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Figure 2-13. Changing the input label to Row Now we will test the My Block by writing a little program. Start a new program and go to the Custom palette (See Figure 2-14) to get a copy of the Display Number My Block.

Figure 2-14. Getting a Display Number block from the Custom palette Complete the rest of the program shown in Figure 2-15. The program first fills the screen with the Smile01 image and then keeps displaying the value of a random number that changes every second on Row 3.

Figure 2-15. Display Number test program When you run the test program, the NXT screen should look something like Figure 2-16. Notice that the program erases the entire row every time without disturbing the rest of the display. No matter how large the value might have been, no part of it will remain from one time to the next.

Figure 2-16. Screen for Display Number test We made Display Number simple to illustrate how to create My Blocks. A more useful block would also allow you to write an identifying label before the number. Also you might want to erase the entire screen at some point. The content of a My Block called Display Value has these added features and is shown in Figure 2-17. The details of how to create this block will be left as an exercise.

Figure 2-17. Display Value My Block Figure 2-18 shows the Display Value block in a test program. The configuration window for the block shows how the Label can be entered, and checking the box in the Clear field will cause the NXT to clear the entire screen before writing the new value.

The screen output of this program is shown in Figure 2-19.

Figure 2-19. Display Value output One last thing you need to understand about My Blocks is where they are kept on your computer. They aren t stored in the same directory as your NXT-G programs ( /LEGO Creations/MINDSTORMS Projects/Profiles/Default). You find them in a further subdirectory (/Blocks/My Blocks). If someone gives you a NXT-G program that contains My Blocks, they must be stored in that directory for everything to work. With first generation NXT-G software, this has to be done by hand. Second-generation NXT-G included a Create Pack and Go tool that made a single distribution (.rbtx) file that contains all the files needed for the program to run. That includes any My Blocks, image files, and sound files. The software also knows how to unpack these distributions and put all the files in the right place.

My Blocks are great for creating subroutines, but at some point you will need to add something so complex that it requires a whole new NXT-G block. Sensors, for example, require low-level NXT control that you just can t get in a My Block. Fortunately, you can easily import new blocks to NXT-G using the Block Import Wizard. We will illustrate the process by importing a couple of blocks essential to building projects in this book. The original MINDSTORMS RCX had a couple of unique sensor types (Temperature and Rotation, for example) that are also supported by the NXT. The NXT software doesn t include these sensors by default because it is not very likely that someone buying a new NXT kit would own them. However, they are very useful for building homebrew sensors. Fortunately, LEGO provides a library of these Legacy Blocks on their MINDSTORMS Support page shown in Figure 2-20.

Figure 2-20. Legacy Block Support download You need to download the library and unzip its contents to a folder on your computer. It doesn t really matter where the files are unzipped as long as you know where they are. The contents of the library can be seen in Figure 2-21. It is the Old Light Sensor and Temp Sensor that we will import. They are known as the Light* and Temperature* Sensors, where the * signifies legacy.