Using LabVIEW to Send Commands via RS232 to ADR Interfaces
LM335 Temperature Measurement
The following application demonstrates how LabVIEW can be used with ADR interfaces, or any ASCII based serial data acquisition and control interface. The application is a simple temperature measurement and plot using an ADR112 and an LM335 solid-state temperature sensor. It is assumed the user has a basic knowledge of LabVIEW, however, no prior experience with LabVIEW is required as long as the user manual is available to help with the generic tasks. Figure 1 shows our final operating panel for the application in operation. The panel allows adjustment of the sample rate via a rotary knob, and displays temperature vs. time in a graph format. A digital reading of present temperature is also provided. The hardware consists of an ADR112 connected to com2, interfaced to an LM335 temperature sensor connected to AN0.
Figure 1: Final Operating Panel
Start labview, and an blank panel will appear called untitled.VI. Save the VI as a file called START . We will now place various controls and indicators on the blank panel that will become the applications operating controls and indicators. Using the CONTROLS menu, place on the following on the panel;
A waveform chart from CONTROLS/ARRAY AND GRAPH/WAVEFORM CHART
A numeric knob from CONTROLS/NUMERIC/KNOB
A vertical switch from CONTROLS/BOOLEAN/VERTICAL SWITCH
A numeric indicator from CONTROLS/NUMERIC/DIGITAL INDICATOR.
Arrange the items as shown in Figure 2 using the pointer to add labels by right clicking on each item and typing it in. Change the scale on the waveform chart vertical axis to 0 to 4095 using the hand icon.
Figure 2. Initial Panel Layout
Use the hot key, CTRL-F to show the control diagram. Arrange the items to the positions shown in Figure 3.
Figure 3. Initial Control Diagram
Before starting to build the program we must understand what we want the program to do for us. We should have a basic knowledge the application hardware and desired operating parameters. This application will require the software to send an RD0 command ( followed by a carriage return ) to the ADR112, causing it to respond with a four digit integer number from 0000 to 4095 representing the temperature data in 12-bit format. The data must then be scaled and displayed on the front panel in degrees Celsius in a digital display and in a waveform chart showing temperature history. The basic program will repeat this operation at a rate determined by our adjustable sample rate knob. For this we will start by selecting a WHILE LOOP using FUNCTIONS/STUCTSANDCONSTANTS/WHILELOOP. position the loop as shown in Figure 4.
Figure 4. Adding the While Loop
We will now add the components required to have the loop execute at an adjustable interval by adding a few more components. Add the following three components;
A wait function timer from FUNCTIONS/TIMEANDDIALOG/WAIT
A multiply function from FUNCTIONS/ARITHMATIC/MULTIPLY
A numeric constant from FUNCTIONS/STUCTSANDCONSTANTS/NUMERICCONSTANT
Position the items as shown in Figure 5 and wire them as shown using the wiring tool. The timer will now delay repeated execution of the loop by 1000 times the setting of the sample rate knob on the front panel.( in ms). If the knob is set at one, the sample rate will be 1 second ( 1000 X 1=1000ms ), if the knob is set at 0.5 the sample rate will be 0.5 seconds ( 1000 X 0.5 = 500ms), etc. The on/off slide switch is wired as shown to enable or disable the loop from running. It will serve as a simple enable/stop function on the front panel.
Figure 5. The Delay
The software in the loop must now be set up to send and receive ASCII data to the ADR112 via Com2. This is accomplished by using a sequencer and a number of serial port functions provided by LabVIEW. The sequence, as the name implies, allows the execution of code in a specific sequence similar to that of a PLC. The sequencer is a series of frames that code is placed into, that determines the actual execution sequence of the code. Add a sequencer to the control diagram using FUNCTIONS/STUCTSANDCONSTANTS/SEQUENCER. position the sequencer as shown in figure 6.
Figure 6. The Sequencer
5. Writing Data to the Serial Port
The first frame will contain the actual serial write function. Also added in this step is the serial port initialize function. Place the serial write function inside frame 0 by selecting it from FUNCTION/SERIAL/SERIALWRITE. Place the serial initialize function outside the while loop as shown in Figure 7 using FUNCTION/SERIAL/SERIALINITIALIZE. Use a numeric constant to set initialize the port to Com2 ( port 1) and set the port for the write operation by wiring as shown. A command string of RD0 must be sent in the write operation and it must be followed by a carriage return for the ADR112 to respond. This is done by selecting a string constant using FUNCTIONS/STUCTSANDCONSTANTS/STRINGCONSTANT. Enter RD0 followed by a carriage return as the string constant and wire as shown in Figure 7.
IMPORTANT ; The default termination character for an ASCII string in LabVIEW is a linefeed character which will not cause the ADR112 to return data. The linefeed must be changed manually to a carriage return. This is done by right clicking on the data and selecting \ CODES DISPLAY . When this is done, RD0\n will be the displayed string data. Use the text icon to change the \n to \r ( carriage return ). If successful, the string will as be shown as in Figure 7 with the \ CODES DISPLAY function enabled.
Figure 7. Serial Write Function
A second step is to be added to the sequencer where the serial read function will be placed. this is done by right clicking on the sequencer frame indicator and selecting ADD FRAME AFTER . The sequencer will appear as shown in Figure 8.
Figure 8. Adding a Step
The data returned from the ADR112 will be read using a serial read with timeout vi. This vi is included with LabVIEW in the samples directory. Place it in the frame along with a Format and Strip function from FUNCTION/STRING/FORMATANDSTRIP. Wire as shown in Figure 9 including, a numeric constant of 5 for number of bytes to receive ( 4 plus CR ), and a string constant of %d to identify the incoming data as integer format. Wire the output of the Format and Strip function to both the Present Temperature display and the Temperature Plot
Figure 9. Reading and Formatting Data
This would be a good time to test the program. Connect a potentiometer to AN0 on the ADR112 and run the VI from the Panel Window. Vary the pot and the display should look something like Figure 10. Vary the sample rate to become familiar with its operation. Do not forget to click on the ON/OFF switch to enable operation.
Figure 10. First Run Through.
Finalizing the application involves converting the ASCII data to Celsius. This is done with a one arithmetic subtraction and one division function added to the second step in the sequencer as shown in Figure 11. These functions are necessary to convert the 12-bit data to Celsius and are explained in the application note Using the LM335 Temperature Sensor
Figure 11. The Final Code
The display can then be customized to give the VI a more pleasing appearance. See your user manual for the various customizing options. The scale of the graph can also set using the hand icon to whatever range is to be measured in the application. Our final version was saved as ADR112.VI as appears as in Figure 12.
Figure 12. Final Panel
This was a simple application using only one analog input port of the ADR112 interface. Additional graphs or use of the digital ports for alarms can be facilitated simply by adding steps to the sequencer with additional write and read operations. All of the functions found on ADR interfaces including, counters, PWM, digital I/O, analog outputs, interuppts and stepper indexers can be utilized with LabVIEW. Programs can be as complex or as simple as the application requires. The people at National Instruments have done an outstanding job developing LabVIEW and it is easy to see why it has become so popular.
Note. The VI used in this example can be downloaded in zip format using the following link;
Ordering LabVIEW ( Students and Educators)
LabVIEW student edition is available for around $100.00 direct from the publisher. This graphical programming software is designed for scientists and engineers who automate laboratories or take industrial measurements. When coupled with the low-cost ADR interfaces. an affordable data acquisition and control hardware/software solution is achieved.
LabVIEW Student Edition, Author, Lisa K. Wells, National Instruments
ORDERING USA 1-800-643-5506 CANADA 1-800-567-3800 ( VISA,MC,AMEX)
ISBN 0-13-210709-0 Macintosh Version
ISBN 0-13-210691-4 Windows Version
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Contact National Instruments for pricing and delivery of full license versions.
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