Information Technology Reference
In-Depth Information
15
+
x
14
+
x
10
+
x
8
+
x
7
+
x
4
+
x
3
+1
The remainder of this polynomial division is the CRC sequence. The algorithm uses a 15-bit
shift register buffer(14:0).
buffer=0;
repeat
nextbit= nextbit
⊕
buffer(14);
buffer(14:1) = buffer(13:0);
buffer(0)=0;
if nextbit then
buffer(14:0) = buffer(14:0)
⊕
(100010110011001b);
endif
until (the start of the CRC SEQUENCE field);
where nextbit denotes the next bit of the bit stream. The resulting buffer will then be stored
in the CRC sequence field.
21.4.2 Remote frame
The receiver for certain data can initiate the transmission of the required data by sending a
remote frame. It has the same fields as a data frame, but does not have a data field. It is iden-
tified with a recessive bit in the RTR bit (a dominant bit identifies a data frame).
For example, if node A transmits a remote frame with the arbitration field set to 123, then
node B could respond with a data frame with the arbitration field also set to 123. This type of
frame is used to implement a request-response type of bus traffic management.
21.4.3 Error frame
Error frames have two different fields, error flags and an error delimiter (eight recessive
bits). There are two types of error flag, these are an active error flag and a passive error flag.
An active error flag has six consecutive dominant bits, whereas the passive error flag has six
consecutive recessive bits (unless they are overwritten by dominant bits from other nodes).
After transmission of an error flag, each node sends recessive bits and then monitors the
bus until it detects a recessive bit. Afterwards it starts transmitting seven more recessive bits.
The error frame thus violates the bit stuffing rules of a CAN message. It is transmitted when
a node detects a fault and causes all other nodes to detect a fault (and they will also send er-
ror frames). After this the transmitter automatically resends the message.
The error frame consists of an error flag, which is 6 bits of the same value (thus violating
the bit-stuffing rule) and an error delimiter, which is 8 recessive bits.
21.4.4 Overload frame
An overload frame contains the two bit fields: overload flag (six dominant bits) and overload
delimiter (eight recessive bits). At the most, two overload frames may be generated to delay
the next data or remote frame. The overload flags form destroys the fixed form of the inter-
mission field. Thus, all other stations detect an overload condition. The conditions that cause
an overload frame are:
•
Receiver internal conditions, which requires a delay of the next data frame or remote
frame.
•
Detection of a dominant bit during intermission.
