IBM 2741
| Company type | Computer terminal |
|---|---|
| Industry | Computer hardware |
| Predecessor | IBM 1050 |
| Founded | 1965 |
| Products | Printing computer terminal |
The IBM 2741 is a printing computer terminal that debuted in 1965. In comparison to the widely used teletypewriter terminals of that era, the 2741 delivered a 50% increase in speed, enhanced print resolution, quieter functioning, interchangeable type fonts, and the ability to print both uppercase and lowercase characters.
While mainly designed for use with the IBM System/360 family, it was also compatible with various IBM and non-IBM systems where faster printing and letter-quality output were essential. The terminal was instrumental in advancing the adoption of the APL language. By the mid-1970s, it was largely superseded by printing terminals employing daisy wheel mechanisms.
Design
The IBM 2741 combines a durable Selectric typewriter mechanism with IBM’s SLT electronics and an RS-232-C serial interface, running at approximately 14.1 characters per second with a data transmission rate of 134.5 bits per second. Unlike typical ASCII terminals, it transmits the most significant bit first.
Supporting 88 printable characters plus space and control codes, the terminal requires shift characters to access its complete character set—this can impact effective printing speed. For instance, printing the phrase "Armonk, New York, U.S." involves inserting 10 shift codes within a total of 32 transmitted characters.
The unit is designed as a compact desk-style enclosure featuring a partially embedded Selectric typewriter and vertically mounted electronics at the rear, contributing to significantly reduced noise levels. It replaced the earlier, bulkier, and costlier IBM 1050 in remote terminal applications. Although the IBM 1050 was engineered for higher duty cycles and frequently served as a console for systems such as the IBM 1130 and IBM System/360, the 2741 was optimized for remote terminal use.
Character codes
Two versions of the IBM 2741 were available: one employing "correspondence coding" and another using "PTT/BCD coding" or "PTT/EBCD coding." These coding schemes correspond to character positions on the typeball, defining tilt and rotate codes required for character printing. Machines with "correspondence coding" could utilize standard office Selectric type elements, whereas those with "PTT/BCD" or "PTT/EBCD" coding necessitated specialized elements offering fewer font variations. The IBM 1050 and its variants exclusively used PTT/BCD coding. Although type elements from either system could be physically swapped, their incompatible coding meant that using an element from one system on another produced illegible output.
Additionally, these IBM 2741 models employed distinct serial interface character codes, requiring host software to identify the terminal type per user session. This was often achieved by prompting users to enter a unique character (such as # or 9) or a standard command like "login" immediately after connection, allowing the host to determine the coding scheme based on received character values.
Line protocol
The communication protocol is straightforward and symmetrical. Each transmission begins with a control character known as "circle D" and concludes with "circle C," starting in lowercase shift mode by default. When incoming data is detected, the local keyboard locks to block user input.
The terminal features a "Receive Interrupt" function that enables an operator to halt transmission from the sending machine by pressing an "Attention" (ATTN) key. This action generates a continuous spacing signal lasting over 200 milliseconds, which triggers a framing error on the receiver side. Upon detecting this condition, the remote system ceases data transmission, switches to receive mode for input from the 2741, and sends a "circle C" (end of message) signal. Once this signal is received locally, the keyboard is unlocked for new input.
This bidirectional protocol theoretically permitted two IBM 2741 terminals to communicate directly without an intervening computer; however, such configurations were rare in practice.
Applications
Originally designed to work with the IBM Administrative Terminal System (ATS/360), the 2741 served as an interactive, multi-user text editing terminal developed in the mid-1960s for the IBM System/360 environment.
Its superior print clarity, swappable typing elements, and the innovative Selectric mechanism encouraged the creation of additional remote terminal systems compatible with the IBM System/360.
APL\360
The IBM 2741 became strongly associated with the APL language. Conceived by Dr. Kenneth Iverson, APL demanded a wide array of special symbols. IBM implemented it as a timesharing environment on the System/360, named APL\360, which required either an IBM 2741 or IBM 1050 equipped with an APL typeball. The keyboard layout included 26 uppercase italic letters that were typed in lowercase mode; shifted keys provided access to numerous special characters, while others were produced by overstriking.
Keyboard configuration for the APL typeball:
ALGOL 68
Similarly to APL, ALGOL 68 utilized many special symbols such as ∨, ∧, ¬, ≠, ≤, ≥, ×, ÷, ⌷, ↑, ↓, ⌊, ⌈, and ⊥ found on the APL Selectric typeball. Despite no formal connection between these languages, this typeball was employed to compose the ALGOL 68 Final Report published in August 1968.
Related machines
The IBM 2740 terminal shared similarities with the 2741 but lacked interrupt and dial-up capabilities. It supported point-to-point, multipoint, or broadcast communication modes and could incorporate a data buffer to enhance performance on multipoint lines at 600 bit/s without being constrained by typing speed.
Subsequent IBM Selectric-based models such as the Communicating Magnetic Card Selectric Typewriter could mimic the functionality of the 2741.
IBM licensed the Selectric mechanism to other manufacturers who produced more affordable clones of the 2741. Some of these were integrated into larger systems rather than marketed as standalone terminals; for instance, Business Computers Ltd in the UK utilized a 2741-style mechanism as the primary user interface for machines during the 1960s and 1970s.
Decline
Selectric-based terminals like the 2741 gradually gave way to ASCII terminals equipped with printers such as the Xerox Diablo 630 "daisy wheel," which delivered comparable print quality alongside enhanced reliability and doubled printing speed (30 characters per second) at a lower cost. These terminals supported multiple fonts—including APL—through interchangeable print wheels and offered full ASCII character coverage. In scenarios where printed output was unnecessary, video display terminals frequently replaced them. Another alternative was the IBM 3767, a terminal featuring a dot-matrix printer capable of printing at speeds of 80 or 120 characters per second.
Function codes
Function codes operated independently from both character sets and shift states.
| Bit value | Code | Description |
|---|---|---|
| C 8 4 | PN | Punch On |
| A 8 4 | BY | Bypass |
| B 8 4 | RES | Restore |
| B A C 8 4 | PF | Punch Off |
| 8 4 1 | RS | Reader Stop |
| A C 8 4 1 | LF | Line Feed |
| B C 8 4 1 | NL | New Line |
| B A 8 4 1 | HT | Horizontal Tab |
| 8 4 2 | UC | Upper Case |
| A C 8 4 2 | EOB | End Of Block |
| B C 8 4 2 | BS | Backspace |
| B A 8 4 2 | LC | Lower Case |
| C 8 4 2 1 | EOT | End Of Transmission |
| A & nbsp ;8& nbsp ;4& nbsp ;2& nbsp ;1 | PRE | Prefix |
| & nbsp ;B& nbsp ;& nbsp ;& nbsp ;& nbsp ;8& nbsp ;4& nbsp ;2& nbsp ;1 | IL | Idle |
| & nbsp ;B& nbsp ;A& nbsp ;C& nbsp ;8& nbsp ;4& nbsp ;2& nbsp ;1 | DEL | Delete |
| & nbsp ;& nbsp ;& nbsp ;& nbsp ;& nbsp ;C& nbsp ;& nbsp ;& nbsp ;& nbsp ;& nbsp ; | Space | Space |
Circle-D assigned a code typically used for printable non-function characters—specifically 8 2 1 (EBCD '#')—which was interpreted as a control code due to its position as the first character transmitted.
PTTC/EBCD code
| Bit value | Lowercase | Uppercase |
|---|---|---|
| B A ;8;;2;;1 | . | ¬ |
| B;;C;;8;;2;;1 | $ | ! |
| ;A;;C;;8;;2;;1 | , | | |
| &&&&& 8&&&2&&1 | # | " |
| &&&&& A&&&&&&&&&@ | ¢ | |
| &&& B A C &&&&& &&& &&& &&& &&& &&& &&& &&& &&& &&& &&& &&& &&& &&& &&& &&& &&& | ||
| - | _ | |
| / | ? | |
| 1 | = | |
| 2 | < | |
| 4 | : | |
| 5 | % | |
| 6 | ' | |
| 7 | > | |
| 8 | * | |
| 9 | ( | |
| 0 | ) | |
| a | A | |
| b | B | |
| c | C | |
| d | D | |
| e | E | |
| f | F | |
| g | G | |
| h | H | |
| i | I | |
| j | J | |
| k | K | |
| l | L | |
| m | M | |
| n | N | |
| o | O | |
| p | P | |
| q | Q | |
| r | R | |
| s | S | |
| t | T | |
| u | U | |
| v | V | |
| w | W | |
| x | X | |
| y | Y | |
| z | Z |