Microprocessors 3 Instrument-computer interfaces

A brief description of computers, microprocessors and computer/instrument interfacing in the context of analytical chemistry is given in the following sections. 

When 8-bit microprocessor-based computers became available, we decided that such a computer, even though slower than minicomputers, would be adequate to operate the OMA and our interface controller with the ultracentrifuge. We now have in the system an Altair 8800 computer with 28K of memory (MITS, Albuquerque, NM), a hard-wired arithmetic board and two minifloppy disk drives (North Star, Berkeley, CA), a 700 ASR Terminal (Texas Instruments, Dallas, TX), and a 7202A Graphic Plotter (Hewlett-Packard, Palo Alto, CA). Appropriate software, written in Basic, has been developed to collect intensity data from the OMA automatically and also to treat and plot the data at the end of the experiment. Details of the system and software will be published elsewhere. We also have an improved illumination system with a 200 W Hg-Xe arc lamp and a Model H-20 monochromator with a holo-... 

Today s laboratories —particularly the larger ones— use a variety of intelligent, microprocessor-controlled, instruments with analogue output and (micro)computers interfaced to one another. It is the fashion in which the interfacing Is done that ensures efficient laboratory computerization (automation). Ziegler [31] established three categories of computerized configurations, namely (Fig. 2.14) ... 

During the latter part of the period, prisms disappeared and more advanced features were built into dispersive spectrometers such as computer interfaces, principally for data acquisition only. Ratio recording became standard in place of the optical null. However, the Perkin-Elmer model 983, introduced in 1982, demonstrated the benefits that computerization could bring to instrument control. Grating rotation was carried out directly by a stepper motor under the command of a microprocessor. With no mechanical cam, position repeatability (+0.005 cm i) was, in consequence, almost as good as that of FT-IR instruments. 

The Ferranti-Shidey viscometer was the first commercial general-purpose cone—plate viscometer many of the instruments still remain in use in the 1990s. Viscosities of 20 to 3 x 107 mPa-s can be measured over a shear rate range of 1.8-18,000 s-1 and at up to 200°C with special ceramic cones. Its features include accurate temperature measurement and good temperature control (thermocouples are embedded in the water-jacketed plate), electrical sensing of cone—plate contact, and a means of adjusting and locking the position of the cone and the plate in such a way that these two just touch. Many of the instruments have been interfaced with computers or microprocessors. 

The development and widespread use of computers and microprocessors in control laboratory instruments has made it possible to fully automate a laboratory, including interfacing instruments directly to a LIMS. In the fully automated laboratory, a sample is logged into a LIMS, then transferred to a laboratory where it is prepared for analysis by a robot, which then transfers it to an autosampler or analyzer. Once analyzed, the data is transferred through a communications link to a device which could convert the raw data into information that a customer needs. For example, in a simple case, a nmr spectrum could be compared to spectra on file to yield an identification of an unknown. In more complex instances, a data set could be compared to standards and by using pattern recognition techniques the LIMS would be able to determine the source of a particular raw material. Once the data is reduced and interpreted, the LIMS becomes the repository of the information. A schematic for such a fully automated laboratory is shown in Figure 2 (6). 

At the bottom end of the scale, many cheap 8-bit microcomputers are finding their way into analytical applications. These include machines such as the Apple and the Commodore PET (which incorporates the IEEE-488 bus) as well as many others. Although computers such as these lack the computing power and sophisticated interfacing abilities of the MINC, there are many applications for which they are quite powerful enough, whilst their low prices enable them to be used in a wide range of situations where computers have not previously been employed for reasons of cost. The impact which microcomputers and microprocessors have had upon analytical instrumentation is reviewed in Ref.7). 

Many laboratory instruments available on the market today contain built-in microprocessors that process data collected on samples and display or send the answer to a computer. In addition, they may have an interface that attaches to an external computer for processing the data generated by the instrument. With regard to experiments or analyses performed frequently, it is often desirable to interface the instrument to a computer to aid in the subsequent analysis of data. Some of the commonly encountered systems following. 

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