RAM 3500 Truck 4WD L6-5.9L DSL Turbo VIN C (2002)
Messages can be broadcast almost simultaneously by modules over the CCD data bus. Therefore, all messages are defined and ranked by a
predetermined priority. When two CCD chips start a message at exactly the same time, non-destructive arbitration occurs between the two CCD chips.
Arbitration will occur based upon the priority code, to determine which message takes priority on the data bus and to prevent data collision. If a CCD
chip senses a message of higher priority being transmitted, it stops transmitting its message. The higher priority message is then transmitted in its entirety
without interruption. The other CCD chips on the data bus do not allow any other messages to be broadcast.
Fig. 9 Message Arbitration
To determine the winner in an arbitration, all messages start with an ID byte which contains the predetermined priority code. In the digital broadcast,
zero is the dominant bit. All ID bytes start with a zero. This is the start of the message. With zeros being the dominant bit, messages starting with more
zeros have a higher priority For example: of the two messages below, Message # 2 loses arbitration at the second bit, where Message # 1 has a zero and
Message # 2 has a one. After the message is broadcast, an idle period occurs while all microprocessors can queue, if necessary, and attempt to broadcast
their messages again.
Message # 1 = 00010110
Message # 2 = 01010101
Message Identification
Because messages are broadcast over the data bus, all modules can receive them, yet not all modules need all messages. In order to enhance
microprocessor speed, unneeded messages are filtered out. The ID byte, along with showing message priority also identifies the data, content and length.
The electronic control module, through its CCD chip transceiver, monitors the ID code of the messages. If the message is not for that particular module,
the message is simply ignored. Once the module recognizes a message that it requires, the rest of the message is monitored and processed.
Transmission Verification
Once a CCD chip transmits a message over the CCD data bus, the message is received by the transmitting module at the same time through the CCD chip
differential transceiver. The module knows the message was broadcast correctly when it receives its own message back. If the message received does not
match the message transmitted, the message is said to be corrupt.
Corruption occurs when the message is incorrectly transmitted on the data bus. Corruption can also occur from interference, wiring problems, or other
data bus problems. In the case of a corrupt message, the module attempts to have the CCD chip re-send the message.
