IBM 2741
The IBM 2741 was a printing computer terminal introduced in 1965. It 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). There were 88 graphic characters and so shift characters were used to allow their encoding in six data bits.
The 2741 supplanted the earlier IBM 1050, which was more expensive and cumbersome, in remote terminal applications. The IBM 1050 and variations were frequently used as console devices for computers such as the IBM 1130 and IBM System/360. By contrast, the 2741 was primarily focussed on remote terminal applications; the primary initial application for the 2741 was the IBM Administrative Terminal System (ATS/360) (for which the 2741 was initially developed and marketed). ATS was an interactive text editing system implemented in the mid-1960s using IBM System/360 assembler language.
The 2741 then encouraged the development of other remote terminal systems for the IBM System/360. APL\360 and ALGOL 68 are two early languages that took advantage of the Selectric print mechanism with its relatively large character set and changeable fonts.
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, so a type element from, say, a System/360 console printer 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. A message began with a circle D control character and ended with a circle C. The message text was initially lower case. When the other end was sending, the local keyboard was locked. Protocol symmetry allowed two 2741s to communicate directly but this was a rare configuration.
Keyboard layout with the APL typeball print head inserted:
As a direct result the characters ∨, ∧, ¬, ≠, ≤, ≥, ×, ÷, ⌷, ↑, ↓, ⌊, ⌈ and ⊥ from the APL typeball print head found their way into the ALGOL 68 programming language standard Final Report (August 1968).
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[edit] Related machines
Some later IBM Selectric-based machines, such as the Communicating Magnetic Card Selectric Typewriter, could emulate the 2741 and could be used in its place.
IBM sold the underlying Selectric mechanism to other manufacturers, who produced 2741 clones at lower cost. There were also integrated into larger systems. For example, the 2741's mechanism formed the principal user interface for a series of machines from the 1960s and 1970s built in the UK by Business Computers Ltd.
[edit] Decline
The 2741 and similar machines were supplanted by 30 char/s ASCII terminals using the Xerox Diablo 630 print mechanism. The IBM 3767 which ran at 80 or 120 char/s was an alternate replacement.
