2

I have a Arduino Pro Mini 3.3V connected to a sim800l chip using a serial connection of 9600bps. When I connect the Arduino to the device, it seems I don't receive anything on the Arduino Rx pin.

I then connected a FTDI that listen the sim800l Tx pin:

enter image description here

The FTDI is able to receive data so this means the sim800l is working well by receiving data from the Arduino and responding accordingly.

So normal logic would say that the Arduino Rx pin is not working but when I connect the Arduino Rx and Tx pin together:

enter image description here

I'm able to receive what I send on Tx on the Rx pin. Why would that be?

I tried with AltSoftSerial and SoftwareSerial. The Arduino is powered by a FTDI connected to my computer via USB. The sim800l is powered by a second FTDI that is also powered by my computer USB. So all component uses the same ground and I verified that with my multi-meter.

Here is the code I use: 

AltSoftSerial sim800Serial(8,9); // RX, TX

void setup()
{
  Serial.begin(9600);
  sim800Serial.begin(9600);
  void loop()
{

while(1)
{
  if(sim800Serial.available())
  {
    Serial.write(sim800Serial.read());
  }

  if(Serial.available())
  {    
    sim800Serial.write(Serial.read());
  }
}
VE7JRO
  • 2,497
  • 15
  • 24
  • 29
Pat
  • 121
  • 3
  • Please complete the circuit with VCC and GND so we understand the complete connection. – Mikael Patel Jan 24 '16 at 00:21
  • As well as the code you wrote to receive data from the device – SoreDakeNoKoto Jan 24 '16 at 00:49
  • `connected to some device Y ` - what device? Part number? Datasheet? – Nick Gammon Jan 24 '16 at 02:20
  • Mikael Patel is right - if you didn't connect the grounds the device may well not work. However the Pro Mini can talk to itself because the grounds are connected internally. – Nick Gammon Jan 24 '16 at 04:03
  • 1
    I added more details to my question that answer yours. Let me know if I need more clarifications. – Pat Jan 24 '16 at 07:11
  • Are those grounds all physically connected in your circuit or are you relying on the grounds provided by the various USB modules being linked via your computer's ground? – Majenko Jan 24 '16 at 12:01
  • 1
    with power off, I measured all ground and they are shorted and have a very little resistance – Pat Jan 24 '16 at 21:32
  • 1
    I also tried putting all devices on the exact same ground without success. – Pat Jan 24 '16 at 21:48

0 Answers0