user80541: I have a Arduino code working on a Adafruit Feather STM32F405 Express, and as long as I test it with Arduino serial Monitor, everything works perfect. As I want to use a serial terminal (a C# application, or the "Terminal Br@y++") through the same COM port, then I can write my commands, there are executed (output pins send expected signal), but I don't received the serial feedback.
My Arduino code :
void setup() {
  Serial.begin(115200); // initialise USB Serial Port
  pinMode(pumpP, OUTPUT);  // Pump Pin is an output
}
void loop() {
    if (Serial.available() > 0) {
      command = Serial.readStringUntil('\n');
    }
    if (command == "do-it"){
        digitalWrite(5, HIGH);
        Serial.println("did it!");
    }
}

In my Arduino Serial Monitor, I write "do-it", and I recieved "did it!" and I can see the Pin 5 changing level.
In all serial application I could use, I can see the pin 5 changing level, but not the "did it!".
I configured Arduino as described on this Adafruit page :
https://learn.adafruit.com/adafruit-stm32f405-feather-express/arduino-ide-setup [ summary: use STM32F4 library, board number = "Adafruit Feather STM32F405", USB support = "CDC ( Generic Serial supercedes USART)", upload method = "STM32CubeProgrammer (DFU)" ]
And I connect the DFU pin to 3,3V during upload, and reset the board after it.
The worst in this story, is that at first time, it worked OK. Since a week or two, I don't get it to work again.

user80541: Thank you @Majenko, you gave the solution :
I checked the box "RTS/CTS" in the "handschake" section of Terminal by Bray, and I got all my serial message back from the STM32.
I guess this is a common think on more that stm32?
how did you know, is that somewhere written in Arduino documentation?
for people like me who do not know what is RTS/CTS, here you are: http://www.brainboxes.com/faq/items/what-is-rts--cts-hardware-flow-control-
or not so straight-forward https://en.wikipedia.org/wiki/RS-232#RTS,_CTS,_and_RTR
Funny to figure out that this "5 pin communication" is virtualise in USB.
@Majenko: if you want to "own" the solution, I let you do write it yourself, I will set your answer as solution.

80541: <p>I have a Arduino code working on a Adafruit Feather STM32F405 Express, and as long as I test it with Arduino serial Monitor, everything works perfect. As I want to use a serial terminal (a C# application, or the &quot;Terminal Br@y++&quot;) through the same COM port, then I can write my commands, there are executed (output pins send expected signal), but I don't received the serial feedback.</p>
<p>My Arduino code :</p>
<pre><code>void setup() {
  Serial.begin(115200); // initialise USB Serial Port
  pinMode(pumpP, OUTPUT);  // Pump Pin is an output
}
void loop() {
    if (Serial.available() &gt; 0) {
      command = Serial.readStringUntil('\n');
    }
    if (command == &quot;do-it&quot;){
        digitalWrite(5, HIGH);
        Serial.println(&quot;did it!&quot;);
    }
}
</code></pre>
<p>In my Arduino Serial Monitor, I write &quot;do-it&quot;, and I recieved &quot;did it!&quot; and I can see the Pin 5 changing level.
In all serial application I could use, I can see the pin 5 changing level, but not the &quot;did it!&quot;.</p>
<p>I configured Arduino as described on this Adafruit page :
<a href="https://learn.adafruit.com/adafruit-stm32f405-feather-express/arduino-ide-setup" rel="nofollow noreferrer">https://learn.adafruit.com/adafruit-stm32f405-feather-express/arduino-ide-setup</a> [ <em>summary: use STM32F4 library, board number = &quot;Adafruit Feather STM32F405&quot;, USB support = &quot;CDC ( Generic Serial supercedes USART)&quot;, upload method = &quot;STM32CubeProgrammer (DFU)&quot;</em> ]
And I connect the DFU pin to 3,3V during upload, and reset the board after it.</p>
<p>The worst in this story, is that at first time, it worked OK. Since a week or two, I don't get it to work again.</p>
 80541: <p>Thank you @Majenko, you gave the solution :
I checked the box &quot;RTS/CTS&quot; in the &quot;handschake&quot; section of Terminal by Bray, and I got all my serial message back from the STM32.</p>
<p>I guess this is a common think on more that stm32?
how did you know, is that somewhere written in Arduino documentation?</p>
<p>for people like me who do not know what is RTS/CTS, here you are: <a href="http://www.brainboxes.com/faq/items/what-is-rts--cts-hardware-flow-control-" rel="nofollow noreferrer">http://www.brainboxes.com/faq/items/what-is-rts--cts-hardware-flow-control-</a>
or not so straight-forward <a href="https://en.wikipedia.org/wiki/RS-232#RTS,_CTS,_and_RTR" rel="nofollow noreferrer">https://en.wikipedia.org/wiki/RS-232#RTS,_CTS,_and_RTR</a></p>
<p>Funny to figure out that this &quot;5 pin communication&quot; is virtualise in USB.</p>
<p>@Majenko: if you want to &quot;own&quot; the solution, I let you do write it yourself, I will set your answer as solution.</p>
