17
TSS463B
4102D–AUTO–03/03
At this point there could be several modules transmitting on the bus, and there is no
possib ility of kno wing if this is the case or not. There fore, the fi rst field in that arbitra tion
can be performed is the identifier field. Since the logical zeroes on the bus are dominant,
and all da ta is transmi tted with the most si gnificant bi t (MSB) first, the fir st module to
transmit a logical zero on the bus will be the prioritized module, i.e., the message that is
tagged with the lowest identifier will have priority over the other messages.
It is, howe ver, possible that two messages tran smitted on the bus will have the same
identifier. The TSS463B, therefore, continues the arbitration of the bus throughout the
whole frame. Moreover, if the identifier in transmission has been programmed for recep-
tion as well, it transmits and receives messages simultaneously, right up until the Frame
Check Sequence (FCS). Only then, if the TSS463B has transmitted the whole message,
it discards the message received. Arbitration loss in the FCS field is considered as a
CRC error during transmission.
This feature is called full data field arbitration, and it enables the user to extend the iden-
tifier. For instance it can be used to trans mit the emi tting modules address i n the first
bytes of the data field, thus enabling the identifier to specify the contents of the fr ame
and the data field to specify the source of the information.
The identifier field of the VAN bus frame is always 12 bits long, and it is always followed
by the 4-bit command field:
•The first bit of the command is the extension bit (EXT). This bit is defined by the
user on transmission and is received and retained by the TSS463B. To conform with
the standard, it should be set to 1 (recessive) by the user, else the frame is ignored
without any IT generation.
•The second bit is the request ACKnowledge bit (RAK). If this bit is a logical one, the
receiving module must acknowledge the transfer with an in-frame
acknowledgement in the ACK field. If it is set to logical zero, then the ACK field must
contain an acknowledge absent sequence.
•The third bit is the Read/Write (R/W). This bit indicates the direction of the data in a
frame.
–If set to zero, it is a ‘write’ message, i.e., data transmitted by one module to
be received by another module.
–If it is set to one, it implies a ’read’ message, i.e., a request that ano ther
module should transmit data to be received by the one that requested the
data (reply request message).
•Last in the command field is the Remote Transmission Request bit (RTR). This bit is
a logical zero if the frame contains data and a logical one if the frame does not
contain data. In order to conform with the standard a received frame included the
combination R/W. RTR = 01 is ignored without any IT generation.
All the bits in the command field are automatically handled by the TSS463B, so the user
does n ot need to be c oncerne d for enc oding and decodi ng thes e bits. The comm and
bits transm itted on the VAN bus are calculated from the cur rent status of the ac tive
message.
After the co mma nd field com es the data field. T his is jus t a sequen ce of byt es tran smit-
ted MSB first. In the VAN standard, the maximum message length is set to 28 bytes, but
the TSS463B handles messages up to 30 bytes.
The next field is the FCS field. This field is a 15 bit CRC checksum defined by the follow-
ing generator polynomial g(x) of order 15:
g(x)= x15 + x11 + x10 + x9 + x8 + x7 + x4 + x3 + x2 + 1