I found out that Microsoft has removed virtual serial devices from the WP8 SDK and from the Metro/Modern UI app SDK.
Certainly M$ hasn't disabled the ability of computers to run BT devices, or to plug in FTDI USB-to-RS232
chips, and access them via terminal emulator software [????]. I don't know about BT, but you should be
able to use some pre-existing tools to access RS232 serial ports, such as Visual C# or Visual Basic.
Hmm, according to the SDK, the System.IO.Ports (C#) is not available (missing assembly reference, but the System.dll reference is not available when adding references) when developing for Windows Phone, Windows RT or for Metro apps - normal desktop programs (outside the Modern UI environment) are unaffected, and Microsoft still allows serial connections on Windows 8. Just not within the app environment forced upon RT and WP8. Serial connections aren't supported at all, whether that's a real one, over Bluetooth, or a virtual one - the functionality is not included. Unless of course I'm very wrong and there's another way around this apparant limitation, in which case please enlighten me

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If you are in the car, why would you want to have to call the car to get information? If you are not in the car, why would you care?
Call as in poll the OBD-II interface - not all vehicles display all that is monitored. One can garner a decent amount of additional information from the interface, including fuel consumption, engine load and so on, which could be pretty useful for cars without a trip computer.
Those are the kind of thing I'm after, but they only implement the CAN-bus, and so are not compatible with many Europe-only vehicles, including mine.