user49126: So, I've finally able to send and receive MIDI using Arduino. 
My next thing to do was sending 5 pin MIDI to rtpMIDI, and vice versa, using ESP8266, but so far I can't even send MIDI correctly. 
So far, I used this library, and it send midi without issues with Arduino Mega. But using Wemos D1, strange thing happens. 

this is output of MIDI OX. Pretty sure I wasn't sending MIDI CC. 
Here's the code so far : 
#include <MIDI.h>

 // Created and binds the MIDI interface to the default hardware Serial port
 MIDI_CREATE_DEFAULT_INSTANCE();

 void setup()
 {
     MIDI.begin(MIDI_CHANNEL_OMNI);  // Listen to all incoming messages
 }

 void loop()
 {
     // Send note 42 with velocity 127 on channel 1
     MIDI.sendNoteOn(42, 127, 1);

     // Read incoming messages
     MIDI.read();
 }

UPDATE : Here goes the settings. 


Any reason why this happens? Please help me. Thanks!
Schematics : 
Basically I just change the Arduino to Wemos D1 Uno. 


user49126: NOTE : Probably not the answer you're looking for, but it works for me. 

Solution : Move the jumper from 5V to Vin. 
Why? I don't know. It just works. 

Please notice that this is Wemos D1 Uno, which has adaptor input, which can receive 9-24V. 
If you're going to use the adaptor, IT MAY BROKE YOUR MIDI DEVICE!
As far as I know, Vin stands for Voltage Input, and everything from the adaptor is routed there. That means if you use 24v adaptor, Vin is 24v!

I'm only planning to use USB as power supply, so yeah. it works for me. 
If you're going to use Vin with external power supply, please use voltage regulator to 5v. Better safe than sorry. 

Anyway, since I'm planning to use 6N137 to receive MIDI also on this board, here goes : 

Input voltages greater than 7 V must be avoided to preclude damage to
  the gate oxide of the input stage. This damage is not necessarily
  permanent, but will adversely affect the expected lifetime of the
  circuit. The gate oxide of AHC devices is only 200 Å thick. An input
  voltage of 7 V corresponds to a field strength over the gate oxide of
  350 kV/cm. Although breakdown of the oxide is expected only at input
  voltages above 10 V, electrons tunnel increasingly into the gate oxide
  at field strengths greater than 350 kV/cm, influencing characteristics
  of the transistors and causing failure.

Source : this question on electronics stackexchange.

49126: <p>So, I've finally able to <a href="https://www.instructables.com/id/Send-and-Receive-MIDI-with-Arduino/" rel="nofollow noreferrer">send</a> and <a href="https://arduino.stackexchange.com/questions/56279/how-do-i-properly-send-and-receive-midi-with-arduino-with-6n138">receive MIDI</a> using Arduino. </p>

<p>My next thing to do was sending 5 pin MIDI to rtpMIDI, and vice versa, using ESP8266, but so far I can't even send MIDI correctly. 
So far, I used <a href="https://github.com/FortySevenEffects" rel="nofollow noreferrer">this library</a>, and it send midi without issues with Arduino Mega. But using Wemos D1, strange thing happens. </p>

<p><a href="https://i.stack.imgur.com/i5EX0.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/i5EX0.png" alt="enter image description here"></a>
this is output of MIDI OX. Pretty sure I wasn't sending MIDI CC. </p>

<p>Here's the code so far : </p>

<pre><code>#include &lt;MIDI.h&gt;

 // Created and binds the MIDI interface to the default hardware Serial port
 MIDI_CREATE_DEFAULT_INSTANCE();

 void setup()
 {
     MIDI.begin(MIDI_CHANNEL_OMNI);  // Listen to all incoming messages
 }

 void loop()
 {
     // Send note 42 with velocity 127 on channel 1
     MIDI.sendNoteOn(42, 127, 1);

     // Read incoming messages
     MIDI.read();
 }
</code></pre>

<p>UPDATE : Here goes the settings. <br />
<a href="https://i.stack.imgur.com/eWgcu.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/eWgcu.png" alt="enter image description here"></a>
<br /><br />
Any reason why this happens? Please help me. Thanks!</p>

<p>Schematics : <br />
Basically I just change the Arduino to Wemos D1 Uno. 
<a href="https://i.stack.imgur.com/jqPZz.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/jqPZz.png" alt="enter image description here"></a></p>
 49126: <p>NOTE : Probably not the answer you're looking for, but it works for me. <br />
<br />
Solution : Move the jumper from 5V to Vin. <br />
Why? I don't know. It just works. <br />
<br />
Please notice that this is Wemos D1 Uno, which has adaptor input, which can receive 9-24V. <br />
If you're going to use the adaptor, <b>IT MAY BROKE YOUR MIDI DEVICE!</b><br />
As far as I know, Vin stands for Voltage Input, and everything from the adaptor is routed there. That means if you use 24v adaptor, Vin is 24v!
<br /><br />
I'm only planning to use USB as power supply, so yeah. it works for me. <br />
If you're going to use Vin with external power supply, please use voltage regulator to 5v. Better safe than sorry. <br />
<br />
Anyway, since I'm planning to use 6N137 to receive MIDI also on this board, here goes : </p>

<blockquote>
  <p>Input voltages greater than 7 V must be avoided to preclude damage to
  the gate oxide of the input stage. This damage is not necessarily
  permanent, but will adversely affect the expected lifetime of the
  circuit. The gate oxide of AHC devices is only 200 Å thick. An input
  voltage of 7 V corresponds to a field strength over the gate oxide of
  350 kV/cm. Although breakdown of the oxide is expected only at input
  voltages above 10 V, electrons tunnel increasingly into the gate oxide
  at field strengths greater than 350 kV/cm, influencing characteristics
  of the transistors and causing failure.</p>
</blockquote>

<p>Source : <a href="https://electronics.stackexchange.com/questions/421556/what-limits-the-maximum-voltage-of-this-optocoupler-6n137">this question on electronics stackexchange.</a></p>
