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IBM 2741

IBM 2741

The 2741 was a low-speed dumb 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.

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.

The 2741 was important because it encouraged the development of remote terminal systems for the IBM System/360. APL360 and ALGOL 68 are two early languages that took advantage of the Selectric print mechanism with its relatively large character set and changeable fonts.

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 lanuage standard Final Report (August 1968).

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.

The 2741 in turn was 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.

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