<p>The <a href="https://en.wikipedia.org/wiki/Lear_Siegler">Lear Siegler</a> <a href="https://en.wikipedia.org/wiki/ADM-3A">ADM-3A terminal</a> is a very important artefact in computing history.</p> <div class="wp-caption aligncenter" id="attachment_3018" style="width: 635px"><a href="https://vintagecomputer.ca/lear-siegler-adm-3a-terminal/"><img alt="" aria-describedby="caption-attachment-3018" class="wp-image-3018 size-large" decoding="async" fetchpriority="high" height="256" sizes="(max-width: 625px) 100vw, 625px" src="https://dave.cheney.net/wp-content/uploads/2017/08/Screen-Shot-2017-08-21-at-14.15.25-1024x420.png" srcset="https://dave.cheney.net/wp-content/uploads/2017/08/Screen-Shot-2017-08-21-at-14.15.25-1024x420.png 1024w, https://dave.cheney.net/wp-content/uploads/2017/08/Screen-Shot-2017-08-21-at-14.15.25-300x123.png 300w, https://dave.cheney.net/wp-content/uploads/2017/08/Screen-Shot-2017-08-21-at-14.15.25-768x315.png 768w, https://dave.cheney.net/wp-content/uploads/2017/08/Screen-Shot-2017-08-21-at-14.15.25-624x256.png 624w, https://dave.cheney.net/wp-content/uploads/2017/08/Screen-Shot-2017-08-21-at-14.15.25.png 1240w" width="625"/></a><p class="wp-caption-text" id="caption-attachment-3018">ADM-3A keyboard (image credit vintagecomputer.ca)</p></div> <p>If you want to know why your shell abbreviates <code>$HOME</code> to <code>~</code>, it’s because of the label on the <code>~</code> key on the ADM-3A. If you want to know why <code>hjkl</code> are the de facto cursor keys in <code>vi</code>, look at the symbols above the letters. The ADM-3A was the “dumb terminal” which <a href="http://www.linfo.org/vi/history.html">Bill Joy used to develop <code>vi</code></a>.</p> <p>Recently the ADM-3A came up in a twitter discussion about the wretched Apple touch bar when Bret Victor dropped this tweet:</p> <blockquote class="twitter-tweet" data-dnt="true" data-width="550"> <p dir="ltr" lang="en">It goes even deeper than that. The designer of vi grew up using keyboard where the "esc" key is to the left of the "Q", like our "tab" key. <a href="https://t.co/CYnlm7VRZv">pic.twitter.com/CYnlm7VRZv</a></p> <p>— Bret Victor (@worrydream) <a href="https://twitter.com/worrydream/status/899417057363546112?ref_src=twsrc%5Etfw">August 20, 2017</a></p></blockquote> <p><script async="" charset="utf-8" src="https://platform.twitter.com/widgets.js"></script></p> <p>Which settled the argument until Paul Brousseau asked:</p> <blockquote class="twitter-tweet" data-dnt="true" data-width="550"> <p dir="ltr" lang="en">What does the "here is" key do?</p> <p>— Paul Brousseau (@object88) <a href="https://twitter.com/object88/status/899481630665883648?ref_src=twsrc%5Etfw">August 21, 2017</a></p></blockquote> <p><script async="" charset="utf-8" src="https://platform.twitter.com/widgets.js"></script></p> <p>Indeed, what does the <code>HERE IS</code>¹ key do? Its prominent position adjacent to the <code>RETURN</code> key implies whatever it does, it is important.</p> <p>Fortunately the answer to Paul’s question was easy to find. The wonderful BitSavers archive has the user manual for the <a href="https://dave.cheney.net/paste/DP2880486F_ADM3A_UM_Apr86.pdf">ADM-3A available</a> (cached to avoid unnecessary bandwidth costs to BitSavers). On page 29 we find this diagram</p> <div class="wp-caption aligncenter" id="attachment_3022" style="width: 353px"><img alt="" aria-describedby="caption-attachment-3022" class="wp-image-3022 size-full" decoding="async" height="137" sizes="(max-width: 343px) 100vw, 343px" src="https://dave.cheney.net/wp-content/uploads/2017/08/Screen-Shot-2017-08-21-at-16.45.52.png" srcset="https://dave.cheney.net/wp-content/uploads/2017/08/Screen-Shot-2017-08-21-at-16.45.52.png 343w, https://dave.cheney.net/wp-content/uploads/2017/08/Screen-Shot-2017-08-21-at-16.45.52-300x120.png 300w" width="343"/><p class="wp-caption-text" id="caption-attachment-3022">Page 29, ADM-3A Users Manual (courtesy bitsavers.org)</p></div> <p>So <code>HERE IS</code>, when pressed, transmits a predefined identification message. But what do to the words “message is displayed in half-duplex” mean? The answer to that riddle lies in the ADM-3A’s A<em>nswerback</em> facility.</p> <p>Scanning forward to page 36, section 3.3.6 describes the configuration of the Answerback facility–programming the identification message transmitted when <code>HERE IS</code> is pressed.</p> <div class="wp-caption aligncenter" id="attachment_3024" style="width: 310px"><img alt="" aria-describedby="caption-attachment-3024" class="wp-image-3024 size-medium" decoding="async" height="284" sizes="(max-width: 300px) 100vw, 300px" src="https://dave.cheney.net/wp-content/uploads/2017/08/Screen-Shot-2017-08-21-at-20.03.33-300x284.png" srcset="https://dave.cheney.net/wp-content/uploads/2017/08/Screen-Shot-2017-08-21-at-20.03.33-300x284.png 300w, https://dave.cheney.net/wp-content/uploads/2017/08/Screen-Shot-2017-08-21-at-20.03.33.png 345w" width="300"/><p class="wp-caption-text" id="caption-attachment-3024">Section 3.3.6, page 36, ADM-3A Users Manual (courtesy bitsavers.org)</p></div> <blockquote><p>Pressing the <code>HERE IS</code> key or receiving an <code>ENQ</code> from the host … causes the answerback message to be transmitted to the host and to be displayed if the terminal is in half duplex mode.</p></blockquote> <p>This is interesting, the <em>remote</em> side can ask the terminal “who are you?”.</p> <p>The <code>HERE IS</code> key is a vestige of a an older facility called <em>Enquiry</em>. Enquiry allowed one end of the connection to query if the remote side was still connected, and if it was, exactly who was connected.</p> <blockquote><p><strong>ANSWERBACK Message</strong><br/> Answerback is a question and answer sequence where the host computer asks the terminal to identify itself. The VT100 answerback feature provides the terminal with the capability to identify itself by sending a message to the host. The entire answerback sequence takes place automatically without affecting the screen or requiring operator action. The answerback message may also be transmitted by typing CTRL-BREAK.</p></blockquote> <p>This description is from the 1978 Digital VT100 user guide. It was certainly a simpler time when the server could ask a terminal to identify itself, and trust the answer.</p> <hr/> <p>Notes</p> <ol> <li>I’ve chosen to write the name of the key in all caps as the base model of the ADM-3A was only capable of displaying upper case letters. If you wanted lower case (above 0x5F hex), that was an optional extra.</li> </ol> <div class="yarpp yarpp-related yarpp-related-website yarpp-template-list"> <!-- YARPP List --> <h3>Related posts:</h3><ol> <li><a href="https://dave.cheney.net/2013/11/15/evaluation-order-oddity" rel="bookmark" title="Evaluation order oddity">Evaluation order oddity</a></li> <li><a href="https://dave.cheney.net/2013/01/15/using-screen-for-lazy-dot-files" rel="bookmark" title="Using screen for lazy dot files">Using screen for lazy dot files</a></li> <li><a href="https://dave.cheney.net/2012/08/19/august-go-meetup-slides" rel="bookmark" title="August Go meetup slides">August Go meetup slides</a></li> <li><a href="https://dave.cheney.net/2014/12/05/friday-pop-quiz-minimum-one-liner" rel="bookmark" title="Friday pop quiz: minimum one liner">Friday pop quiz: minimum one liner</a></li> </ol> </div>