Arduino connects to the PC using an USB port. Anyway, that USB connection is actually used like a Serial (RS232)
connection (remember the old 56K external modems? most of them where connected using a Serial interface). The Arduino IDE uses it to upload our programs to the board but the serial connection can also be used for any other kind of communication.
Arduino also has digital pins 0 (RX) and 1 (TX) which can be used to directly connect Serial interfaced wires into Arduino. They deliver the same signals sent on the USB Serial interface.
Step 1: Step 1: Parts List and Wire Diagram
Parts list: two Arduino , two Arduino UNO, three Dupont wires
Step 2: Step 3: Softmodem for Arduino
There are some versions about Softmodem Arduino library, you can click
Step 3: Step 3: Some Problems About Softmodem.h File
I chose Softmodem-005.zip, after downloading ，input to Arduino and when
compiler ,there are some errors (I use 1.5.6 version of the Arduino IDE), can modify the SoftModem.h file under SoftModem.h file ,the results as follows:
code in here :
Step 4: Step 4：softmodem.cpp File
softmodem.cpp file is here :
Step 5: Step 5: Sample Code
The code is very simple, it is a Arduino sending data, another is Arduino receiving data.
Code one is responsible for sending data:
code is here: