LABORATORY USE OF A MICROCOMPUTER (Last Revision: August 27, 2000) ABSTRACT: A program, LabVIEW, is used to enable a microcomputer to acquire and manipulate data and to generate output voltages. TEXT REFERENCE: Sime, Rodney J., "Physical Chemistry - Methods, Techniques and Experiments," Saunders Pub., Philadelphia, PA, 1990. Chapt. 9-10. OTHER REFERENCES: LabVIEW Manuals. GENERAL DESCRIPTION AND THEORY: A microprocessor is a small computer element. It has all the elements of a computer except for the memory and input/output devices and it is contained in a very small package. In the case of the 8088 or 80286, which are the microprocessors for the IBM PC and AT, respectively, the package is about one by two inches. (The 80386, 80486 and pentium are a little larger.) In spite of its small size the microprocessor has most of the computing power associated with larger computers, however, because of its small size and low cost it is now being used in the construction of many modern chemical instruments. Instruments such as mass spectrometers, nmr spectrometers, gas chromatographs, etc., often come equipped with a microprocessor and memory to accomplish many of the tasks that used to require a large amount of sophisticated electronic hardware. The advantage of the microprocessor is that the electronic package is always the same for each instrument. It is the software or programs for the microprocessor which are different in each of the instruments. Another advantage of the software is that it can be changed easily (much more easily than hardware) and so improvements in instrumentation can often be accommodated by simply changing the program for the microprocessor. In addition, the microprocessor allows an instrument to be "customized" for a particular analysis or purpose at relatively low cost, in fact it can often be accomplished by one of the laboratory workers. Since these microprocessors are becoming so prevalent in chemical instrumentation, it is important that chemistry students have the opportunity to get familiar with the operation and use of the microprocessor in the laboratory. A processor or microprocessor is only part of a computer. It is the part that contains the logic and arithmetic functions together with some special memory locations called registers. The instructions that enable the computer to compute, read information, operate switches, etc., are executed in the processor section of the computer. However, a computer is composed of several additional components besides the processor. A sketch of a typical computer arrangement is shown in figure one below:
Memory RAM Input/ Memory Keyboard Output Processor ROM Control Timer/Clock Printer Aux. Memory Disk Figure 1. Computer System As can be seen from the figure, memory and input/output functions are usually part of any computer system. The memory serves the function of holding the list of instructions for the processor and also stores data for the programs. There are several different kinds of memory that are used by a computer. The memory immediately accessible to the processor for reading from and writing to is called random access memory (RAM). This form of memory is necessary when the memory contents are to be changed during the execution of a program. Another form of memory is read only memory (ROM) from which a processor may receive instructions or data but on which it cannot write anything. ROM memory is often used in microcomputer systems to contain programs which will used over and over again. It is most useful to put the monitor or operating control program in ROM so that it is always available and cannot be destroyed by another program writing over it. A mini-operating system for the IBM is contained in ROM but the main operating systems are read in from a magnetic disk. A magnetic disk is an auxiliary memory system. It is usually much larger than the RAM memory but not always. Auxiliary memory in the form of a floppy disk is quite portable and can be used to store and transport programs and data from one computer system to another. Probably one of the most important parts of the computer system is the input/output control system since it is by means of this section that the processor communicates with the outside world either through devices such as typewriter or keyboard-display system or to the instrument control world of electrical signals. The microcomputer system will do the former as it is delivered from the company. It is able to do the latter by means of ICS AI08 I/O or National Instruments AT-MIO-16 boards for data acquisition and control applications. Among other features, these boards contain timers, analog to digital and digital to analog converters. The timer is used to enable the computer system to operate according to definite time signals. For example, a timer will allow the computer to take readings every ten seconds if the program provides for this. The microcomputer has an interrupt system which allows the timer to interrupt a program in execution so that a particular operation
can be carried out by an "interrupt program." When the interrupt program has complete its action, the original program will continue from where it was interrupted. Most computer systems are rated in terms of the number of bytes in the memory. A byte consists of eight bits. A bit is the fundamental unit of the computer memory and is either a 0 or a 1. Since electronic instruments are stupid they can only tell if something is on (1) or off(0). This means that computers can only use a binary number system. Therefore all information that goes into a computer has be changed into binary representation. Decimal numbers are changed into their corresponding binary form and alphabetic characters are represented by a binary code called ASCII. It may seem obvious to you that both data and instructions are represented by a set of binary digits, and that what is a computer instruction in one case will be just a piece of data in another case. The difference to the processor lies in when they are delivered to the processor. If the set of binary digits comes to the processor during the instruction fetch cycle it is an instruction, while if it comes to the processor during a data fetch cycle it is a piece of data. The processor will execute a series of instructions in order, one after another unless told to deviate from this order by either a branch or jump instruction. A special register called the program counter stores the location of the next instruction to be called. There are other registers to be found in a processor. One which is always found is the accumulator or main register. It is often called the A register and here is where the arithmetic operations are carried out. Another type of register which is useful is the index register. There are many uses of an index register. One use to address a series of data by constructing the address from the contents of the index register and from the address part of the computer instruction code. The index register may also be used to count down the number of times that a certain operation or series of instructions are carried out. When the register is counted down to zero or a negative value, a branch instruction may be used to go to another part of the program. We have mentioned that the computer uses only binary numbers. This presents a problem to the programmer who normally does not think in binary. There is no easy conversion from binary to decimal. However, there is an easy conversion from binary to octal or hexadecimal. Most systems now use the hexadecimal notation for binary numbers. The hexadecimal system starts with the single digit, 0, and goes the single digit, F, (0 to 15 in decimal) before starting the two digit sequence at 10 (16 in decimal). The conversion is given below for numbers from 1 to 16: Hexadecimal 0 1 2 3 4 5 6 7 8 9 A B C D E F 10 Decimal 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 Tables for the conversions are available from a number of sources. EQUIPMENT:
The microcomputer system used is equipped with the following features: 1) 32 Megabytes of RAM memory. 2) ROM memory containing a minimum operating system. 3) Input/output in the form of a keyboard/display. 4) Auxiliary storage in the form of a hard disk and floppy disk drives. 5) A data acquisition board equipped with analog to digital and digital to analog converters and several timers. 6) A pentium computer chip which includes a floating point processing unit. PROGRAMS: The computer program used is the LabVIEW program developed by the National Instrument Company. LABORATORY PROCEDURE: Before using the computer system look to see that the computer board is connected to the flat cable coming out of the back of the computer. The LabVIEW program may be started by double clicking the LabVIEW icon on the display. Part I: The Use of the LabVIEW Program
The linked image cannot be displayed. The file may have been moved, renamed, or deleted. Verify that the link points to the correct file and location. The opening panel of LabVIEW is shown above and this is the starting point for each section of the tutorials covered in this experiment. For this introductory experiment you will follow the procedures outlined in the LabVIEW Quick Start Guide. In this guide you should carry out the procedures described in Chapters 1-3 and Chapters 5-6. Omit Chapter 4 since there are no instruments attached to the computer. When you get to chapter 3, read the material in part II of this manual. Part II: Data Acquisition and Voltage Output One of the important functions of a computer system for data acquisition is the ability to sample and store data at variety of rates, from very small intervals of time to long periods of time. The data recorded can be saved on a floppy disk for later use or further manipulation. The data acquisition system used in the IBM PC AT can sample eight different analog signals. The signal can be either AC or DC but it must be in the range of -5 to +5 volts. The A to D converter used in the data acquisition system is located on a board in the computer housing. It is a 12 bit analog to digital converter. The A to D converter can function either as a 12 bit converter or an 8 bit converter depending on the signal received from the computer program. The A to D converter starts the conversion of the voltage value when it receives a signal from the computer. When the conversion is complete (the analog values is converted to an 8 or 12 bit digital value) the converter sends a signal to the computer. Then the computer can read the digital value from the converter and store it in the computer memory or write the value to a floppy disk. In the case of the data acquisition used in this experiment the A to D converter is multiplexed to eight different lines. This means that the converter can be switched to each of eight different lines so that eight different analog signals can be monitored simultaneously. In this part of the experiment, the voltages of several batteries will be determined. The connection block is illustrated below in Figure 1. Connect one battery to
The linked image cannot be displayed. The file may have been moved, renamed, or deleted. Verify that the link points to the correct file and location. channel 0 - ACH0 and ACH8 the polarity will not matter since the value will be read as either negative or positive. Connect the second battery to channel 1 - ACH1 and ACH9 When the connections have been made continue with Chapter 3. If you have time at the end of the period you can try to set up and use the output channels listed below: channel 0 - AO Gnd and DAC0 or to channel 1 - AO Gnd and DAC1 Figure 1 It is important to gain familiarity with the system since it will be used in several other experiments.