Unable to issue AT commands to SIM900

[kasparsm]

Hi All,

Facing issues establishing AT connection to SimCom SIM900 module.
Using Mega 2560.
Pins:
Mega pin 16 (Tx2) -> Sim900 pin 8 (Rx)
Mega pin 17 (Rx2) -> Sim900 pin 7 (Tx)

On Sim900 Serial communication jumpers set to Software.

Power supply: 9v 1A (AC-> DC converter)

Tried various Baud rates, ranging from 9600 till 115200 with no effect.
Passing AAAAT (in case with auto-baud enabled), AT results in nothing.

According to leds on SIM900, module is able to establish network connection.

Downgraded to older IDE version (1.6.5) - Didn't helped.

Any advice will be appreciated.

Sketch code:

int baudRate=115200;

void setup() {
  delay(1000);
  Serial.begin(baudRate);
  Serial2.begin(baudRate);
  Serial.println("Initializing...");
}

void loop() {
  printBack();
}

void printBack() {
  delay(500);
  while (Serial.available()) {
    Serial2.write(Serial.read());
  }
  delay(500);
  while (Serial2.available()) {
    Serial.write(Serial2.read());
  }
}

Thanks

[JohnLincoln]

Try removing those 'delay(500)' statements

[kasparsm]

Hi John,
Thanks for your reply!

Removed, but no luck.
Below is what's happening in Serial monitor

Fells like some communication exchange is going on, but not on command.

[xfpd]

That looks like a BAUD mismatch. Verify sketch, monitor and device are using the same (115200). Sending AAAT with a mismatch would have no effect on the other end.

[kasparsm]

Hi, thanks for your response.

Yes, verified that. Baud rates for bot Serial communcations are in sync, and serial monitor also uses defined configuration, but the problem is that I don't know device settings, apart from documentation for this module, that states that autobaud is by default.
I'm wondering if there's a way to check if module receives any input issued via serial port? In documentation I see there are debug ports as well, however I'm unable to understand which exact pins are those on my device...

At least If I could verify that chip is faulty, then I could move on...

[xfpd]

Does your documentation discuss the two TX/RX (red box in drawing, lower-left/J11/J12 in picture)?

[kasparsm]

Not really. At least was unable to find that... probably was looking wrongly...
https://simcom.ee/documents/SIM900/SIM900_Hardware%20Design_V2.05.pdf

[xfpd]

You show tx2/rx2 as 16/17. Do tx/rx wires connect to 0/1 pins?

Power supply: 9v 1A (AC-> DC converter)

SIM modules can use 2A.

[kasparsm]

Do tx/rx wires connect to 0/1 pins?

I played around with SIM900 tx/rx pins. Switched jumber between software/hardware serials and that didn't helped.

Indeed, it can use up to 2A, but my understanding was that it can also be used in combination with 9v 1A. 2A are crucial when GPRS or Calls are used. I might be wrong on this, therefore plan is to provide 5v 2A and see if that makes a difference.
Don't have appropriate voltage/current source now, therefore will have to figure out something.

[kasparsm]

Ok, issue resolved.
Turns out that indeed it was power supply issue. 9v 1A will not do the work.
Used 5v 2A and all is OK

Thanks to @JohnLincoln @xfpd for help!