erminal that has only half- duplex capability wastes some of the available bandwidth. The fol- lowing list can serve as a guideline for CRT terminal selection. Can it display enough characters? Are the displayed characters large enough? Are the characters easy to read? Is it flicker-free? Is the image bright enough? Is the image suitably protected from external glare? Is the display rate fast enough? Can it handle vectors or other graphic features? Is a modem or acoustical coupler built in? Is full-duplex or half-duplex transmission used? What cursor movements are possible? Does it have selectable tabs or other formatting features? What editing capabilities are available? Can different codes be used, for flexibility? Is there a nonprinting feature for keying-in security codes? Is there a cryptography feature? Is there a physical lock and key? 4.2 PRINTING TECHNOLOGIES In spite of the promise of a "paperless society," printers and other hard copy devices have continued to advance both in the market place and in technological innovations. In a modern communications sys- tem, either for data processing or office applications, printers have become one of the key interfaces between operator and machine. For a printer to function satisfactorily requires more than the making of marks on paper by a printing mechanism; it needs a paper TERMINAL TECHNOLOGY 57 transport mechanism, electronic controls, and a means for inputting information to the printer, such as a keyboard or some sophisticated communication devices. Printers can be categorized into impact and non-impact printers. The impact printers use the kinetic energy of a rapidly moving ham- mer, character ball, or other actuators to drive an ink ribbon against paper for ink transfer to the paper (so-called front printing). The printing can also be done by impacting the paper against the ribbon backed by a platen (so-called back printing). Since the impact force of the printing element can be transmitted through several sheets of paper and carbon paper, it can simulta