Hardware Reference
In-Depth Information
Interframe space
Frame
Frame
Suspend
transmission
Intermission
Bus idle
Figure 13.11
■
Interframe space for error-passive nodes
signaling of an overload condition. The period of bus idle may be of arbitrary length. The bus
is recognized to be free, and any node having something to transmit can access the bus. A mes-
sage, pending during the transmission of another message, is started in the first bit following
intermission. When the bus is idle, the detection of a dominant bit on the bus is interpreted as a
start of frame
.
After an error-passive node has transmitted a frame, it sends 8 recessive bits fol-
lowing intermission, before starting to transmit a new message or recognizing the bus as idle. If,
meanwhile, a transmission (caused by another node) starts, the node will become the receiver of
this message.
A node uses filter(s) to decide whether to work on a specific message. Message filtering is
applied to the whole identifier. A node can optionally implement mask registers that specify
which bits in the identifier are examined with the filter. If mask registers are implemented,
every bit of the mask registers must be programmable; in other words, they can be enabled or
disabled for message filtering. The length of the mask register can comprise the whole identifier
or only part of it.
The point in time at which a message is taken to be valid is different for the transmitters
and receivers of the message. The message is valid for the transmitter if there is no error until
the end of frame. If a message is corrupted, retransmission will follow automatically and ac-
cording to the rules of prioritization. In order to be able to compete for bus access with other
messages, retransmission has to start as soon as the bus is idle. The message is valid for the
receiver if there is no error until the last but 1 bit of the end of frame.
The frame segments including start-of-frame field, arbitration field, control field, data field,
and CRC sequence are encoded by
bit stuffing
. Whenever a transmitter detects 5 consecutive
bits of identical value in the bitstream to be transmitted, it automatically inserts a complemen-
tary bit in the actual transmitted bitstream. The remaining bit fields of the data frame or remote
frame (CRC delimiter, ACK field, and end-of-frame field) are of fixed form and not stuffed. The
error frame and overload frame are also of fixed form and are not encoded by the method of bit
stuffing.
The bitstream in a message is encoded using the
non-return-to-zero
(NRZ) method.
This means that during the total bit time the generated bit level is either dominant or
recessive.
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