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Instrumentation F.E. Woodard W.S. Woodward
”.N. Reilley inan Labmatorb of Chemistry y of North Carolina el, N.C. 27514
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During the past decade, a series of rapid advances in microprocessor technology has made it possible for many scientists to reap the benefits provided by inexpensive, distributed intelligence in instrument control and in the acquisition, reduction, and display of data. Much of this progress has been exhibited in a new breed of commercial instruments that incorporates embedded microcomputer systems. However, in some laboratories separate microcomputers are also used for interfacing existing or one-of-a-kind instruments as well as for accomplishing tasks traditionally assigned to larger computers (e.g., number crunching or word processing). At present, availahle microcomputer systems range from inexpensive “personal” computers to a group of microcomputers specifically designed for use as laboratory data acquisition systems. Selecting a cost-effective computer from this colleotion is a perplexing problem, and its solution cannot be based solely upon initial prices. This is largely because the total cost of implementing a particular application (whether measured in dollars or hours) can be so large that it com0003-2700181/A351-1251$01 0010
@ 1981 American Chemical Society
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nletelv overshadows the initial cost of the computer system. Often a large measure of this implementation cost arises from the necessity for extensive development of application-specific hardware and software. This dilemma is usually the consequence of a poor matching of tool and task. Thus, a thorough assessment of both the tasks expected and the capabilities of various computers should precede the selection of a laboratory system. This article focuses on attributes of microcomputer systems that affect their usefulness in a laboratory environment. In addition to presenting general concepts, some comments are made regarding the implementation of these concepts, using a microprocessor-based data acquisition system developed at the University of North Carolina (UNC) as an example, primarily because of our first-hand knowledge of and experience with its properties in a variety of laboratory applications ( 1 4 ) .
Laboratory Computer Hardware Modular Computer Design. If a microcomputer is to be a truly commonplace laboratory tool, it must be
both inexpensive and versatile. This can be achieved in part by dividing the computer into hardware modules that facilitate the assembly of application-specific configurations. Evaluating a modular computer is not always straightforward. Its effectiveness cannot be determined solely from the performance of individual modules. Equally important is how readily the modules are merged to form a single instrument. Factors which affect this include: the way in which functions have been distributed among modules, the design of the system bus that interconnects the modules, and the availability of the software required to orchestrate the activity of various ensembles of those modules. Thus, careful selection of hardware and almost certainly some software development are requisite if one is to mold an assemblage of modules from several vendors into a single, well-integrated computer system. (A reasonable starting point is to investigate the architecture of the system bus and the functions it will support. A comparison of several popular microbuses [i.e., LSI-11,MULTIBUS, S100, STD, and ZBI] appears in Reference
ANALYTICAL CHEMISTRY, VOL_.53, NO. 11, SEPTEMBER 1981
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nan !!action
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Irial-Typically RS232C
irallel Digital
Figure 1. Human interaction provisions and fundamental computer elements (i.e.. CPU.memory, system DUS. and power supplies) in the UNC microcomputer. The asynchronous serial interlace and the ASCII keyboard interface are on a single printed circuit board
5. Laboratory computer systems using SlOO and MULTIBUS backplanes are described in References 6 and 7.) We have implemented this modularity concept at UNC using 4.5- X 10-in printed circuit cards. In a single card hay up to 22 modules can he interconnected by insertion into a 44-conductor bus. The bus consists of eight bidirectional data lines, 16 address lines, eight control lines, a 2-MHz clock, regulated +5- and f15-V power supplies, and grounds. As shown in Figures 1,2,3, and 4, the present collection of modules consists of: a central processing and system control unit (CPU), a 64K-byte memory array, peripheral input/output (110)controllers, instrument I10 devices, and modules for connecting the microcomputer to computer networks. One of the problems concomitant with purchasing or designing a microcomput,er is that after several years it no longer represents the state of the art. A modular computer design affords some relief to this problem. When a module becomes obsolete, often it can be replaced with a newer module, leaving the rest of the computer intact. This means the useful lifetime of the computer hardware can be extended considerably. One example is the memory module used in the UNC microcomputer. The 1252A
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computer originally used 4K dynamic memory chips and took four memory modules to get 64K bytes of memory. After 16K memory chips were introduced, a new module was designed so that 64K bytes of memory now reside on a single printed circuit card. The CPU module has been redesigned several times. It was originally based on an 8008 microprocessor, hut later was replaced with an 8080. Currently we are through the prototyping stage of a Z-80-based CPU. This module provides a number of enhancements, including a two-fold increase in the program execution rate and an increase in the video graphics resolution (256 X 512 using video refresh memory [16K X 91 located on the CPU module). Because the 8080 instruction set is a subset of the Z-80 instruction set, existing software is upwardly compatible. (Note that, because the new CPU is a very recent addition, all references to system performance elsewhere in the text pertain to the 8080 CPU). Computer-Instrument Interaetions. The ability of a general-purpose laboratory computer system to control and acquire data from a multitude of different laboratory instruments is certainly one of the most significant characteristics of the system, probably even more important than its computational speed. A reason for this is that
ANALYTICAL CHEMISTRY, VOL. 53, NO. 11, SEPTEMBER 1981
since computational tasks are usually associated with postexperimental data interpretation, they need be completed only in amounts of time commensurate with the operator’s patience. On the other hand, requirements regarding the data acquisition rate, the data resolution, or the accuracy of computer timing between events are often dictated by the system under study. Hence, inadequacies in this regard can render a data acquisition system useless to a study. During the past few years, the UNC microcomputer has been utilized in a wide variety of applications. These range from gas chromatography, where a relatively slow data acquisition rate was appropriate, to electrochemical relaxation rate measurements, which demanded high data throughput. In these applications, most of the computer-instrument interactions were effected by using modules selected from a “library” of standard I10 modules. The ensuing discussion of these modules should provide some insight into the hardware necessary for a truly versatile data acquisition system. Integrating Analog-to-Digital Converter. The real workhorse of the I 1 0 modules is the integrating ADC. This ADC is somewhat unique in that it does not require a track-and-hold am-
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ANALYTICAL CHEMISTRY, VOL. 53, NO. 11. SEPTEMBER 1981
1253A
plifier. In fact, the result of a measurement by this device is a 16-bit binary integer whose value is linearly related to a Ym-s integration of the input voltage. This integration technique discriminates against high frequency noise with essentially complete rejection of noise occurring a t frequencies that are integer multiples of 60 Hz.A second noise-reducing feature of the integrating ADC is that it has differential rather than single-ended inputs. This lessens the sensitivity of the ADC to common mode noise introduced by ground loops, a plague that is usually rampant whenever two instruments are connected. Albeit the maximum data throughput of the integrating ADC is only 20 pointsls, it has proven to be quite adequate for a host of applications, including gas chromatography, automatic titrations, and conventional infrared spectrometry. In applications where the data span a wide dynamic range, the 16-bit resolution (Le.. one part in 65 536) provided by this ADC is particularly desirable. Most laboratory instruments measure a dependent variable, which reflects the behavior of a chemical system being investigated, as a function of some independent variable that the instrument either controls or measures (e.g., absorption vs. wavelength in conventional infrared spectrometers or thermal conductivity vs. time in some gas chromatographs). Included on this ADC module is a 20-Hz clock that operates in synchronization with conversions by the ADC. This permits accurate timing between consecutive measurements of some dependent variable. Thus, this module is most useful in instances where the independent variable is time, a known function of time, or is controlled by (and thus known by) the computer. If the independent variable is varied by the laboratory instrument in a complicated fashion, it may be necessary to measure both the dependent and independent variables simultaneously using two ADCs or one ADC preceded by two track-and-hold amplifiers. In such situations the integrating ADC may not be suitable, even though its data throughput and resolution are adequate. Fast Analog-to-Digital Converter. Often a data acquisition system is required to collect an ensemble of data by performing a series of consecutive measurements of a varying analog voltage. If the elements of the ensemble must be acquired in rapid succession, the fast ADC module is employed. Depending on the performance desired, this module can be equipped with one of several differenb commercially available 12-bit ADCs (successive approximation type) with 1254 A
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Figure 2. UNC microcomputer modules that ailow computer interactions with laboratory instruments
ANALYTICAL CHEMISTRY, VOL. 53, NO. 11. SEPTEMBER 1981
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Shimadzu Capillary-type lsotachophoreticAnalyzer IP-2A is applicable to a wide range of fields, such as: Chemistry: Analysis of intermediates. or-
ganic and inorganic acids. metallic ions, and surfactants. F&: Analrjis of such constituents and additives as organic acids, amino acids, and proteins. Pharmamuiiml fields: Determination of the effenive components and impurities in antibiotics, crude drugs. and electrolytes. Moniioring of industrial waste water. Medical and biochemical fields; Analysis of body fluids and metabolites for organic acids, amino acids, polyamines. uncleotides. peptides, and proteins for clinical and diagnonic purposes.
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I Figure 3. Means of mass storage available for the UNC microcomputer analog-to-digital conversion times of 2,8, or 20 ps. Unfortunately, the time required for each measurement is not determined solely by the conversion time of the ADC hut includes contributions due to the time necessary for data transfers between the ADC and memory and for decisions related to the control of the activity of the ADC. To minimize this timing overhead, once the CPU has established a mode of operation for the ADC, its activity can be supervised by another module-the 110sequencer. Each time the sequencer triggers the ADC, a specified ooeration is executed and theresults are routed to memory using DMA (Direct Memorv Access). Several modes of operation-are discussed in the following sections. Track-and-Hold Amplifiers. As manv as four auad track-and-hold modules can be used in conjunction with the fast ADC module, providing the ADC with access to analog data from up to 16 channels. An ADC ooeration entails either the conversionbf a voltage from a single analog channel or miltiple conversions that start a t a specified channel and proceed one after another @ t h e zeroth channel. In a multiple conversion operation, although the conversions occur in a serial manner, the results represent simul. taneous measurements. This is because the ADC puts all the track-andhold amplifiers in the hold state a t the same time. As discussed earlier, the ability to do simultaneous voltage 1256A
measurements is particularly advantageous for applications requiiing measurements of both dependent and independent variables. Hardware Averoger. Because each track-and-hold amplifier provides the fast ADC a “snapshot” of the input voltage rather than an average of that voltage, this ADC is more sensitive to high frequency noise than the integrating ADC described earlier. However, if the noise is truly random, the signal-to-noise ratio of the data can be improved by a factor equal to the square root of n by repeating the experiment n times and averaging the collected ensembles. The optional hardware averager module is designed to do this ensemble averaging as the data is being collected and at a pace much faster than could be achieved using the 8080 microprocessor to perform the necessary arithmetic. The averager, operatine under the direction o? the fast AD3 module, procures an &hit value from the ADC and adds it to (or subtracts it from) the corresponding element in an array of 8.16, 24, or 32-bit words in memory. Each word is transferred to and from the averager using DMA. Digital-to-Analog Converters. The most common task for the quad 12-bit DAC module is driving analog X-Yrecorders; however, occasionall; i t serves to control one or more independent variables in an experiment (e.g., staircase voltammetry). Like the fast ADC module, the DAC module can
ANALYTICAL CHEMISTRY, VOL. 53, NO. 11, SEPTEMBER 1981
work under the direct supervision of the CPU or under external supervision by either the I10 sequencer module or a laboratory instrument. External control of this module is realized by manipulating the trigger lines provided for each of the four DACs. When a DAC is triggered, a new setting for that DAC is fetched from memory under DMA. After all bits in that setting (eight or 12, depending on the mode of operation) are available on the module, they are parallel transferred into the appropriate DAC buffer register. Setting a buffer register in this fashion prevents output glitches that occur if a 12-bit DAC setting is altered piecemeal as each byte of the new setting arrives on the data bus. In the event that several DACs are triggered concurrently, they are loaded sequentially, starting with the DAC assigned the highest channel number. Inputloutput Sequencer. The I10 sequencer module probably contributes more than any other module toward extending the capacity of the UNC microcomputer to effect bighspeed analog interactions with laboratory instruments. It affords flexible management of the real-time activities of the quad DAC and fast ADC modules a t speeds far exceeding the capabilities of the 8080 microprocessor to manage such activities. It can also serve as a 16- or 32-bit submicrosecond resolution timer for events occurring up to 35 min apart. During periods of sequencer-con-
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*
Laboratories
Computer Room
Telephone
Telephone
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Microcomputer
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‘q== 75-ohm Coaxial Cable
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LI Remote Mainframe
Figure 4. Communication schemes used to connect the UNC microcomputerto host computers. X’s correspond to microcomputers or other laboratory stations. The loop communication network ( 4 , 141 Dermits serial data transfers between a host cornouter and up to 256 laboratory stations at a rate of 50K bytesls trolled I/O, the primary function of the sequencer is to sequentially trigger a series of up to eight events after an interval which is timed by a 16-bit counter on the module (maximum duration: 32.7 ms, resolution 0.5 rs).Immediately after an interval has timedout, as the sequencer begins to trigger the appropriate series of events, it also begins to time the next interval. Thus, timing continuity is preserved. Associated with this I/O activity is a special program that the CPU stores in memory prior to activating the sequencer. Under DMA, this program is accessible to the It0 sequencer, fast ADC, hardware averager, and quad DAC modules. Possible constituents of the program include DAC settings, 1258A
storage locations for results from the fast ADC or the hardware averager, 16-bit delay values, which are required if the interval duration is to he altered, and a “program byte” for each interval that specifies which events the sequencer is to trigger after that interval. The sequencer converses with itself as well as with other modules or laboratory instruments via a bus comprised of eight GO lines and a READY line. An event is triggered by a GO pulse on the appropriate line. Events that can be triggered on other modules include ADC conversions as well as DAC resettings. In addition, the sequencer can trigger itself to load a new 16-bit delay, to load a program byte,
ANALYTICAL CHEMISTRY, VOL. 53, NO. 11, SEPTEMBER 1981
or to terminate its activity. Digital InputtOutput Interfaces. Currently many manufacturers provide digital I/O ports on their instruments in an effort to make those instruments more compatible with laboratory computers. Unfortunately, however, an industry-wide standard interface has not been universally adopted. Several of the interfaces now in use are described below (8). IEEE-488 Interfacing Bus. This parallel interface, originated by HewLett-Packard, allows &bit, bidirectioual data transfers between an array of devices interconnected by a common bus. Physical and electrical characteristics as well as the protocol governing device interactions are meticulously
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