Timeline for Why was the return key symbol ↵ drawn differently from the motion of a CR-LF?
Current License: CC BY-SA 4.0
21 events
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| Apr 21, 2022 at 16:35 | comment | added | supercat | On an ASR-33 teletype, if the print head isn't near the left edge of the paper, sending a CR followed by a printable character will result in the latter character being printed while the print head is in motion. If LF were sent before CR, it would be necessary to follow the CR with a non-printing character to prevent this. While one could use the sequence LF+CR+NUL between lines, this would add an extra 1/10 second to transmission time for every line sent. Teleprinter output wasn't designed to be particularly attractive, so any smudging that might result would have been tolerable. | |
| Apr 21, 2022 at 16:31 | comment | added | supercat | Early teleprinters needed to print each character at a fixed amount of time (typically less than 1/10 of the time required to transmit a character) after the character came over the wire. Trying to print a character while the print head was moving would have far worse effects than any kind of smudging that could otherwise be affected by operating sequence. Note that line printers are completely different from teleprinters, and low-speed "computer printers" don't have the same timing constraints as teleprinters, since a computer can hold off on sending text until the print mechanism is ready. | |
| Apr 21, 2022 at 16:19 | comment | added | Lou Knee | @supercat I don't see that: the moment the print head starts its CR motion it will potentially smudge fresh print, so to me it would always make more sense to do the LF first. There again, I'm prioritising quality over speed, and my experience is (was) primarily consumer dot-matrix printers rather than datacentre "line printers" - maybe those had better head designs or better quality paper? | |
| Apr 19, 2022 at 18:18 | comment | added | supercat | @LouKnee: If resetting the carriage will take longer than advancing the paper, and the actions can overlap but can't be started simultaneously, starting the CR first will be mechanically better than starting the LF first. | |
| Oct 3, 2020 at 17:11 | comment | added | Lou Knee | @chromatix I've long forgotten the proper name for the things, but ribbon-based typewriters/printers had thin metal shields either side of the slot for the type/print-head/whatever to punch through, to keep loose ribbon off the paper and smooth out creases from the paper: tiny scraps of paper - often fragments of tractor strip - got stuck behind those a lot. But grot trapped behind the leading shield will at least stay clean. Grot under the trailing shield will smudge the text, but on a typewriter the user would notice and clean up within a few characters. Maybe "reduce" rather than "avoid". | |
| Oct 3, 2020 at 7:18 | comment | added | Chromatix | @LouKnee Surely if that was a concern, the same grot would smudge the ink immediately after impact? | |
| Oct 2, 2020 at 13:07 | comment | added | Lou Knee | Note that mechanically LF first followed by the carriage moving is the better order, to avoid grot around the print head from smudging the fresh text... | |
| Jul 15, 2020 at 16:37 | comment | added | supercat | @JdeBP: Some devices may have required a long delay, and MS-BASIC offered an option to configure pad characters after an LF, but I don't recall any devices using pad characters between CR and LF. Maybe some did, but CR+LF, in that order, is by far the most common multi-character newline sequence. | |
| Mar 20, 2020 at 21:08 | comment | added | JdeBP |
Actually, the text traditionally sent by the host was not CR+LF. See for example the superbee-xsb terminal type. It was CR+long-delay+LF or CR+lots-of-pad-characters+LF.
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| Mar 20, 2020 at 0:27 | comment | added | Russell Borogove | ...with brokenness set to 22 and ribbon ink set to 250. | |
| Mar 20, 2020 at 0:20 | comment | added | Russell Borogove | @supercat Mechanical tolerances, while quite good, were just sloppy enough that the second pass would be shifted enough to give a really good bold -- much better than a harder physical strike. Check out uniqcode.com/typewriter | |
| Mar 20, 2020 at 0:17 | comment | added | supercat | @RussellBorogove: How did the effectiveness of that compare with simply hitting the keys harder? Unless a ribbon was really faded, effective bolding would require shifting the second strike relative to the first. | |
| Mar 19, 2020 at 23:55 | comment | added | Russell Borogove | @supercat Bolding by typing a line, throwing the carriage return lever, rolling the paper back a line, and retyping the line was also a common operation on typewriters. ;) | |
| Mar 19, 2020 at 22:53 | comment | added | supercat | @RussellBorogove: I know such things were common under Unix, but of course the invention of ASCII substantially predates Unix. | |
| Mar 19, 2020 at 20:18 | comment | added | Russell Borogove | @supercat Bolding via typing a line, issuing CR without LF, then retyping the line was a common operation on ttys, and the vestiges of it still appear sometimes in Unixy systems. | |
| Mar 19, 2020 at 17:05 | comment | added | supercat | @EricTowers: My first thought was that the ability to underline text using CR without LF would have been recognized as useful, but then I remembered that underline wasn't part of the original ASCII set. | |
| Mar 19, 2020 at 17:03 | comment | added | Eric Towers | @supercat : Many TTYs implicitly LFed on an explicit CR (duplicating typewriter behaviour). On those models, CR and CRLF were already equivalent. | |
| Mar 19, 2020 at 15:32 | comment | added | supercat | @RossRidge: I wonder if there would have been any extra cost to assigning the CR and LF codes so that any code matching 000 1xx1 would reset the carriage and any code matching 000 101x would advance the paper? Then 000 1011 would naturally have reset the carriage and advanced the paper. | |
| Mar 19, 2020 at 14:32 | comment | added | user722 | @CaptainMan Because the carriage return action usually takes takes longer than the line feed action, so you want it to start first even it's only by the fraction of a second it takes to transmit one character. | |
| Mar 19, 2020 at 14:20 | comment | added | Captain Man |
This begs the question why it's traditional for the text to be CR LF instead of LF CR since that is what actually happens.
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| Mar 19, 2020 at 5:32 | history | answered | Chromatix | CC BY-SA 4.0 |