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Serial Breakout Cables: Not Created Equal

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#1 systems_glitch


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Posted 08 July 2012 - 10:14 AM

I wasted 3 or 4 hours last night debugging one of my homebrew 8085 projects. Specifically, I was trying to figure out why I was getting garbage out of the new etched-PCB serial I/O board. Not only was there garbage from the new board, but also the known-good prototype! Turns out the cable I was using to go from the 5x2 header on the board to a DB-9M wasn't standard pinout!

Modern 5x2 to DB-9M cables follow the same pinout as the actual DB-9M: pin 1 on the dual row is pin 1 on the DB-9M, pin 2 is pin 2, et c. Apparently this was not always the case with old breakout cables from old x86 motherboards! I grabbed one out of the box 'o cables not realizing this. The particular cable I pulled out follows a "staggered" pattern; that is, pin 1 goes to pin 1, pin 2 goes to pin 6, pin 3 goes to pin 3, pin 4 goes to pin 7, et c. It's easy to figure these out by opening the strain relief housing on the back of the connector.

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