state of the node.

www.national.com 114