Appendix B Example Bus REACCESS Procedure

A method for reaccessing the bus can be described by the following example:

1. First crash occurs for the current attempt to access the bus.
2. Each device wishing to access the bus then waits its predefined bus access time (as described in the section entitled Network Parameters, Bus Contention). 
3. Second crash occurs for the same attempt to access the bus.
4. Any device that has experienced two consecutive crashes in its attempt to transmit the same message shall follow the bus access procedure defined in the section entitled Network Parameters, Mesage Priority,  but with the bus access time calculated as follows:

   Ta = Ti + [2*Tb]*[P2+1]

   Ti and Tb are defined in the subsection entitled "Bus Access Time."

   P2 is a three-bit psuedo random number such as the three least significant bits of the stack pointer.

   For example, if 18 is the location of stack pointer register, the contents of this register is the stack pointer. This value will be ANDED with 0007, which results in a number from 0 to 7. P2 would, therefore, be a value from 0 to 7.

5. If any more consecutive crashes occur, the procedure described in 4, above, is repeated.

This example addresses the recognized possibility that two or more devices could continue to crash if their priorities were the same. The above method would greatly reduce the possibility of a third crash with the same device or devices.