© 2004 Microchip Technology Inc. DS70074C-page 26-19
Section 26. Appendix
Appendix
26
B.8 Remote Frame
Normally data transmission is performed on an autonomous basis with the data source node
(e.g., a sensor sending out a data frame). It is possible, however, for a destination node to
request the data from the source. For this purpose, the destination node sends a “remote frame”
with an identifier that matches the identifier of the required data frame. The appropriate data
source node will then send a data frame as a response to this remote request.
There are 2 differences between a remote frame and a data frame, shown in Figure B-4. First,
the RTR bit is at the recessive state and second, there is no data field. In the very unlikely event
of a data frame and a remote frame with the same identifier being transmitted at the same time,
the data frame wins arbitration due to the dominant RTR bit following the identifier. In this way,
the node that transmitted the remote frame receives the desired data immediately.
B.9 Error Frame
An error frame is generated by any node that detects a bus error. An error frame, shown in
Figure B-5, consists of 2 fields, an error flag field followed by an error delimiter field. The error
delimiter consists of 8 recessive bits and allows the bus nodes to restart bus communications
cleanly after an error. There are two forms of error flag fields. The form of the error flag field
depends an the error status of the node that detects the error.
If an error-active node detects a bus error then the node interrupts transmission of the current
message by generating an active error flag. The active error flag is composed of six consecutive
dominant bits. This bit sequence actively violates the bit stuffing rule. All other stations
recognize the resulting bit stuffing error and in turn generate error frames themselves, called
error echo flags. The error flag field therefore consists of between six and twelve consecutive
dominant bits (generated by one or more nodes). The error delimiter field completes the error
frame. After completion of the error frame, bus activity returns to normal and the interrupted
node attempts to resend the aborted message.
If an error passive node detects a bus error, then the node transmits an error passive flag
followed again by the error delimiter field. The error passive flag consists of six consecutive
recessive bits, and therefore the error frame for an error passive node consists of 14 recessive
bits. From this, it follows that, unless the bus error is detected by the bus master node that is
actually transmitting, the transmission of an error frame by an error passive node will not affect
any other node on the network. If the bus master node generates an error passive flag, then this
may cause other nodes to generate error frames due to the resulting bit stuffing violation. After
transmission of an error frame, an error passive node must wait for 6 consecutive recessive bits
on the bus before attempting to rejoin bus communications.
B.10 Interframe Space
lnterframe space separates a proceeding frame (of whatever type) from a following data or
remote frame. lnterframe space is composed of at least 3 recessive bits, called the intermission.
This is provided to allow nodes time for internal processing before the start of the next message
frame. After the intermission, the bus line remains in the recessive state (Bus idle) until the next
transmission starts.
To Next Page
Loading...
