When the fluid is low, the trans can't build enough pressure to engage the clutches. The trans has no direct way to measure fluid pressure. What happens when the clutches wont engage, is that the whole gear stack starts to spin up due to friction. The TCM rightfully interprets this as a slipping condition, and goes into soft LHM. When the ignition is reset it resumes normal operation.
Soft limp being that is resets when the fault is removed and the ignition is cycled. The trans will default to 2nd and reverse safe mode, which protects it from nuking itself. Of course if you don't have enough fluid, you wont get any drive at all.
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There are 2 ea.
It is safe to assume that the above applies to the drive train in general which includes the transmission.I'm no expert. I don't even understand much of what you guys are describing.
For certain one simple reason is reliability.
There have been a few threads here where the CAN B communication was compromised, but the engine ran fine. Most times the CAN B not being right was only noticed because a scan tool wouldn't connect/poll the modules. The OBDII port is a likely trouble maker because it is subject to consumer use. [Added: Aftermarket wires/components tagged on to the CAN system are another.] Keeping the critical engine operation (CAN C} separate helps with reliability.
As you point out, there's likely more complex reasons also.
vic
I agree that K-line is used for scan tool communication. The K-line communication to the the outside world must be derived internally from somewhere.
The canbus standard is a 2 wire bus. A high and low pair are used for a single bus. To my knowledge there are no canbus wires in the T1N OBDII plug. Unlike the NCV3 the T1N has a single canbus for all modules. The NCV3 uses 3 canbus segments, AFAIK, only one is used for dignostics, one for critical operations/safety, and another for body and minor stuff like radio.
vic