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So, I recently discovered a quite painful mistake in my PCB regarding the PGA socket numbering. I have used a row numbered I in my footprint, while the package has no such row. The numbering of the package rows goes alphabetically, from A till H, skips I, and then continues on with J. I was not aware of this. Consequently, the actual row J on the package has no connection, row K is connected to the pins of row J instead, and so on ...

When looking up if I had just stumbled across an exotic PGA package, or if this really was a convention, I found evidence that it probably is the latter. Here are a few datasheets where every time the row I is missing:

This leads me to 2 questions:

  1. Is it indeed an industry standard to skip row I?
  2. Why is this done? I immediately thought of confusion between the number 1 and the letter I. Or confusion between capital I and lowercase l ? But the first does not make sense because you know the first symbol must be a letter, and the latter is easily avoided if you always use capital letters... (or if you consequently use lowercase).
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  • \$\begingroup\$ Don't assume that the uppercase I, lowercase l, and digit 1 have a significantly different appearance in all possible fonts. \$\endgroup\$ Mar 9, 2022 at 17:12
  • \$\begingroup\$ @ElliotAlderson Absolutely! I am also not saying that. I mean that you can easily deduce from context if the first symbol in the pin designator represents a number, a lower case letter, or an upper case letter. Once you know that, there is no confusion possible anymore. \$\endgroup\$
    – mxt3
    Mar 9, 2022 at 19:48
  • \$\begingroup\$ @ElliotAlderson The US military seems to prefer sans-serif fonts. Eg. MIL-STD-130. The differences are less distinct without serifs. \$\endgroup\$ Mar 9, 2022 at 19:50

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This is also standard in military-style connectors, no doubt for the reason you state (to prevent confusion between '1' and 'I' as well as '0' and 'O'). 'Q' is also skipped. Image from this answer but ultimately from MIL-STD-1560.

enter image description here

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  • \$\begingroup\$ That's even more remarkable! With what would one confuse the letter O in this case? There is no number (no zero) at all in the numbering of the contacts in that connector... \$\endgroup\$
    – mxt3
    Mar 9, 2022 at 19:46
  • \$\begingroup\$ If you never use I, O, Q you never have to worry about whether there are numbers involved or not. \$\endgroup\$ Mar 9, 2022 at 19:51

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