I am currently using a MG2639 Cellular Shield with a PIC24F host.
I am able to send AT commands from the PIC24F host to the MG2639, and I have confirmed this with my signal analyzer. The transmit pin on the PIC24F host idles HIGH and works as expected (this is how the UART is configured).
But I have noticed that the TX signal from the MG2639 to the RX pin on the PIC24F idles LOW, and I receive nothing whatsoever back from the MG2639. Even if I received nothing back from the MG2639, SHOULDN'T THE TX PIN ON THE MG2639 IDLE HIGH????