CAN Basic Architectures - CAN System Engineering - page 13

a. A receiver detects an error → REC = REC + 1
except the error is a Bit - Error that is detected while the receiver sends an Active
Error Flag or an Overload Flag .
b. A receiver detects a dominant bit after sending an Error Flag → REC = REC + 8.
c. A transmitter starts an Error Flag → TEC = TEC + 8
Exception 1 : When a transmitter is in Error Passive state and detects an Acknowled-
gment-Error (no dominant bit in the ACK Slot ) and it does not detect a dominant
bit while sending its Passive Error Flag , the TEC is not changed.
Exception 2 : If a transmitter detects a Bit - Error on a recessive stuff-bit in the Arbi-
tration Field before the RTR bit, the TEC is not changed.
d. A transmitter detects a Bit - Error while sending an Active Error Flag or an Over-
load Flag → TEC = TEC + 8.
e. A receiver detects a Bit - Error while sending an Active Error Flag or an Overload
Flag → REC = REC + 8.
f. Any CAN node tolerates up to seven consecutive dominant bits after sending
an Active Error Flag , Passive Error Flag, or Overload Flag . After detecting the
14th consecutive dominant bit (in case of an Active Error Flag or an Overload
Flag ) or after detecting the eighth consecutive dominant bit following a Passive
Error Flag , and after each sequence of additional eight consecutive dominant
bits, every transmitter increases → TEC = TEC + 8 and every receiver increases
→ REC = REC + 8.
g. A transmitter with TEC > 0 will decrement after each successful transmission of
a Data Frame or a Remote Frame → TEC = TEC − 1.
h. A receiver with REC > 0 will decrement after each reception of a fault-free Data
Frame or Remote Frame → REC = REC - 1.
Exception 1 of rule c) was introduced to avoid complicating the initialization phase
of a network. After a system is started, the initialization times of the individual
CAN nodes in a distributed control network will—under normal circumstances—
be different. If we suppose now that the first CAN node that has completed its
initialization sends a frame, it will always get an Acknowledgment - Error since there
is no other CAN node ready to give an acknowledge, with the transmission at-
tempt automatically repeated. This would mean that the Transmit Error Counter
is incremented by 8 at each attempt, until it finally reaches the Bus Off threshold
and the CAN operation is stopped. One can easily imagine that the initialization
of a network would be very difficult under these circumstances. Therefore, this
Acknowledgment-Error is treated differently.
After reaching the Error Passive state, the Transmit Error Counter is no longer
incremented in the case of an Acknowledgment-Error —if there is no other node
sending an Active Error Flag . This means that the first transmission attempt will be
repeated until a second node finishes its initialization and gives an acknowledge.
One single error may cause the application of more than one rule, e.g. under the
following circumstances:
If CAN node A, that is currently receiving a frame, detects a local error and then
sends an Active Error Flag , this Error Flag will not completely overlap with the