Design and Analysis of CAN Networks for Vehicles 2000-01-2585
CAN based protocols for on- and off-highway applications are very popular. Designers of these systems need to know prior to implementation if the communications design will meet all the timing requirements, especially when critical machine control is involved. An analysis of the system that is accurate and easy to understand is needed. Further, tools are needed to allow “what if” analysis of varying messaging and priority schemes. The methods described in this paper are useful for standard protocols like SAE J1939 and ISO 11783 as well as for proprietary protocols.
A communication modeling method using Rate Monotonic Analysis is possible. The theory and analysis method developed by others is applied to the J1939 situation. Some equations are modified to make it more realistic such as the calculation of stuff bits. The model is easily implemented with a spreadsheet. The very process of developing the model reveals certain key design features that are required of the CAN communication software drivers to prevent unbounded priority inversion. This information is used to determine the correct usage of the CAN objects, queuing methods and message parsing to achieve the best possible communications performance. Using the model allows the bus loading to be determined. This measure alone does not give enough information to guarantee a stable and robust system. To go further, the model allows the individual message latencies and system spare capacity to be determined. The measure of spare capacity is used to optimize message priorities to allow for maximum system expansion while guaranteeing the system performance.
An example system is analyzed and optimized using a spreadsheet to implement the mathematical model. Conclusions can be drawn from the analysis regarding the usefulness and appropriateness of various message protocol features.