00194614-08 Trainingsdoku. SG X-Serie_X4i SW70x (AL2)_EN.pdf - 第90页
Communication and Control CAN Bus Protocol CAN Bus Student Guide SIPLACE X-Serie and X4I SW70x (AL2) 90 In CAN networks, there is no a ddressing o f subscribers or stati ons in the co nventional sense, but instead, prior…
Communication and Control
CAN Bus CAN Bus Protocol
89 Student Guide SIPLACE X-Serie and X4I SW70x (AL2)
4.3.2.2 CAN Bus Arbitration [ ➙ 89]
CAN Bus Protocol
4.3.2 CAN Bus Protocol
CAN Bus Protocol
▪ Start
This bit indicates the beginning of a telegram and is a dominant bit. After this bit is set, no other user
of the CAN bus is able to send.
▪ Address field (11 bit identifier)
The 11 bit address identifier value determines the bus access. The lower value has the highest
priority.
▪ Control field
The 4 lowest bits in the 6 bit field show the data length of the following data field in bytes (DLC: Data
Length Code.
▪ Data field
Contains the information actually required and can be from 0 byte to 8 byte. The transfer of a byte
begins with the most significant bit (the bit with the highest value).
▪ Data control field CRC
Consists of a 15 bit check sequence (CRC sequence + CRC delimiter = CRC Field - Cyclic
Redundancy Check) and a recessive delimiter bit. The redundant information in the control
sequence allows the receiver to check whether the message received has been falsified by
interference.
▪ End
Each data telegram is terminated by a sequence of 7 recessive bits.
11 Bit Iden tifier
4.3.2.1 11 Bit Identifier
11 Bit Identifier
The CAN bus system is using the 11 Bit identifier for addressing the different CAN objects
An 11 Bit identifier (address) identifies the type, priority, source and /or target of the message.
This identifier also controls the bus access (arbitration).
CAN Bus Arbitration
4.3.2.2 CAN Bus Arbitration
Arbitration (arbitration means decision)
Communication and Control
CAN Bus Protocol CAN Bus
Student Guide SIPLACE X-Serie and X4I SW70x (AL2) 90
In CAN networks, there is no addressing of subscribers or stations in the conventional sense, but
instead, prioritized messages are transmitted. Whenever the bus is free, any unit may start to transmit a
message. In general, a subscriber can only occupy the bus if this is free. The bus subscriber can detect
the bus occupation state by analyzing a certain time period within which the bus must be inactive.
When multiple nodes begin to send a message at the same time, a selection phase (arbitration phase)
is used to decide which node may remain on the bus.
Bus access conflicts are resolved by including a message arbitration field (as a default the 11 bit
identifier is used).
The basis of bit-wise arbitration is the differentiation of 2 physical bus levels, a dominant one (low) and
a recessive bit (high).
A free bus is always on the recessive level. A DATA FRAME prevails over the REMOTE FRAME. During
arbitration every transmitter compares the level of the bit transmitted with the level that is monitored on
the bus. If these levels are equal the unit may continue to send.
When a recessive level is sent and a dominant level is monitored, the unit has lost arbitration and must
withdraw without sending one more bit. At the end of arbitration, the only subscriber left on the bus is the
one whose message has the lowest identifier value (logical zero is a dominant level). The lower the
identifier value is, the higher the priority of a message is.
When the bus is free any unit may start to transmit a message. The message sent by this subscriber is
not destroyed here i.e. it is a loss-free arbitration.
Flow chart bus arbitration
The following example shows arbitration by bit-wise scanning of the identifier by the 2 subscribers.
CAN bus arbitration example
Communication and Control
CAN Bus CAN Bus Structure
91 Student Guide SIPLACE X-Serie and X4I SW70x (AL2)
If bus subscriber A and B want to transmit, they begin to do so after the start-of-frame bit and compare
in each case the bits sent and received. Since"0" dominates on the bus, bus subscriber B recognizes
that the fourth bit differs from the bits sent and therefore withdraws from the bus until the next start-of-
frame. Bus subscriber A does not recognize a difference and therefore continues to transmit. Messages
with high priority therefore have an identifier which begins with several "0"s.
There are two bus states possible during arbitration: dominant and recessive.
Errors on the CAN Bus
4.3.2.3 Errors on the CAN Bus
Error frames
▪ What are error frames?
Error frames are sent by the individual subsystems, if a command does not correspond with the
coding rules or if it has been corrupted i.e. when a CAN telegram has 6 or more consecutive bits with
the same level (high or low).
If a command is recognized by a subscriber, this subscriber immediately informs the other
subscribers and the telegram sender, by issuing an error frame.
Upon receipt of the error frame, all subscribers reject the telegram received and begin to send their
own error frames.
Once the bus is free again, the commend is resent.
▪ If multiple error frames are issued, this indicates that a physical bus error has occurred. If too many
error frames are recognized during operation, a detailed analysis of the CAN signals is required.
CAN Bus Structure
4.3.3 CAN Bus Structure
NOTICE
► In SIPLACE X machines, the machine controller is a box PC. This also contains the COM
unit.
► From approx. 2008 machines will not have separate illumination control as this is realized
in the stationary cameras from version 4 onwards. The NCs are addressed and the sensors
monitored by the CAN node module. The NC is therefore reintegrated into the CAN bus
system.
NOTICE
X series/X4I with SW 702
In these machines the first BoxPC assumes the control function for the entire machine. The
second BoxPC is used as a Vision computer in these cases.