Network Parameters

Network Topology - The network interconnect shall use a common or global bus.

Network Access - The method of access to the network is random.

Bus Access - A transmitter shall begin transmitting a message only after an idle state has continuously existed on the bus for at least one bus access time. The transmitter must verify that the idle state continues to exist immediately prior to initiating a transmission (that is, within one-half bit time).

Bus Access Time - Bus access time is a time duration equal to the minimum time of an idle line plus the product of two bit times and the message priority. The relationship can be expressed as:

Ta = Ti + [2 * Tb] * P

Where:

Ta = Bus access time

Tb = Bit time, or period of one unit of information

P = Message priority

Ti = Minimum time duration of an idle line

NOTE: The minimum time duration of an idle line is defined in the Glossary. However, a transmitter that cannot distinguish between a stop bit and any other high logic state may not assume that Ti has elapsed until it has received 19 consecutive high logic bits.

Message Priority Assignment - All messages will be assigned a priority from 1 to 8 as follows: priority 1 and 2 will be reserved for messages that require immediate access to the bus. Priority 3 and 4 will be reserved for messages that require prompt access to the bus in order to prevent severe mechanical damage. Priority 5 and 6 will be reserved for messages that directly affect the economical or efficient operation of the vehicle. Priority 7 and 8 will be reserved for all other messages not fitting into the previous priority categories. The applications document shall define the priority associated with each message. In the event that more than one priority could be assigned to a particular message, the application document shall define each priority and the circumstances in which the priority is assigned.

Bus Contention - All transmitters shall monitor the message identification portion of their message to determine if another transmitter has attempted to gain access to the bus at the same time. If a transmitter detects a collision, the transmitter shall relinquish control the of the bus after completing the transmission of the current character, or sooner if possible. After relinquishing control, it is recommended that the transmitter become a receiver, using the received MID at the beginning of the incoming message. The transmitter may attempt to regain access to the bus after a bus access time has elapsed. An example bus reaccess procedure is shown in Appendix B.