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The 3 inputs (TXD, RTS, DTR) go through RS-232 conversion circuitry before going to
the Huawei module. They SHOULD NOT be inverted from the normal RS-232 signaling.
In other words, TXD is 0 volts when no data is flowing, and a positive voltage on RTS is
a request to send. RS-232 level signaling can be used, and logic levels can also be
used. A low should be .6 volts or lower and a high should be 2.4 volts or higher. There is
typically .5 volts input hysteresis and there's 5K typical input resistance.
When all RS-232 lines need to conform to RS-232 signaling conventions, the DE-9
serial connector can be used.
Power Lines
There are two Ground pins that are shorted together in the modem.
3.92 volts comes out of the modem's switching power supply and connects to the
modem's circuitry including the Huawei module. The 3.92 volts is NOT available only if
either VIN doesn't receive proper power or the header's VIN_ENABLE pin is low.
VIN_ENABLE is pulled up to VIN through a 10K resistor on the modem board. To bring
VIN_ENABLE low, consider using a N-channel Mosfet.
VMID comes from the Huawei MU609, supplying 1.8 volts nominal.
VIN is the input to the modem's switching supply. It connects through power inductors to
the modem's power jack. When the power jack is used, a board connected to the
header can draw power from VIN. When the power jack is not used, a board connected
to the header can supply power to the modem's switching power supply. VIN should be
4.7 volts to 60 volts, with the ability to deliver approximately 6 watts of power. Circuitry
external to the modem can, for instance, be normally powered by a DC power cube and
can be powered by a battery when the DC power cube is not supplying power.
USB Lines
USB D+ and USB D- connect to the Huawei module.
GPIO Lines
The header's 4 GPIO lines are controlled by the AT^IOCTRL command. This command
can be used to set the pins as an output or to read the pins when set to an input. In input
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