An arduino program that will bridge a UART and USB serial interface using an Arduino and software serial.

SerialBridge.ino

/*
  This is a basic Arduino program to read the UART serial interface on an old
  SpeedStream 5360 DSL modem I have. The code sets up a UART software serial
  interface using the Arduino's 2 and 3 pins as RX and TX.
  
  This program bridges the serial interfaces between USB and the UART software
  serial. This will allow communication between the 3 devices.
 
 This example code is in the public domain.
 
 */
 
 #include <SoftwareSerial.h>

// These constants won't change.  They're used to give names
// to the pins used:
const int TX = 3;  // Transmit
const int RX = 2; // Receive

SoftwareSerial myserial(RX, TX);


void setup() {
  
  Serial.begin(9600);
  while (!Serial) {
    ; // wait for serial port to connect. Needed for native USB port only
  }
  
  // initialize serial communications at 9600 bps:
  myserial.begin(9600); 
  
  Serial.println("Hello from Arduino");
}

void serialEvent()
{
  String TXData = Serial.readStringUntil('\n');
  
  if (TXData.length())
  {
    Serial.println("");
    Serial.print("<- ");
    Serial.print(TXData);
    Serial.println("");
    myserial.print(TXData);
  }
}

void loop() {
  myserial.listen();
  
  // Receive data
  while (myserial.available() > 0)
    Serial.write(myserial.read());
}