Hardware Reference
In-Depth Information
End-of-frame or
Error delimiter or
Overload delimiter
Interframe space
or
Overload frame
Overload frame
Overload flag
Overload delimiter
Superposition of
overload flags
Figure 13.9
■
Overload frame
No more than two overload frames may be generated to delay the next data frame or
remote frame. The overload flag consists of 6 dominant bits. The format of an overload frame
is similar to that of the active-error flag. The overload flag's form destroys the fixed form of
the
intermission field
. As a consequence, all other nodes also detect an overload condition
and each starts to transmit an overload flag. In the event that there is a dominant bit detected
during the third bit of
intermission
locally at some node, it will interpret this bit as the start
of frame.
The overload delimiter consists of 8 recessive bits. The overload delimiter has the same
form as the error delimiter. After the transmission of an overload flag, the node monitors the
bus until it detects a transition from a dominant to a recessive bit. At this point of time every
bus node has finished sending its overload flag and all nodes start transmission of 7 more reces-
sive bits in coincidence.
Data frames and remote frames are separated from preceding frames by a field called
inter-
frame space
. In contrast, overload frames and error frames are not preceded by an interframe
space, and multiple overload frames are not separated by an interframe space.
For nodes that are not error-passive or have been receivers of the previous message, the
interframe space contains the bit fields of
intermission
and
bus idle
, as shown in Figure 13.10.
The interframe space of an error-passive node consists of three subfields:
intermission, suspend
transmission,
and
bus idle
, as shown in Figure 13.11.
The intermission subfield consists of 3 recessive bits. During intermission, no node is
allowed to start transmission of the data frame or remote frame. The only action permitted is
Interframe space
Frame
Frame
Intermission
Bus idle
Figure 13.10
■
Interframe space for non error-passive node or receiver of previous
message
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