CAN Basic Architectures - CAN System Engineering - page 11

• Acknowledgement - Error
A transmitter did not detect a dominant bit in the ACK Slot , which means that the
message was not recognized as fault-free by any other CAN node.

1.2.4.2 Error Handling

After one of the errors described above is recognized, each CAN node is notified by
an Error Frame . The Error Frame overwrites other frames and is detected by the
violation of the bit-stuffing rules. As a result of an Active Error Flag , which consists
of actually only six dominant bits, other CAN nodes see a Stuff-Error and therefore
also start to send an Error Frame . This effect occurs when the error is not a global
error (detected simultaneously by all CAN nodes), but a local error (detected only
by one CAN node or by a group of CAN nodes) and leads to a possible superposi-
tion of Error Flags up to a length of 12 dominant bits. The Error Frame is termi-
nated by an Error Delimiter (a series of eight recessive bits).
The error handling is done in the following order:

a. An error is detected.
b. An Error Frame is sent by any CAN node that has detected the error.
c. The message currently in progress is rejected by all CAN nodes.
d. The error counters (see fault isolation) in each CAN node are affected according
to the fault confinement rules.
e. The disturbed message is repeated.

1.2.4.3 Fault Isolation

The CAN protocol defines—with the aim of fault isolation—specific methods to
prevent local disturbances from disturbing the CAN bus with multiple dominant
Active Error Flags . For this purpose, three states are defined for the CAN nodes:

• Error Active
This is the normal state of a CAN node, in which messages can be sent and recei-
ved. In the event of a fault, an Active Error Flag , consisting of dominant bits, is
transmitted.
• Error Passive
This state is reached after several errors have been detected on the CAN bus. In this
state, the CAN node may continue to send and receive messages, but in the case
of an error, only a Passive Error Flag is sent, consisting of recessive bits. Conse-
quently, a CAN node that is Error Passive cannot impede the other CAN traffic.
It will mark only errors in its own transmitted messages. This may happen when
the node sees a local disturbance. Such a CAN node that terminated its own
transmitted frame switches—after the Error Delimiter— for 8 bits into a Suspend
state where it cannot start a frame, but can receive every message.