General Submission : Multi-Packet Messages and the DM_RV
Martin Perlot, RV-C Administrator, submits the following replacement text for section 3.2.5.10
Encoding multiple faults in a multiple packet message
Although similar protocols suggest using the multi-packet protocol when multiple faults are present, this practice has been deprecated in RV-C. Table 3.2.5.10 defines the former practice for the benefit of designers for whom compatibility with older products is necessary. New designs should not use this format.
When multiple faults are present in a product, a DM_RV shall be broadcast for each fault. This specifically allows the alarm level (red, yellow) to be communicated for each SPN/FMI combination, which is often essential for proper analysis. Each transmission shall be repeated every 5000ms while the fault is active.
(Table 3.2.5.10 follows, marked DEPRECATED)
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Justification
There are two reasons to deprecate the use of multi-packet messages: First, many simple devices parse the DM_RV solely to obtain the general status of a device, and implementing the necessary buffers to reliably parse multi-packet messages puts a significant burden on the design. (A full implementation of the scheme theoretically requires 253 buffers of 1785 bytes each!) Second, as noted in the text, the multi-packet DM_RV format is ambiguous regarding the severity of each failure, which is clearly an important piece of diagnostic information.