[NTLK] USB to Newton Interconnect cables

[Jake Bordens]

>I don't know which serial channel the USB-001 uses. If it's using channel 0, then it would conflict.

I believe it uses SerChan3, but what's more important is whether it obeys the SerChanSel3 signal.   This prevents both an internal device and an external device from attempting to address the same serial channel at the same time.

The SerChanSel0 signal actually shuts down the LTC1323 driver in the newton, and thus disables the external port when an internal device is using the serial port.  NewtonOS also uses this to shutdown the driver when not in use, to save power, I guess.

Serial3 has no internal driver, and so it is up to the devices (internal and external) to properly obey this signal.  its possible for both an internal and an external device to try and drive the serial signals at the same time-- if the hardware developer was lazy and did not put the outputs into High-Z state when they're not allowed to drive the port.
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