TSS461C
12
Rev. D (22 Feb 01)
VAN BUS
SEQUENCE
VAN BUS
SEQUENCE
VAN BUS
SEQUENCE
NUMBER OF
PRESCALED
CLOCKS
NRZ
“
0
”
NRZ
“
1
”
MANCHESTER
“
0
”
MANCHESTER
“
1
”
0
8
16
24
32
Figure 11. Data Encoding.
The IFS is defined to be a minimum of 64 prescaled clocks periods. The TSS461C, accepts an IFS of zero prescaled
clocks for the reception only of a SOF sequence.
Once the bus has been determined as being free, the module must now, if it is an autonomous module, emit a SOF
sequence or, if it is a synchronous access module, wait until it detects a preamble sequence.
Up till this point there can be several modules transmitting on the bus, and there is no possibility of knowing if this
is the case or not. Therefore the first field in wich arbitration can be performed is the identifier field. Since the logical
zeroes on the bus are dominant, and all data is transmitted with the most significant bit (MSB) first, the first 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, however, conceivable that two messages transmitted on the bus will have the same identifier. The TSS461C
therefore continues the arbitration of the bus throughout the whole frame. More, if the identifier in transmission has
been programmed for reception as well, it transmits and receives messages simultaneously, right up till the Frame
Check Sequence (FCS). Only then, if the TSS461C has transmitted the whole message, does it discard 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 identifier. For instance it can be
used to transmit the emitting modules address in the first bytes of the data field, thus enabling the identifier to specify
the contents of the frame 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 immediately 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 TSS461C. 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.
Third we have the Read/Write bit (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 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 comination R/W. RTR = 01 is ignored without any IT generation.