Arduino with TOSLINK serial communication

Projects General Guidance
1

Hi guys,

I have an Arduino Nano hooked up to a FCR684205T TOSLINK transmitter and FCR684205R TOSLINK receiver, which are connected together with a 10m long TOSLINK cable.

A simple SoftwareSerial example code is running on the Nano:

#include <SoftwareSerial.h>

SoftwareSerial mySerial(2, 3); // RX, TX

void setup()
{
  // Open serial communications and wait for port to open:
  Serial.begin(9600);
  while (!Serial) {
    ; // wait for serial port to connect. Needed for Native USB only
  }


  Serial.println("Goodnight moon!");

  // set the data rate for the SoftwareSerial port
  mySerial.begin(9600);
  mySerial.println("Hello, world?");
}

void loop() // run over and over
{
  if (mySerial.available())
    Serial.write(mySerial.read());
  if (Serial.available())
    mySerial.write(Serial.read());
}

These are the waveforms on the pins 2 and 3 of the Nano when I just press the "Send" button on the serial monitor, without typing anything (yellow is TX, going into a MOSFET, which ties the TOSLINK transmitter's Vin to ground, blue is the TOSLINK receiver's output):

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Everytime I send something I get three '␦' characters, then as much '␦' characters as long the string I sent is.

I heard that these TOSLINK components works over 100kHz, but if I have such a nice signal at 9600bps then why can't I read the characters I just sent? I tried higher baudrates with the same results...

Any ideas would be appreciated!

68808126_659730514521987_6769486392620744704_n.jpg
68808126_659730514521987_6769486392620744704_n.jpg3024×4032 713 KB

2

Have you tried bringing the two Arduino together and connecting with a short wire (and common ground)? Does it work correctly like that?

3

I use a single Arduino as transmitter and receiver. Is this a problem?

4

Ah, sorry, I should have payed more attention. As far as I know, software serial cannot transmit and receive at the same time. You cannot use the hardware serial on Uno because it is also connected to the usb-serial adaptor chip and you can't connect to outputs together without risk of damage.

A better idea would be an Arduino with extra hardware serial ports, such as Mega, Leonardo or Pro Micro.

Or you could try AltSoftSerial.

5

Thank you! I tried sending from a FT232 module to a Mega and replaced the MOSFET with a level shifter IC. Under 57600 bps now works! I'm surprised because I thought they only work over 100kHz.

I want to increase the speed to at least 500000 bps, but the pulses already gets shorter/longer at the receiver around 57600 bps, so I rarely read the same character I sent.

This issue is no longer related to Arduino I think, but I would be really thankful for any tips to improve the communication speed!

6

SmileXS4:
I tried sending from a FT232 module to a Mega and replaced the MOSFET with a level shifter IC. Under 57600 bps now works! I'm surprised because I thought they only work over 100kHz.

You didn't post a schematic, so we can't give any feedback on your MOSFET circuit or this mystery "level shifter IC". What was it you thought only worked over 100KHz?

SmileXS4:
the pulses already gets shorter/longer at the receiver around 57600 bps

Randomly? What gets longer and what gets shorter?

SmileXS4:
I would be really thankful for any tips to improve the communication speed!

Post that schematic!

7

Well, it looks like these TOSLINK components are really designed for over 100kHz! I don't know why they worked at 9600 bps with the 74HC14 IC, however they work now without it even at 2M bps!

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Here's the schematic which contains the 74HC14 IC that I no longer use:

800×401 25.1 KB

Thank you for the suggestions again, PaulRB! I'll be back if some new problems occur or if I finish this part of my project. :slight_smile: