Product Code Database
The IBM 2741 is a printing computer terminal that was introduced in 1965. Compared to the teletypewriter machines that were commonly used as printing terminals at the time, the 2741 offers 50% higher speed, much higher quality printing, quieter operation, interchangeable type fonts, and both upper and lower case letters.
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.
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.
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.
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.
The 2741's existence encouraged the development of other remote terminal systems for the IBM System/360, particularly systems that could benefit from the high print quality, interchangeable typing elements, and other advantages of its Selectric mechanism.
Keyboard layout for use with the APL typeball:
Some later IBM Selectric-based machines, such as the Communicating Magnetic Card Selectric Typewriter, can emulate the 2741 and be used in its place.
IBM sold the underlying Selectric mechanism to other manufacturers, who produced 2741 clones at lower cost. Some of these were integrated into larger systems instead of being sold as standalone terminals. For example, a 2741-type mechanism formed the principal user interface for a series of machines from the 1960s and 1970s built in the United Kingdom by BCL Molecular.
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| End of transmission |
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| Delete |
| Space |
Circle-D used a code assigned to a printing, non-function character – 8 2 1 (EBCD '#'). It was identified as a control code based on its position as the first character in a transmission,
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