IBM 2741

It was used primarily with the IBM System/360 series of computers, but was used with other IBM and non-IBM systems where its combination of higher speed and letter-quality output was desirable. It was influential in the development and popularity of the APL programming language.

It was supplanted, starting in the mid-1970s, primarily by printing terminals using daisy wheel mechanisms.

Design

The IBM 2741 combines a ruggedized Selectric typewriter mechanism with IBM SLT electronics and an RS-232-C serial interface. It operates 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). In contrast to serial terminals employing ASCII code, the most significant data bit of each character is sent first.

As with the standard office Selectrics of the day, there were 88 printing characters (not quite enough for the entire EBCDIC or ASCII printing character set including the lower case alphabet) plus space and a few nonprinting control codes, more than can be represented with six data bits, so shift characters are used to allow the machine's entire character set to be used. This could cause a significant reduction in the print speed since printing "Armonk, New York, U.S." requires 10 shift characters resulting in a total of 32 characters transmitted to print 22 characters.

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. This allowed a significant reduction in the noise it generated. 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.

Character codes

The IBM 2741 came in two different varieties, one using "correspondence coding" and the other using either "PTT/BCD coding" or "PTT/EBCD coding". These refer to the positioning of the characters around the typeball and, therefore, the tilt/rotate codes that have to be applied to the mechanism to produce a given character. A "correspondence coding" machine can use type elements from a standard office Selectric (i.e. elements used for "office correspondence"). "PTT/BCD coding" and "PTT/EBCD coding" machines need 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 are physically interchangeable, but code-incompatible, so a type element from, say, a System/360 console printer (a variety of IBM 1050) produces gibberish on a "correspondence coding" 2741 or an office Selectric, and vice versa.

The two varieties of IBM 2741 use 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. One way this was accomplished was by having the user type a unique character such as # , 9 or a standard command such as "login" immediately after connecting. The host software would recognize which code was used by the value of the characters it received.

Line protocol

The protocol is simple and symmetric. Each message begins with a control character called "circle D" in the documentation, shown as , and ends with a "circle C" . Each message was assumed to begin with the shift mode in lower case.

When the remote end is sending, the local keyboard is locked.

The "Receive Interrupt" feature allows the operator to interrupt the sending machine and regain control by pressing a special "Attention" key (labeled ATTN). This key causes the 2741 to send a continuous "spacing condition" for 200 or more milliseconds. This will be recognized by the receiving system as a framing error (a start bit that is not followed by a stop bit in the expected time). (The break key on ASCII terminals works the same way: continuous spacing is a "break condition" used to signal the remote end of an interruption.) If the attention signal is honored, it causes the remote system to stop sending data, prepare to receive data from the 2741, and send a "circle C", meaning "end of message". Upon receipt of the "circle C" the local 2741 unlocks its keyboard and the operator can send another input to the system.

Protocol symmetry allows two people using 2741s to communicate with each other with no computer in between, but this was a rare configuration.