Other than the cynical reason, do we have any good engineering reasons why Resmed went to the hugely non-standard 24-volt-with-3.3volt-sense power supply?
Googling for the patent, I find this:
Power management in respiratory treatment apparatus
US 20140366876 A1
I see that they do the obvious, which is to prioritize power to the fan over the less immediate needs of the heater and humidifier, but that has nothing to do with the non-standard 24volt supply, nor the 3.3-volt sense resistor.
Deep in the patent description is the line "Optionally, a pull up resistor may be implemented to code information about a component coupled with the bus such that different resistors may indicate different components or different component functionality"
It goes on to explain that the pull-up resistor is in the power supply and that its value is sensed by the "analog sensing elements of the transceiver
To me, the use of the word "transceiver" is strange, but they must be using it in a non-standard way, as I've only seen the word used in terms of radio transmissions. They seem to be using the word "transceiver" merely as the items sending and receiving signals on a non-radio-frequency communications bus.
Basically the patent explains, in words, the circuit used, as it says "if the pull-up resistor represents the type of power supply, a detection of 3.9 K ohms may be interpreted by a master controller that the power supply is an infinite supply (mains).
This is interesting, because it intimates that ResMed machines can handle the 120Volt 60HZ mains, which, if true, I was wholly unaware of.
The patent goes on further to say "A detection of 2.7K ohms may be interpreted as a 90 Watt power supply.
", which, incidentally, is the wattage of "my" 24-volt power supply.
Additionally, they outline that "A detection of 1.8K ohms may be interpreted as a 60 Watt power supply
" and that "A detection of 1.0K ohms may be interpreted as a 30 Watt power supply
I find it odd that these actual numbers are listed in a patent, simply because there is nothing inherent in the resistor values that determines the wattage, and patents are generally rather purposefully vague on such essentially-meaningless intrinsic values.
The patent does state the obvious, which is that "Other voltage levels and coding schemes may also be utilized to code information about the power supply unit in this analog manner.
", so, that bolsters my surprise that they bothered to list the actual resistor values.
As would be expected, in addition to the analog resistor sensing, the patent does vaguely mention that "Optionally, digital messaging may also be utilized to detect information about the power supply unit or other components of the bus.
", which simply means that the patent is trying to garner rights over putting "anything" in the power supply line to provide once-removed information about said power supply.
Of course, a digital signal presupposes a "smart" power supply, which is exactly what they hint at when they mention in closing that "a data message with information from circuits or memory of the power supply unit or based on signals from the detectors or sensors of the power supply unit may be sent from a processor of the power supply unit to a system level controller or processor.".
If anyone can make sense of this more so than I just did, I would be appreciative, as I really don't see why they have such a need to make the power supply non standard.
For example, a Lenovo Thinkpad W510 senses the power supply WITHOUT a pull-up resistor, so, certainly there are other ways of doing things that don't in and of themselves make the power supply non standard.