TRS-80 curiosity

On Fri, 14 Apr 2017, Bill Gunshannon via cctalk wrote: Possibly very similar, Have you counted the pin numbers to determine which traces the parts are connected to? If the component resistors, transistors and capacitors perform compatible "buffering" functions, they may be functionally compatible. Both designs sound like kludges to deal with problems that surfaced when straight-through cables were used. If you count pins and identify the components, or take a picture, then Allison might be able to identify the details! That's due to a fluke in TRS-80 history. Even before the EI was available, RS had a "screen printer" that connected to the 40 pin expansion bus of the CPU. "To avoid confusion", and to avoid explaining what a bus was, RS therefore, called that 40 pin connector the "Screen Printer Port". http://www.trs-80.org/trs-80-screen-printer/ It sold for $599 (as much as the computer!) That continued to be available for a while after the EI came out. Owners of it "had to be" supported. The EI has a continuation of the 40 pin expansion bus. But, rather than try to explain it, RS simply showed them a picture of where the EI 40 pin expansion bus "continuation"/"output" connector was, and told them to plug their 40 pin screen printer into that connector. "To avoid confusion", and to avoid explaining what a bus was, RS therefore, called that 40 pin connector the "EI Screen Printer Port". Because RS had PROBLEMS with the EI, http://www.trs-80.org/radio-shack-expansion-interface/ they ended up coming out with multiple versions of the cabling, including the "Screen Printer" cable specifically including "buffered" cable with a box. I think that the screen printer cable was 40 pin to 40 pin? They also made a 40 pin to 34 pin adapter for connecting "Centronics style" printers to the CPU bus. It also would probably have been labelled "screen printer port", http://www.trs-80.org/printer-interface-cable/ One 50 pin to 40 pin adapter would be the one to connect the screen printer to the model 3. Insane thing to do, but we, the TRS-80 users, were rarely accused of sanity. Another 50 pin to 40 pin adapter would be Model 1 to hard disk. The connector on that for the model 1 would probably have been labeled "EI Screen Printer Port". All of these cables would have been easiest to build as a board with two gold-plated card-edge connectors, and then use female card-edge to female card-edge cables for connection (except 26-1401 female card-edge to "blue-ribbon" for "Centronics style" printers). But, RS cut corners and did not gold-plate the EI connectors! That came back to bite them on the ass (similarly to the way Apple's crappy sockets bit them), and therefore, rather than risk another card-edge bad connection, some of their stuff was made with attached cables, instead. -- Grumpy Ol' Fred cisin at xenosoft.com