{"id":1260,"date":"2017-10-11T20:41:13","date_gmt":"2017-10-11T19:41:13","guid":{"rendered":"http:\/\/www.randomorbit.co.uk\/?p=1260"},"modified":"2017-10-11T20:45:47","modified_gmt":"2017-10-11T19:45:47","slug":"retrochallenge-201710-purely-by-coincidence","status":"publish","type":"post","link":"https:\/\/www.randomorbit.co.uk\/?p=1260","title":{"rendered":"RetroChallenge 2017\/10 \u2013 Purely by coincidence"},"content":{"rendered":"<p>So here&#8217;s a thing. I recently met up with an old acquaintance who I hadn&#8217;t seen in years and as we&#8217;re both retro-computing fans we were talking about old machines.<\/p>\n<p>I have just received an email from an old acquaintance of mine who is a dedicated retro-computing fan and has spotted my plight in these pages after following me on Twitter (@acollins22 if you&#8217;re interested).<\/p>\n<p>He mentioned that he had had a similar problem on a Nascom a few years ago and it was caused by a buffer failing. Now I know this is wishful thinking but it&#8217;s worth a look.<\/p>\n<p>The chip in question was an 81LS97, a 3-STATE Octal Buffer.<\/p>\n<p><a href=\"http:\/\/www.randomorbit.co.uk\/wordpress\/wp-content\/uploads\/2017\/10\/81ls97.png\"><img loading=\"lazy\" decoding=\"async\" class=\"alignleft size-medium wp-image-1262\" src=\"http:\/\/www.randomorbit.co.uk\/wordpress\/wp-content\/uploads\/2017\/10\/81ls97-300x270.png\" alt=\"\" width=\"300\" height=\"270\" srcset=\"https:\/\/www.randomorbit.co.uk\/wordpress\/wp-content\/uploads\/2017\/10\/81ls97-300x270.png 300w, https:\/\/www.randomorbit.co.uk\/wordpress\/wp-content\/uploads\/2017\/10\/81ls97.png 558w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/a>I&#8217;m not the most familiar with this to be honest, a keen amateur. My take on this is that Y1 will equal A1 when _G1_ is low. At other times it will be tri-stated. The other pairs (A2-Y2, A3-Y3 and A4-Y4 will do the same).<\/p>\n<h2>Is this right.<\/h2>\n<p>I wired up my old Blackstar logic analyser to IC47, the 81ls97 as follows&#8230;<\/p>\n<ul>\n<li>CH0 &#8211; _G1_<\/li>\n<li>CH1 &#8211; A1<\/li>\n<li>CH2 &#8211; Y1<\/li>\n<li>CH3 &#8211; A2<\/li>\n<li>CH4 &#8211; Y2<\/li>\n<li>CH5 &#8211; A3<\/li>\n<li>CH6 &#8211; Y3<\/li>\n<li>CH7 &#8211; A4<\/li>\n<\/ul>\n<p>It&#8217;s set to trigger on CH0 being low. This is what we get.<\/p>\n<p><a href=\"http:\/\/www.randomorbit.co.uk\/wordpress\/wp-content\/uploads\/2017\/10\/20171011_191528.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"alignleft size-medium wp-image-1259\" src=\"http:\/\/www.randomorbit.co.uk\/wordpress\/wp-content\/uploads\/2017\/10\/20171011_191528-300x225.jpg\" alt=\"\" width=\"300\" height=\"225\" srcset=\"https:\/\/www.randomorbit.co.uk\/wordpress\/wp-content\/uploads\/2017\/10\/20171011_191528-300x225.jpg 300w, https:\/\/www.randomorbit.co.uk\/wordpress\/wp-content\/uploads\/2017\/10\/20171011_191528-768x576.jpg 768w, https:\/\/www.randomorbit.co.uk\/wordpress\/wp-content\/uploads\/2017\/10\/20171011_191528-1024x768.jpg 1024w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/a>What I see here is CH0 going low and so the outputs are enabled. Surely that means that CH4 should echo CH3 but as you&#8217;ll see, it doesn&#8217;t.<\/p>\n<p>Does that mean I have a faulty 81LS97 or a faulty understanding of what&#8217;s going on. It seems a coincidence that a friend tells me how he fixed his Nascom and mine has the same fault but the trace above suggests just that.<\/p>\n<p>Spooky.<\/p>\n","protected":false},"excerpt":{"rendered":"<p>So here&#8217;s a thing. I recently met up with an old acquaintance who I hadn&#8217;t seen in years and as we&#8217;re both retro-computing fans we were talking about old machines. I have just received an email from an old acquaintance of mine who is a dedicated retro-computing fan and has spotted my plight in these &hellip; <a href=\"https:\/\/www.randomorbit.co.uk\/?p=1260\" class=\"more-link\">Continue reading <span class=\"screen-reader-text\">RetroChallenge 2017\/10 \u2013 Purely by coincidence<\/span> <span class=\"meta-nav\">&rarr;<\/span><\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_coblocks_attr":"","_coblocks_dimensions":"","_coblocks_responsive_height":"","_coblocks_accordion_ie_support":"","footnotes":""},"categories":[47,129,33,98,127],"tags":[],"class_list":["post-1260","post","type-post","status-publish","format-standard","hentry","category-c93-blog-posts","category-nascom-1","category-c84-retro-tech","category-retrochallenge","category-retrochallenge-201710"],"featured_image_src":null,"author_info":{"display_name":"andy","author_link":"https:\/\/www.randomorbit.co.uk\/?author=1"},"_links":{"self":[{"href":"https:\/\/www.randomorbit.co.uk\/index.php?rest_route=\/wp\/v2\/posts\/1260","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.randomorbit.co.uk\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.randomorbit.co.uk\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.randomorbit.co.uk\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.randomorbit.co.uk\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=1260"}],"version-history":[{"count":1,"href":"https:\/\/www.randomorbit.co.uk\/index.php?rest_route=\/wp\/v2\/posts\/1260\/revisions"}],"predecessor-version":[{"id":1263,"href":"https:\/\/www.randomorbit.co.uk\/index.php?rest_route=\/wp\/v2\/posts\/1260\/revisions\/1263"}],"wp:attachment":[{"href":"https:\/\/www.randomorbit.co.uk\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=1260"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.randomorbit.co.uk\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=1260"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.randomorbit.co.uk\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=1260"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}