Design
The IBM 2741 combined a ruggedized Selectric typewriter mechanism with IBM SLT electronics and an RS-232-C serial interface. It operated at about 14.1 characters per second with a data rate of 134.5 bits/second (one start bit, six data bits, an odd parity bit, and one and a half stop bits). As with the standard office Selectrics of the day, there were 88 printing characters plus space and a few nonprinting control codes, more than can be represented with six data bits, so shift characters were used to allow the machine's entire character set to be used.
The machine was packaged into its own small desk, giving the appearance of square tabletop with a Selectric typewriter partly sunken into the surface, with the electronics in a vertically oriented chassis at the rear. It supplanted the earlier IBM 1050, which was more expensive and cumbersome, in remote terminal applications. The IBM 1050 and its variations were designed for a higher duty cycle and so were frequently used as console devices for computers such as the IBM 1130 and IBM System/360. By contrast, the 2741 was primarily focused on remote terminal applications.
The IBM 2741 came in two different varieties, some using "correspondence coding" and the others using "PTT/BCD coding." These referred to the positioning of the characters around the typeball and, therefore, the tilt/rotate codes that had to be applied to the mechanism to produce a given character. A "correspondence coding" machine could use type elements from a standard office Selectric (i.e. elements used for "office correspondence"). "PTT/BCD coding" machines needed special elements, and did not have as wide a variety of fonts available. The IBM 1050 and its derivatives were only available in PTT/BCD coding. The two element types were physically interchangeable, but code-incompatible, so a type element from, say, a System/360 console printer (a variety of IBM 1050) would produce gibberish on a "correspondence coding" 2741 or an office Selectric, and vice versa.
The two varieties of IBM 2741 used different character codes on the serial interface as well, so software in the host computer needed to have a way to distinguish which type of machine each user had.
The protocol was simple and symmetric. Each message began with a control character called "circle D" (depicted in the manual as a capital D with a circle around it) and ended with a "circle C." Each message was assumed to begin with the shift mode in lower case.
When the other end was sending, the local keyboard was locked. An "attention feature" allowed the operator to interrupt the sending machine and regain control (much in the manner of "control-C" in many ASCII systems) by pressing a special key.
Protocol symmetry allowed two 2741s to communicate with each other with no computer in between, but this was a rare configuration.
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