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TSS463-AA
4205B–AUTO–12/04
tion can be performed is the identifi er field. Since the logical ze roes on the b us are
domina nt, and all dat a is transmi tted with the most signific ant bit (MSB) fi rst, the first
module t o tran smit a log ical z ero on th e b us will be th e prior itized module , i.e. the me s-
sage that is tagged with the lowest identifier will have priority over the other messages.
It is possible that two messages transmitted on the bus will have the same identifier. The
TSS463AA, therefore, continues the arbitration of the bus throughout the whole frame.
Moreov er, i f the ide nti fie r in tr an sm is sion has bee n pr ogr am med fo r rece pti on as we ll , i t
transmits and receives messages simultaneously, right up until the Frame Check
Sequence (FCS). Only then, if the TSS463AA has transmitted the whole message, it
discard the message recei ved. Arbi tration los s in th e FCS fi eld 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 TSS463AA. 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 another
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 T ransmission 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 includes the
combination R/W. RTR = 01 is ignored without any IT generation.
All the bit s in the command field are autom atically hand led by the TSS 463AA, so the
user ne ed not to be conce rned for encod ing and dec oding of thes e bits. The c ommand
bits transm itted on the VAN bus are calculated from the cur rent status of the ac tive
message.
The data field comes after the command field. This is just a sequence of bytes transmit-
ted MSB first. In the VAN standard the maximum message length is set to 28 bytes, but
the TSS463AA 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
The division is done with a rest initialized to 0x7FFF, and an inversion of the CRC bits is
performed befo re transmi s sion.