344
OPUS Projektor Manual
CAN
3. Bits 23 ... 16 define the "PDU Format": If "PDU format" is less than 240 then "PDU
specific" is 0 in the PGN definition and is replaced by the destination address in the
CAN identifier (PDU1). If "PDU format" is greater or equal to 240 then "PDU
specific" just extends the PGN number.
4. Bits 15 ... 8 are "PDU specific" and either define the destination address (PDU1) of
the message or just extend the PGN number (PDU2).
Please note that the definitions above limit the number of valid PGNs:
PGN 0x000712 (==1810) is invalid since 0x07 == 7 is less than 240 and therefore the last
byte has to be 0.
A valid PGN would be 0x000700.
Another side effect of the above definitions is that all PGNs with "PDU format" >= 240 are
broadcasts because there is no room for a destination address in their CAN identifier.
Number Formats
J1939 typically transmits all values as unsigned with offset in metric units e.g. most
temperatures are transferred in one byte with -40°C offset and 1°C/bit resolution. This
means 50 is sent on CAN to indicate +10°C.
For most number formats the higher range is reserved and in almost every case "all bits
set" indicates "invalid" or "don't care". I.e. in the example above one could not transfer
temperature 215°C since the respective value on CAN would be 255 == 0xFF where all
bits are set.
Value transmission of all standardized PGNs happens in "little endian" format i.e. "least
significant byte first". Nevertheless some manufacturers use "big endian" format in their
proprietary messages.
Address Arbitration
Every ECU on a J1939 network has to claim its own address. The address consists of one
byte and either uses predefined suggestion from SAE, e.g. 0x00 for the engine ECU or is
dynamically negotiated at runtime.
In static networks addresses can be pre-configured but should nevertheless be claimed.
The tool allows to configure ECUs with static addresses where only the address is defined.
Alternatively addresses can be claimed dynamically i.e. each ECU claims an address and
if there should be conflicts the one with the lower J1939 name wins.
J1939 name is a 64 bit number which is supposed to uniquely identify each ECU (at least
on the network they are all connected to). It is sent as the data bytes of the address claim
PGN 0x00EE00 == 60928. Since the fields of this 64 bit number are not byte aligned the
address claim PGN can be considered a "worst case" scenario for little endian
transmission.
Transport Protocol
J1939 transport protocol is used whenever the amount of data exceeds 8 bytes so that
more than one CAN message is required to transfer them. J1939 transport protocol
supports transferring up to 1785 bytes.
Our program automatically handles the transport protocol but requires all data transferred
to be byte aligned in this case.
To Next Page
Loading...