Servo - April 2009

signals up to 5 MHz and can also be configured as an external trigger.

Also on the digital side is a +2.5 VDC at 1 mA terminal, a + 5 VDC at 200 mA, and a ground terminal. The +2.5 VDC 1 mA terminal is for calibrating the A-to-D channels. On the analog side of the devices are 16 terminals, as well. There are four analog input

FIGURE 3

ports, AI0 through AI3, each with a positive, negative, and ground terminal. When these ports are configured as differential inputs, they have 12 or 14 bit resolution (depending on the model). When they are configured as single-ended inputs, a total of eight individual analog voltages can be converted at one bit less in resolution. There are also two analog output channels,

AO0 and AO1; these have a 12-bit resolution and can output an analog voltage from 0V up to 5V at a maximum of 150 Hz. (The analog features of the unit will not be covered until Part 4 of this series.)

Before you connect the unit to your PC, you must first install the NI-DAQmx drivers from the two CDs included with the unit. After the drivers are installed, a pop-up menu should appear when you plug the unit into a USB port on your computer (see Figure 1). From this menu, you can test the USB device, as well as several other choices. If you are having problems with your unit later on, you may try to resolve these problems through this menu; for now, we will close this pop-up menu and begin building a VI for the USB hardware.

FIGURE 4

front panel, place an array by selecting: Modern>Array, Matrix & Cluster>Array from the controls palette (see Figure 2). Next, place a toggle switch in the box to the right of the array by selecting: Express>Buttons and Switches>Toggle Switch (see Figure 3). Now, stretch the array with the toggle switch in it down until four toggle switches are visible, as shown in Figure 4.

Using the pointer finger cursor, select this from the tools palette; click on the bottom toggle switch in the array. If you do this correctly, the toggle switches will change from a grayed-out color to a more solid color. Add a stop button to the front panel at this time by selecting: Express>Buttons and Switches>Stop. Now, you will need to work in the block diagram window to complete the VI.

Write Some Code

Build A Framework

Let’s make a simple VI that can be used to output four digital signals to control four LEDs connected to Port0. Begin by opening LabVIEW and selecting Blank VI. If you are having trouble starting a new VI, refer to Part 1. On the

In the block diagram window, click and drag out a while loop by selecting: Programming>Structures>While loop from the functions palette. Be sure to place both the stop button and the array icons inside the while loop. Wire the stop button to the stop icon in the lower right corner of the while loop. You will now need to use the DAQ Assistant to set up the USB unit for use by selecting: Express> Output>DAQ Assistant.