Omron CJ1W-CORT21 Operation Manual page 50

1 Features and System Configuration
Bit Stuffing
The coding of the individual bits is tested at bit level. The bit representation used by CAN is NRZ
(non-return-to-zero) coding, which guarantees maximum efficiency in bit coding. The synchroniza-
tion edges are generated by means of bit stuffing, i.e. after five consecutive equal bits the sender
inserts into the bit stream a stuff bit with the complementary value, which is removed by the receiv-
ers. The code check is limited to checking adherence to the stuffing rule.
 Error Flag
If one or more errors are discovered by at least one unit (any unit) using the above mechanisms, the
current transmission is aborted by sending an Error Flag. This prevents other units accepting the
message and thus ensures the consistency of data throughout the network.
 Re-transmission
After transmission of an erroneous message has been aborted, the sender automatically re-
attempts transmission (automatic repeat request). There may again be competition for bus alloca-
tion. As a rule, retransmission will begin within 23 bit periods after error detection; in special cases
the system recovery time is 31 bit periods.
However effective and efficient the method described may be, in the event of a defective unit it might
lead to all messages (including correct ones) being aborted, thus blocking the bus system if no mea-
sures for self-monitoring were taken. The CAN protocol therefore provides a mechanism for distin-
guishing sporadic errors from permanent errors and localizing unit failures (fault confinement).
This is done by statistical assessment of unit error situations with the aim of recognizing a unit's own
defects and possibly entering an operating mode where the rest of the CAN network is not nega-
tively affected. This may go as far as the unit switching itself off to prevent messages erroneously
recognized as incorrect from being aborted.
 Bus Off
With respect to fault confinement a CAN unit is in one of three states, the number of the state
increases with the severity of the error:
• error active
• error passive
• bus off
An error active unit can normally take part in bus communication and sends an Active Error Flag
when an error has been detected.
An error passive unit must not send an Active Error Flag. It takes part in bus communication but
when an error has been detected only a Passive Error Flag is sent. Also after a transmission, an
error passive unit will wait before initiating a further transmission.
A bus off unit is not allowed to have any influence on the bus. (E.g. output drivers switched off.)
 Error Counters
For fault confinement two counters are implemented in every CAN unit:
• Transmit error counter
• Receive error counter
For respectively a transmit error and a receive error the corresponding counter increases, normally
the receive counter is increased by one for every error, the transmit error counter is normally
increased by eight. See the CAN bus specification for a detailed description of increasing the error
counters, and the exceptions in increasing the counters.
A CAN unit is error passive when the transmit error counter equals or exceeds 128, or when the
receive error counter equals or exceeds 128. A CAN unit is bus off when the transmit error counter
is greater than or equal to 256.
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CJ-series User Defined CAN Unit Operation Manual for NJ-series CPU Unit (W517)