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I have made a very basic test circuit for the opto coupler H11L1 ('similar'datasheet: http://www.datasheet4u.com/datasheet/H/1/1/H11L1_MotorolaInc.pdf.html))

I use the test circuit:



![schematic](https://i.sstatic.net/YeztS.png)





[![Circuit][1]][1]

The LED and 470 ohm resistor is only for testing.
U1 is the Arduino (software serial pin 9/11) and U2 is the H11L1.

The LED is flashing similar to the RX led on the Arduino.

Also I tested before in an even simpler circuit that the signals from the H11L1 pass through pin 4 (although inverted, which seems to be normal).

However, I don't receive any data in the Arduino (I tried different Arduinos, different speeds, different H11L1).

The data I receive from the (hardware) serial/debug terminal:

 0
 0
 0
 0

etc.

My sketch:

 #include <SoftwareSerial.h>

 SoftwareSerial mySerial(9, 11); // RX, TX

 void setup()
 {
 Serial.begin(9600);
 mySerial.begin(300);
 }

 void loop()
 {
 mySerial.write('A');
 while (mySerial.available())
 {
 Serial.println(mySerial.read());
 }
 
 delay(100);
 }

How can I get the correct signal? (meaning 'A' are printed instead of 0's?


 [1]: https://i.sstatic.net/z0218.jpg

Update
======

New sketch (not having the duplex problem):


 #include <SoftwareSerial.h>

 SoftwareSerial mySerial(12, 13); // RX, TX

 void setup()
 {
 
 Serial.begin(115200);
 mySerial.begin(115200);
 }
 
 void loop()
 {
 Serial.print("Write: A ");
 mySerial.write('A');
 
 while (mySerial.available())
 {
 Serial.print((int) mySerial.read());
 }
 
 Serial.println("");
 
 delay(100);
 }

 
Output:

 Write: A
 Write: A
 Write: A
 Write: A

etc.


Update
========
Problem solved: I put it in an answer for clearity.

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