Detecting And Signaling Errors - Omron CJ1W-CORT21 Operation Manual

 CRC Field
The CRC field contains the CRC sequence, followed by a delimiter. this CTC-delimiter is a single
recessive bit. The frame check sequence is derived from a cyclic redundancy code best suited for
frames with bit counts less then 127 bits (BCH code). In order to carry out the CRC calculation the
polynomial to be divided is defined as the polynomial, the coefficients of which are given by the
destuffed bit stream consisting of Start Of Frame (SOF), arbitration field, control field, data field and
for the 15 lowest coefficients, by 0. This polynomial is divided (the coefficients are calculated mod-
ulo-2) by the generator-polynomial:
15 14
• X + X
The remainder of this polynomial division is the CRC sequence transmitted over the bus.
 Ack Field
The ACK field comprises the ACK slot (1 bit) and the ACK delimiter (1 recessive bit). The bit in the
ACK slot is sent as a recessive bit and is overwritten as a dominant bit by those receivers which
have at this time received the data correctly (positive acknowledgement). Correct messages are
acknowledged by the receivers regardless of the result of the acceptance test.
 Intermission
The end of the message is indicated by End Of Frame. Intermission is the minimum number of bit
periods separating consecutive messages. If there is no following bus access by any unit, the bus
remains idle (bus idle).
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Detecting and Signaling Errors

 Error Detection
Unlike other bus systems, the CAN protocol does not use acknowledgement messages but instead
signals any errors that occur. For error detection the CAN protocol implements three mechanisms at
the message level:
Cyclic Redundancy Check (CRC)
The CRC safeguards the information in the frame by adding redundant check bits at the transmis-
sion end. At the receiver end these bits are re-computed and tested against the received bits. If they
do not agree there has been a CRC error.
Frame Check
This mechanism verifies the structure of the transmitted frame by checking the bit fields against the
fixed format and the frame size. Errors detected by frame checks are designated "format errors".
ACK Errors
As mentioned above, frames received are acknowledged by all recipients through positive acknowl-
edgement. If no acknowledgement is received by the transmitter of the message (ACK error) this
may mean that there is a transmission error which has been detected only by the recipients, that the
ACK field has been corrupted or that there are no receivers.
 Bit Level Errors
The CAN protocol also implements two mechanisms for error detection at the bit level:
Monitoring
The ability of the transmitter to detect errors is based on the monitoring of bus signals: each node
which transmits also observes the bus level and thus detects differences between the bit sent and
the bit received. This permits reliable detection of all global errors and errors local to the transmitter.
CJ-series User Defined CAN Unit Operation Manual for NJ-series CPU Unit (W517)
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1 Features and System Configuration
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