.NET/Java PDF, Tiff, Barcode SDK Library

Figure 13-22. ULN2003A Like the PCF8574, the ULN2003A only sinks current. Because the reset condition of a PCF8574 output is 1, the reset condition of any loads connected through a ULN2003A will be on. The output current capability of each amplifier actually depends on how many of the amplifiers are on at any given time. You can usually count on at least 200 mA per amplifier, and with 12V that works out to a minimum coil resistance of about 60 . Going much below this runs the risk of overheating the part. A stepper motor circuit combining the ULN2003A and the PCF8574 is shown in Figure 13-23. Pullup resistors R3 R6 are 3.3k are necessary because the ones inside the PCF8574 don t quite provide enough current for the inputs of the ULN2003A. Connecting the correct color wire from the stepper to the ULN2003A is critical. The colors shown are only for the 35DBM10B2U-L (Digi Key #403-1034-ND) and will vary with model and style of stepper motor. It is a good idea to include a power switch so you can turn off the 12V when the motor is not in use.

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Venkman offers a plethora of information that is divided into eight windows. The default layout consists of a large pane showing the selected source code. Smaller windows are arranged vertically on the left side of the window. Venkman s command-line interface resides on the bottom of the window under the Source Code pane. You can drag each pane with the mouse and dock them at other locations within the main window. You can also add each pane as a separate tab to an existing pane. For example, to make the Loaded Scripts tab be a tab within the Local Variables pane, simply drag and drop the Loaded Scripts tab to the Local Variables tab. You can also undock the small window panes from the main window by clicking the docking button located on the left side of the pane s title bar, as shown in Figure 7-10. Docking the pane back to the main window is as easy as clicking the docking button again.

s you saw in 3, you can use F# for pure functional programming. However, some issues, most notably I/O, are almost impossible to address without some kind of state change. F# does not require that you program in a stateless fashion. It allows you to use mutable identifiers whose values can change over time. F# also has other constructs that support imperative programming. You ve already seen some in 3. Any example that wrote to the console included a few lines of imperative code alongside functional code. In this chapter, you ll explore these constructs, and many others, in much more detail. First, you ll look at F# s unit type, a special type that means no value, which enables some aspects of imperative programming. Next, you ll look at some of the ways F# can handle mutable state, that is, types whose values can change over time. These are the ref type, mutable record types, and arrays. Finally, you ll look at using .NET libraries. The topics will include calling static methods, creating objects and working with their members, using special members such as indexers and events, and using the F# |> operator, which is handy when dealing with .NET libraries.

Pin 4 Green R1 R2 U1 16 Pin 5 Yellow Pin 6 Blue 14 15 VDD SCL SDA 4 P0 5 P1 6 P2 7 P3 R3 R4 R5 R6 U2 1 IN1 2 IN2 3 IN3 4 IN4 9 COM OUT1 OUT2 OUT3 OUT4 GND 8 16 15 14 13 Yellow Black Orange Brown 12V

Any function that does not accept or return values is of type unit, which is similar to the type void in C# and System.Void in the CLR. To a functional programmer, a function that doesn t accept or return a value might not seem interesting, since if it doesn t accept or return a value, it does nothing. In the imperative paradigm, you know that side effects exist, so even if a function accepts or returns nothing, you know it can still have its uses. The unit type is represented as a literal value, a pair of parentheses (()). This means that whenever you want a function that doesn t take or return a value, you just put () in the code: #light let main() = () In this example, main is a function because you placed parentheses after the identifier, where its parameters would go. If you didn t this, it would mean main is not a function and instead just a value that is not a function. As you know, all functions are values, but here the difference between a function and a nonfunction value is important. If main were a nonfunction value, the expressions within it would be evaluated only once. Since it is a function, the expressions will be evaluated each time it is called.