Information Technology Reference
In-Depth Information
After transmission of an overload flag, nodes monitor the bus until they detect a transition
from a dominant to a recessive bit. Then every node start transmission of seven more reces-
sive bits.
21.4.5 Interframe spacing
Data frames and remote frames are separated from preceding frames by an interframe space.
It contains intermission (three recessive bits) and bus idle bit fields. During intermission, no
station can start to transmit data. The period of bus idle is of any length (a free bus condi-
tion). After this, the detection of a dominant bit on the bus is interpreted as a start of frame.
21.5 Fault confinement
With respect to fault confinement a unit may be in one of three states:
•
Error active - these nodes can normally take part in bus communications and sends an
active errorr flag when an error has been detected.
•
Error passive - these nodes must not send an active error flag. They take part in bus
communication but when an error has been detected they only send a passive error flag.
•
Bus off - these nodes are not allowed to have any influence on the bus.
For fault confinement, two counts are implemented in every bus unit. The first is the transmit
error count. For example:
•
A transmitter sending an error flag, increases it by 8.
•
A transmitter detecting a bit error while sending an active error flag or an overload Tflag,
increases it by 8.
•
Successful transmission of a message, decreases by 1 (unless it is already 0).
The second is the receive error count. For example:
•
A receiver detects an error, increase it by one.
•
A receiver detecting a dominant bit as the first bit after sending an error flag, increased
by eight.
•
A receiver detects a bit error while sending an active error flag or an overload flag, in-
creased by eight.
•
After the successful reception of a message, decreased by one.
Nodes are initially error active. An error active node transmits active error flags
when it de-
tects errors. A node becomes error passive when the transmit error count equals or exceeds
128, or when the receive error count equals or exceeds 128. An error passive
node transmits
passive error flags when it detects errors. A node is bus off when the transmit error count is
greater that or equal to 256. A node which is bus off will not transmit anything
on the bus at
all.
