Many small computers use teletype machines as console typewriters. The ASR-33 teletype has not performed well, but it has recently been improved. The ASR-35 and KSR-35 have excellent records, and the newer ASR-37 and KSR-37 (15 characters/sec) are very good. The IBM Selectric has had a mixed reliability record which is, however, improving. In every case, expert routine maintenance is required.
12. Index Registers
These are a valuable asset to efficient programming. At least one, and preferably more, is desirable, especially in the medium and large computers.
13. Line Printers
These are of great use for obtaining a permanent ("hard copy") record, especially when large volumes of output are produced; however, they are expensive, usually costing $20,000 or more (including interfacing). In order to avoid tying up a large central processor during typewriter output of masses of data, a line printer is not only very useful, it is essential for efficient operation (and to spare the typewriter). A line printer can be immensely helpful and can save much time in the process of developing and debugging programs. The cost, however, will often preclude its addition to a modest system. If the system has an IBM-compatible tape drive, the computer output can be written on tape and later carried to a computing center for printing. Several industrial concerns are known to be working on new types of printers, some being dry-copy, nonpercussive types. One type which has already been marketed, the Inktronic printer, operates by spraying ink at the paper from small tubes. The characters are well formed. It operates at about 120 characters per second and costs $5600. Conveniently, it requires standard Teletype interfacing, and it can be ordered with an optional keyboard. Although it has exhibited a few new-product ailments in its first 8 months or so of use, it shows promise of becoming a very useful device. Another printer operating on a similar principle has just appeared—the A.B. Dick Company's Videojet printer, priced at about $4900.
14. Plotters
The overwhelming favorite is still the incremental machine called the Calcomp plotter. It costs about $5000 and is easily interfaced to many computers. It is very accurate (about 0.01 in. in each direction) and provides valuable output to the experimenter. It can be programmed to plot experimental points and theoretical curves together on white paper in India ink, relieving draftsmen of considerable work and doing a more precise job. Other incremental plotters are now on the market, e.g., the Houston Instruments version. Varian has developed an electrostatic plotter to sell for about $15,000.
15. Cathode-Ray Tube Displays
At least four types are in use. The standard scheme involves the displaying of bright spots under control of the computer, which has generated appropriate words to cause x and y deflections of the spot when those words have been transformed by ADC's in the CRT unit. The pattern is rewritten continuously. A light pen held against a particular part of the display pattern can be used to signal the computer. This scheme works well but may produce a flickering image if the computer is interrupted frequently to handle higher priority jobs or if the display is so complicated that the rewriting period exceeds 1/30 sec. The expensive hardware option called a character generator is considered not worthwhile unless large amounts of text are to be displayed. On a 10 in. x 10 in. raster a matrix of dots 1024 x 1024 is sensible.
A second scheme involves a disk or drum on which the computer writes the words to generate the pattern. Separate reading heads send the words to the CRT unit. Thus the display, automatically rewritten over and over, is updated from time to time by the computer. The light-spot cursor and joy-stick method replace the light pen in this case. (In passing, it is worth remarking that a light pen is only as effective as the computer program allows it to be, that the effort of programming for light-pen control is usually not trivial, and that a substantial amount of core storage may be required. A means of display control perhaps not so popular as it should be is sense-switch control.)