send command via rs232

You can't talk to two devices using the same set of pins.

You can send data to numerous devices connected in parallel to the serial pins. Receiving data can have issues.

How can I Take a wire from the RS232 TX output and connect it to the RS232 TX input?

Why are you having trouble with concept?
You use wire, you solder it from pin 2 to pin 3 on the 9 way D-type connector on your TTL to RS232 converter.
You connect the other pins of this converter to the pins you had before. You then can test if you are talking to this converter or not.

I only want to comunnicate with external device and control it. I don't care what method to use. I'am looking in some books, Google and forums and I cannot comunnicate yet with the device.

I only want to comunnicate with external device and control it.

Yes and you have implemented something that if you have actually wired it up like you say should work. What we are trying to do is to help you test it to see where you have gone wrong.

One way to do this is a loop back test but you have trouble in doing what was asked of you.
Your DFROBOT RS232-TTL Converter should be connected to some pins other than 0 & 1. This should be being driven by new software serial. You say it is not working so a loop back test would rule out wiring problems and software problems.
The DFROBOT RS232-TTL Converter has a 9 pin D-type socket on the end I am asking you to connect pin 2 & pin 3, You might need to have a plug to put the wires on or you can do it on the PCB side.

When you have confirmed that this setup is working you can then go on to investigate other things that could go wrong like a miss match in the baud rate or data format between your DFROBOT and the thing you want to connect it to.

Thank you very much. I'll tried and I'll tell you.

I'm here again.
I have connected pin 2 to 3 to make a loopback. And when I test it I don't get response.
I have changed cables and not work.
This is the code I'm trying to test:

#include <SoftwareSerial.h>
SoftwareSerial MyDevice(7,8); // I don't know which is TX and RX...
void setup() {
// begin serial:
Serial.begin(9600);
MyDevice.begin(9600);
}
void loop() {
if (Serial.available() > 0) {

switch (Serial.read()) {
case 'a':
MyDevice.print("1!");
Serial.println("sent 1"); //to test in S.Monitor
break;
case 'b':
MyDevice.print("5!");
Serial.println("sent 5"); //to test in S.Monitor
break;
default:
break;
}
}
}

thank you very much.

SoftwareSerial MyDevice(7,8);  //  I don't know which is TX and RX...

Then, why the hell don't you try to find that out? Look at the source code!

I have connected pin 2 to 3 to make a loopback. And when I test it I don't get response.

Hardly surprising, since your code does not use pins 2 and 3.

I mistake my response.
I solder TX with RX that is the pinout of RS232 connector. This is my scheme:

DFROBOT TX-> Arduino pin 7
DFROBOT RX-> Arduino pin 8
DFROBOT VCC -> Arduino 5V
DFROBOT GNG -> Arduino Gnd

solder in DFROBOT DB9 pinout TX and RX (2 and 3)

Dear all:
I have changed to an Arduino Mega.
I have tried the loopback and is OK. The character that I send, I receive it.
I have changed my code. because Arduino Mega. This is my new code:

void setup()
{
Serial.begin(9600);
Serial3.begin(9600);
}

void loop()
{
if (Serial3.available()) {
Serial.print((char)Serial3.read());
}
if (Serial.available()) {
Serial3.println((char)Serial.read());

}
}
I have connected my device Extron to DFROBOT RS232 through to Serial3 of Arduino,
Then, I sent this command: ! (admiration), and the device must respond the input selected.
But I have no response from the device.
When I send the command: !, I see that Rx of DFROBOT light, but doesn't light the TX light. I mean, I don't get response from the Extron device.
Can you help me please?
Many thanks

Is the bit rate correct?
Is the character format (8,n,1) OK for the receiver?

Baud rate is 9600 for Arduino and Extron device.
Other parameters, Do I have to configure it writing code?
How can I do it?

Parameters for Extron device are:
Parity bit : none
Data bit: 8
Stop bit: 1
Handshake: none.
Thank you very much

Parameters for Extron device are:...........

That is the same as the arduino so there is nothing extra to do.

When I send the command: !, I see that Rx of DFROBOT light, but doesn't light the TX light. I mean, I don't get response from the Extron device.

This would suggest you are not sending the correct command. Your code is wrong try this:-

void setup() 
{
  Serial.begin(9600);
  Serial3.begin(9600); 
 }

void loop()                     
{
  if (Serial3.available()) {
      Serial.write((char)Serial3.read());
  }
  if (Serial.available()) {
      Serial3.write((char)Serial.read());
   
  }
}

Extron device

Any datasheet ? I would like to see it. Maybe you need to read the datasheet of the so call "Extron device"

Ok... You want to send ---> ! Is it ASCII code ? or a binary code ? If I have a device to received a data, I need to know :

Baud rate
8 bits
Parity or Not
Stop bits
Start bits

So your device is : 9600 baud - 8 bits - no parity - 1 stop bit OK
Code sequence into the device ? Answer in the datasheet <-- Maybe ASCII into the device ? Answer in the datasheet <-- Maybe

You see what I mean ....

Thank you to all.
Here you can download the manual:
http://media.extron.com/download/files/userman/68-791-01_IN1508_UG_H.pdf
At the end of the manual are the codes to control it and some samples.

So the first thing I would do is to see if it is talking to you, the book says:-

When a power-up occurs, the switcher responds by sending the following message to the host:
Copyright 2005, Extron Electronics, IN1508, Version x.xx] Version x.xx is the firmware version number.

So write some code to see this message by transferring it from the software serial input to the hardware serial output and displaying it on your monitor.

Thank you very much.
I have tried with the new code and When I connect Extron I don't get anything.
Must be a problem with the cable? Do you know the pinout?
I don't know how to solve this.

I don't know how to solve this.

You solve it by using the right tools. Use an oscilloscope to see that your device is sending data to you. You use it also to see that you have got the baud rate right and that the signal is the right way up. It should go to a logic one after it has finished sending stuff.
If all that checks out you look again at your software serial stuff and see it is recieving.

How I said in a above message, I connected the device right to a PC and hyperterminal and I get the the message from Extron device.
I have no an oscilloscope is to expensive for me. Is there any other to posibility to test it? or oscilloscope software?

Do you know other cheap serial devices to make trying with Arduino and serial?
Many thanks

Tell us what equipment you do have, and what equipment you know how to use.