{"id":1880,"date":"2011-10-25T23:15:10","date_gmt":"2011-10-26T03:15:10","guid":{"rendered":"http:\/\/www.willegal.net\/blog\/?p=1880"},"modified":"2011-10-25T23:15:10","modified_gmt":"2011-10-26T03:15:10","slug":"brain-board-74ls244-grounding-issue","status":"publish","type":"post","link":"http:\/\/www.willegal.net\/blog\/?p=1880","title":{"rendered":"Brain Board 74LS244 Grounding Issue"},"content":{"rendered":"

The Brain Board review in Juiced GS included a description of a bad 74LS244. I have had one other report of problems with the Brain Board in an Apple IIe. In that second case, I had that person send back his board, twice. Though I wasn’t able to reproduce the problem in my Apple IIe with his board, I tried two different repairs, the second of which seems to be successful. The first was a simple swap of the 74LS244, which helped, but didn’t completely eliminate the problem.<\/p>\n

After further analysis, I determined that the grounding of the 74LS244 is less than optimal on the Brain Board and switching on of the 74LS244 causes a lot of noise on the ground. Apple boards that use a similar data bus driver circuit (Apple FW card & Super Serial) also exhibit this switching noise. The stock Brain Board is somewhat worse in this regard. I found that adding an additional ground wire on the 74LS244 reduces noise to levels lower than the Apple FW board and appears to solve the problem.<\/p>\n

This intermittent operation is typically seen as random characters input while at the console prompt, while not actually typing on the keyboard. If you notice this problem, add a wire between the ground pin of the 74LS244 and the ground lead of the decoupling cap near the other end of the edge connector.<\/p>\n

Here is an image of a board with the fix applied.
\n

\"brainboard<\/a>

brainboard 74LS244 ground wire fix<\/p><\/div><\/p>\n

One more word about this problem. Though I didn’t know of any issue with the ground, when laying out the production Brain Board, I considered shortening the ground connection to the 74LS244, running it across the center of the board, instead of the top. This change might have prevented this issue. However, I decided that, since, in several years of operation of the prototype “Brain Board”, I had seen no issues, that it was unnecessary. This is a case where the “if it ain’t broke, don’t fix it” approach, was the wrong approach. For now, the one wire fix will have to do. If I make another run of these boards, the ground trace will be widened and rerouted using the shortened path I considered before.<\/p>\n

Also because of this issue, I’m going to have to make major changes to the layout of the protoboard that I have described earlier in this blog.<\/p>\n","protected":false},"excerpt":{"rendered":"

The Brain Board review in Juiced GS included a description of a bad 74LS244. I have had one other report of problems with the Brain Board in an Apple IIe. In that second case, I had that person send back … Continue reading →<\/span><\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[4,3,14,7],"tags":[],"class_list":["post-1880","post","type-post","status-publish","format-standard","hentry","category-apple1","category-apple2","category-brain-boardwozanium","category-apple"],"_links":{"self":[{"href":"http:\/\/www.willegal.net\/blog\/index.php?rest_route=\/wp\/v2\/posts\/1880","targetHints":{"allow":["GET"]}}],"collection":[{"href":"http:\/\/www.willegal.net\/blog\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"http:\/\/www.willegal.net\/blog\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"http:\/\/www.willegal.net\/blog\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"http:\/\/www.willegal.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=1880"}],"version-history":[{"count":31,"href":"http:\/\/www.willegal.net\/blog\/index.php?rest_route=\/wp\/v2\/posts\/1880\/revisions"}],"predecessor-version":[{"id":1912,"href":"http:\/\/www.willegal.net\/blog\/index.php?rest_route=\/wp\/v2\/posts\/1880\/revisions\/1912"}],"wp:attachment":[{"href":"http:\/\/www.willegal.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=1880"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"http:\/\/www.willegal.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=1880"},{"taxonomy":"post_tag","embeddable":true,"href":"http:\/\/www.willegal.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=1880"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}