Need Help Debugging Arduino MEGA 2560 and GPS Shield

Projects Programming
1

I have an Arduino mega that is connected to a CIROCOMM GPS Shield

Background: I made this work once using example code from the internet. However it recently stopped working and has never worked since. I don't believe this is equipment problems because I have both a spare Mega and a spare GPS Shield. The symptoms are the same regardless of what combination of equipment I use. I really do not know what is going on or how to debug this system. Can someone please help explain some techniques that will allow me to debug this?

Problem: The problem appears to be in the Serial3 connection. Serial3.available() never returns true. How can I figure out what is going on here?

////// BEGIN CODE ///////

// WIRING:
// 1. GPS is powered and Grounded. After about 20 seconds the GPS achieves lock.
// GPS lock is indicted by a solid red light.
// 2. GPS TX (blue wire) is connected to Mega 14 (Tx3)
// GPS RX (yellow wire) is connected to Mega 15 (Rx3)

#include <TinyGPS.h>
#define GPSBAUD 4800
TinyGPS gps;
void getgps(TinyGPS &gps);

void setup()
{
Serial.begin(115200);
Serial3.begin(GPSBAUD);
Serial.println("GPS TEST CODE ");
}

void loop()
{
Serial.println("entering loop");
if(Serial3.available()) { // PROBLEM IS HERE!!! - Never is available
Serial.println("serial3 available"); // THIS LINE NEVER PRINTS
} else {
Serial.println("serial3 not-available"); // THIS LINE ALWAYS PRINTS
}

delay(1000);
while(Serial3.available()) // While there is data on the RX pin...
{
int c = Serial3.read(); // load the data into a variable...
if(gps.encode(c)) // if there is a new valid sentence...
{
getgps(gps); // then grab the data.
}
}
}

////// END CODE ///////

2

Without seeing a datasheet on the device you are using, I would question this:

// WIRING:
// 1. GPS is powered and Grounded. After about 20 seconds the GPS achieves lock.
// GPS lock is indicted by a solid red light.
// 2. GPS TX (blue wire) is connected to Mega 14 (Tx3)
// GPS RX (yellow wire) is connected to Mega 15 (Rx3)

Normally the Tx on one device is connected to the Rx on the other device.

3

Correction:

Below is wiring:

// WIRING:
// 1. GPS is powered and Grounded. After about 20 seconds the GPS achieves lock.
// GPS lock is indicted by a solid red light.
// 2. GPS RX (blue wire) is connected to Mega 14 (Tx3)
// GPS TX (yellow wire) is connected to Mega 15 (Rx3)

4

If the wiring is correct, then I would check the GPS Tx line with my o-scope for timing and voltage levels.

add: I would change the code temporarily to just read the GPS output and send it directly to the serial monitor.

void loop()
{
 while(Serial3.available())     // While there is data on the RX pin...
 {
     int c = Serial3.read();    // load the data into a variable...
     Serial.write(c);
 }
}