IFM ecomat 100 R 360 Series, Системное руководство

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page 31
6.3. CAN errors and error handling
The error mechanisms described below are automatically
processed by the CAN controller integrated in the plc. This is not
influenced by the user. He must/should only react to errors
signalled in the application software.
Goal of the CAN error mechanisms:
•
Ensuring uniform data objects in the whole CAN network
•
Permanent function of the network also in case of a faulty
CAN participant
•
Distinction between temporary and permanent disturbance of
a CAN participant
•
Locating and automatic switch-off of a faulty participant in 2
steps (error-passive, bus-off). This gives a temporarily
disturbed participant a "break".
To give the interested user an overview of the operating
characteristics of the CAN controller in case of an error, a
simple description of the error handling will be given below. After
the error detection the information is processed automatically
and is available to the programmer as CAN error bits in the
application software.
Error message If a bus participant detects an error condition, it immediately
sends an error flag, thus causing the abort of the transmission
or rejection of the correct messages already received by the
other participants. This ensures that all participants are provided
with correct and uniform data. Since the error flag is transferred
immediately, the sender can immediately start to repeat the
disturbed message as opposed to other field bus systems
(which wait until a defined acknowledgement time has elapsed).
This is one of the most important features of CAN.
One of the fundamental problems of the serial data transmission
is that a permanently disturbed or faulty bus participant can
block the whole system. This would be a danger especially for
the error handling method of CAN. To exclude this case, a
mechanism is required which detects a faulty participant and
switches it off from the bus, if necessary.
Error counters
To do so, the CAN controller incorporates a transmission error
counter and a reception error counter. They are counted up
(incremented) for each erroneous transmission or reception. If a
transmission was correct, these counters are counted down
again (decremented).
However, in case of an error these error counters are
incremented more than they are decremented in case of no
error. During a certain time period this can lead to a substantial
increase of the counts even if the number of undisturbed
messages is greater than the number of disturbed messages.
But longer time periods without errors reduce the counts again.
Thus the counts are a measure for the relative frequency of
disturbances.