IBM, sonic delay lines, and the history of the 80×24 display

What explains the popularity of terminals with 80×24 and 80×25 displays? A recent blog post "80x25" motivated me to investigate this. The source of 80-column lines is clearly punch cards, as commonly claimed. But why 24 or 25 lines? There are many theories, but I found a simple answer: IBM, in particular its dominance of the terminal market. In 1971, IBM introduced a terminal with an 80×24 display (the 3270) and it soon became the best-selling terminal, forcing competing terminals to match its 80×24 size. The display for the IBM PC added one more line to its screen, making the 80×25 size standard in the PC world. The impact of these systems remains decades later: 80-character lines are still a standard, along with both 80×24 and 80×25 terminal windows.

In this blog post, I'll discuss this history in detail, including some other systems that played key roles. The CRT terminal market essentially started with the IBM 2260 Display Station in 1965, built from curious technologies such as sonic delay lines. This led to the popular IBM 3270 display and then widespread, inexpensive terminals such as the DEC VT100. In 1981, IBM released a microcomputer called the DataMaster. While the DataMaster is mostly forgotten, it strongly influenced the IBM PC, including the display. This post also studies reports on the terminal market from the 1970s and 1980s; these make it clear that market forces, not technological forces, led to the popularity of various display sizes.

Some theories about the 80×24 and 80×25 sizes

Arguments about terminal sizes go back decades,5 but the article 80x25 presented a detailed and interesting theory. To summarize, it argued that the 80×25 display was used because it was compatible with IBM's 80-column punch cards,1 fits nicely on a TV screen with a 4:3 aspect ratio, and just fit into 2K of RAM. This led to the 80×25 size on terminals such as the DEC VT100 terminal (1978). The VT100's massive popularity led to it becoming a standard, leading to the ubiquity of 80×25 terminals. At least that's the theory.

It's true that 80-column displays were motivated by punch cards4 and the VT100 became a standard,2 but the rest of this theory falls apart. The biggest problem with this theory is the VT100's display was 80×24, not 80×25.3 In addition, the VT100 used extra bytes of storage for each line, so the display memory did not fit into 2K. Finally, up until the 1980s, most displays were 80×24, not 80×25.

Other theories have been expressed on Software Engineering StackExchange and Retrocomputing StackExchange, arguing that 80×24 terminals resulted from technical reasons such as TV scan rates, aspect ratios, memory sizes, typography, the history of typewriters, and so forth. There is a fundamental problem with theories that 80×24 is an inevitable consequence of technology, though: terminals in the mid-1970s had dozens of diverse screen sizes such as 31×11, 42×24, 50×20, 52×48, 81×38, 100×50, and 133×64.11 This makes it clear that technological limitations didn't force terminals into a particular size. To the contrary, as technology improved, most of these sizes disappeared and terminals were largely 80×24 by the early 1980s. This illustrates that standardization was the key factor, not the technology.

I'll briefly summarize why technical factors don't have much impact on the terminal size. Although US televisions used 525 scan lines and 60 Hz refresh,9 40% of terminals used other values.6 The display frequency and bandwidth didn't motivate a particular display size because terminals generated characters with a wide variety of matrix sizes.8 Although memory cost was significant, DRAM chip sizes quadrupled every three years, making memory only a temporary constraint. The screen's aspect ratio wasn't a big factor because the text's aspect ratio often didn't match the screen's ratio.7 Of course technology had some influence, but it didn't stop early manufacturers from creating terminal sizes ranging from 32×8 to 133×64.

The rise of CRT terminals

At this point, a bit of history of CRT terminals will help.11 Many readers will be familiar with ASCII terminals, such as stand-alone terminals like the DEC VT100, serial terminal connections via a PC, or the serial port on boards such as the Arduino. This type of terminal has its roots in teleprinters, electro-mechanical keyboard/printers that date back to the early 1900s. The best-known teleprinter is the Teletype, popular in newsrooms as well as computer systems in the 1970s. (The Linux device /dev/tty

is named after the Teletype.) Teletypes typically printed 72-character lines on a roll of paper.10

In the 1970s, replacing teleprinters with CRT terminals was a large and profitable market. AT&T introduced the Teletype Model 40 in 1973, a CRT terminal with an 80×24 display.12 Many other companies introduced competing CRT terminals, and "Teletype-compatible" became a market segment. By 198111 these terminals were being used in many roles besides replacing teleprinters and the name shifted to "ASCII terminals". By 1985, CRT terminals were a huge success with 10 million terminals installed in the US.

But there's a parallel world of mainframe terminals, a world that may be unfamiliar to many readers. In 1965, IBM introduced the IBM 2260 Display Terminal, which placed IBM's "stamp of approval" on the CRT terminal, which had previously been "somewhat of a novelty."6 This terminal dominated the market until IBM replaced it with the cheaper and more advanced IBM 3270 terminal in 1971. Unlike asynchronous ASCII terminals that transmitted individual keystrokes, these terminals were block oriented, efficiently exchanging large blocks of characters with a mainframe. The 3270 terminal was fairly "intelligent": a 3270 user could fill in labeled fields on the screen, and then transmit all the data at once by pressing the "Enter" key. (This is why modern keyboards often still have the "Enter" key.) Sending a block of data was more efficient than sending each keystroke to the computer, and allowed mainframes to support hundreds of terminals. In the next sections, I'll discuss the 2260 and 3270 terminals in detail.

The chart below6 shows how the terminal market looked in 1974. The market was ruled by IBM's 3270 terminal, which had obsoleted IBM's 2260 terminal by this point. With 50% of the market, IBM essentially defined the characteristics of a CRT terminal. Teleprinter replacement was a large and influential market; the Teletype Model 40 was small but growing in importance. Although DEC would soon be a major player, it was in the small "Independent Systems" slice at this point.

The IBM 2260 video display terminal

The IBM 2260 was introduced in 1965 and was one of the first video display terminals.14 It filled three roles: remote data entry (in place of punching cards), inquiry (e.g. looking up records in a database), and as a system console. This compact terminal weighed 45 pounds and was sized to fit on a standard office typewriter stand. Note the thickness of the keyboard; it reused the complex keyboard mechanism of the IBM keypunch.13

You might wonder how IBM could produce such a compact terminal with 1965 technology. The trick was that the terminal held just the keyboard and CRT display; all the control logic, character generation, storage, and interfacing was in a massive 1000 pound cabinet (below).15 This cabinet contained the circuitry to handle up to 24 display terminals. It generated the pixels for these terminals and send video signals to the terminals, which could be up to 2000 feet away.

One of the most interesting features of the 2260 is the sonic delay lines used for pixel storage. Bits were stored as sound pulses sent into a nickel wire, about 50 feet long. The pulses traveled through the wire and came out the other end exactly 5.5545 milliseconds later. By sending a pulse (or not sending a pulse for a 0) every 500 nanoseconds, the wire held 11,008 bits. A pair of wires created a buf