Reference designators for electrical and electronics parts and equipment

06

Appendix E: KiCad Anomalous Handling of Suffix Letters.

The programmers who have developed KiCad have only allowed for suffix letters to be used with multiple-element parts.

The programmers who have developed KiCad have only allowed for suffix letters to be used with multiple-element parts. An example is a quad 2-input NAND gate. The reference designator for this part would be U# and each gate would be assigned a suffix letter A, B, C, and D. Thus on a schematic diagram each of the gates can be used in different places on the schematic, but with the suffix letters you know that it is one part. And this is how the part would be listed on a parts list.

The ASME Y14.44 standard, however, allows for the use of suffix letters, “...also used to identify several separate connectors...” Here are three examples and the recommended workaround:

  1. 1
    Given that you have a fuse with a fuse holder. The reference designator of the fuse would be F# and the fuse holder would have a reference designator of XF#. The problem is if you use a pair of fuse clips. Then the reference designators of the two fuse clips would be XF#A and XF#B, and this is how these parts would be listed on a parts list, so that you have two fuse clips. However, KiCad cannot handle suffix letters of individual parts. If you try to add them to the reference designator you’ll end up with XF#A1 and XF#B1. Not so good because reference designator A1 would indicate a separable assembly and B1 would indicate a motor. My recommendation is to use reference designators XF#E1 and XF#E2. The E class letter in this case would mean a terminal or miscellaneous electrical part.
  2. 2
    Given the scenario presented in ASME Y14.44 Fig. 2(e), where there is a printed circuit board assembly (PBA) with reference designation of A5 that has two sets of fingers that plug-in to two different connectors. The PBA is reference designated A5 and so the sockets that mount A5 would be reference designated XA5A and XA5B, which again KiCad can not handle. My recommendation would be to use reference designations XA5J1 and XA5J2.
  3. 3
    If you wanted to replace one resistor with two resistors either in parallel or series you would use suffix letters. Let’s say you wanted to replace R7, a 51 Ω resistor, with two 100 Ω resistors in parallel, without disturbing the rest of the series of reference designated resistors. You would reference designate the two resistors as R7A and R7B. However, again KiCad can not handle this situation. My recommendation would be to reference designate the two resistors as R7R1 and R7R2.
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1. Reference designators for electrical and electronics parts and equipment
2. Appendix A: A list of pertinent applicable standards
3. Appendix B: Class Designation Letters
4. Appendix C: List of nonconforming class letters
5. Appendix D: System subdivision
6. Appendix E: KiCad Anomalous Handling of Suffix Letters.
7. Appendix F: Scenario Questions and Answers.

About the author

The articles in this series are written by Lawrence W. Joy (Larry)
[email protected]
Michigan USA.

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