user85885: I want to connect an Arduino Uno to an Arduino Nano, so that it can send signals. The problem is that the Arduino Uno has a motor shield connected to it, and it takes up all pins of the Arduino.
This is a picture of the whole motor shield contraption if it helps:

Does anybody know any solution for this?

user61851: Pins 0 and 1 are serial pins which are also connected to usb, and you use them when you are programming your board or when you send anything to serial. It is unlikely that any shield uses it unless it has to and it although has a benefit of being able to listen to what the Arduino sends.
I think hitching a ride on pins 1 and 0 (rx/tx) would be the easiest. SO you can print or write to them the same way as you respond to yourself.
Just a warning that when you are sending data between two boards and Print numbers they come as symbols of the numbers, not the numbers themselves So Serial.print(7) would arrive to the next arduino as number 55 or symbol '7'. If you are sending numbers between two arduinos you can use write Serial.write(7); which does not change data, but your serial monitor on a computer wont read it right, or you can send them as symbols, but then the arduino listening may have to convert number symbol to the number. (sooner or later you will have to create yourself a simple function to convert symbols to numbers in your project...

85885: <p>I want to connect an Arduino Uno to an Arduino Nano, so that it can send signals. The problem is that the Arduino Uno has a motor shield connected to it, and it takes up all pins of the Arduino.</p>
<p>This is a picture of the whole motor shield contraption if it helps:</p>
<p><a href="https://i.stack.imgur.com/UAv4k.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/UAv4k.png" alt="enter image description here" /></a></p>
<p>Does anybody know any solution for this?</p>
 61851: <p>Pins 0 and 1 are serial pins which are also connected to usb, and you use them when you are programming your board or when you send anything to serial. It is unlikely that any shield uses it unless it has to and it although has a benefit of being able to listen to what the Arduino sends.
I think hitching a ride on pins 1 and 0 (rx/tx) would be the easiest. SO you can print or write to them the same way as you respond to yourself.</p>
<p>Just a warning that when you are sending data between two boards and Print numbers they come as symbols of the numbers, not the numbers themselves So <code>Serial.print(7)</code> would arrive to the next arduino as number 55 or symbol '7'. If you are sending numbers between two arduinos you can use write <code>Serial.write(7);</code> which does not change data, but your serial monitor on a computer wont read it right, or you can send them as symbols, but then the arduino listening may have to convert number symbol to the number. (sooner or later you will have to create yourself a simple function to convert symbols to numbers in your project...</p>
