Data Reliability Of The Can Protocol - Omron CJ1W-CORT21 Operation Manual

 Error Reset
In bus off mode, the CAN unit has its communication disabled. The user program in the CPU can
enable the communication using a message command. Enabling the communication will reset both
the receive and transmit counter.
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Data Reliability of the CAN Protocol

 Reliability
The introduction of safety-related systems in automobiles brought with it high requirements for the
reliability of data transmission. The objective is frequently formulated as not permitting any danger-
ous situations for the driver to occur as a result of data exchange throughout the whole life of a vehi-
cle.
This goal is achieved if the reliability of the data is sufficiently high or the residual error probability is
sufficiently low. In the context of bus systems data, reliability is understood as the capability to iden-
tify data corrupted by transmission faults. The residual error probability is a statistical measure of the
impairment of data reliability: It specifies the probability that data will be corrupted and that this cor-
ruption will remain undetected. The residual error probability should be so small that on average no
corrupted data will go undetected throughout the whole life of a system.
 Residual Error
Calculation of the residual error probability requires that:
• the errors which occur are classified, and
• that the whole transmission path is described by a model.
This calculation of the residual error probability of CAN results in a maximum bit error probability,
which is approximately 0.02 - in the order of 10
CAN is determined as a function of:
• the bit error probability for message lengths of 80 to 90 bits,
• for system configurations of, for instance, five or ten nodes
• and with an error rate of 1/1000 (an error in one message in every thousand).
Based on the residual error probability it is possible to calculate the maximum number of undetect-
able errors for a given CAN network.
 Undetectable Errors
For example, if a CAN network operates at a data rate of 1 Mbps/s, at an average bus capacity utili-
zation of 50 percent, for a total operating life of 4000 hours and with an average message length of
80 bits, then the total number of messages transmitted is: 9 x 10
The statistical number of undetected transmission errors during the operating life is thus in the order
of less than 10
error rate of 0.7 s, one undetected error occurs every thousand years (statistical average).
CJ-series User Defined CAN Unit Operation Manual for NJ-series CPU Unit (W517)
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. Or, with an operating time of eight hours per day on 365 days per year and an
1 Features and System Configuration
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(0.02 * 10 ). The residual error probability of
10
.
1
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