Why is KiCad regularly changing my IC References?

Hi guys. I have a fairly simple schematic I am working on and I have it laid out exactly as I want it. It splits logically across a few sheets which seems to cause no problem. It is based on a few 74series chips and a number of opto-isolator chips. The logic chips actually used are 74Fxx series, (actually 04 and 06), which are not in the libraries but I assumed I could use the existing 74LSxx and simply change the Value field to suit.

The optos cause no problem at all but the logic chips have an irritating quirk. I have them all referenced as IC10:A-G, IC11:A-G etc with their sections split across the sheets and they seem happy, I have no errors or warnings at all when running the ERC. Then when I’m working, seemingly from nowhere, I find that they have been renumbered as IC10:1A-1G. Kicad adds an additional “1” in before the letters. It isn’t always the entire set of gates, sometimes it is only a few at a time. Occasionally I have even seen this appear as a “2” on the power G section with the gates still “1”.

I could simply accept it and renumber but I am manually porting an existing PCB from a previous old piece of incompatible software to KiCad so keeping the same numbering is preferable if not essential for backwards compatibility.

I have no idea what is going on here, this should be a straightforward schematic layout but it has this irritating quirk. Can anyone suggest what is going wrong please?

KiCad has a quirk in that the References always have to end in a number. KiCad is not satisfied if the references are merely “unique”. I assume your A to G are from the unit, and that makes the colon the last character of the reference itself. If you remove the colon then the base name of the reference will be IC10 and that is OK because it ends in a number.

Also, do yourself a favor and get used to the industry standard practice of labeling IC’s with U. This is also the default in KiCad, so if you just accept these defaults, it is less work for you. It is also shorter, so easier to place the silkscreen text on dense PCB’s.

Ahahaha! Thanks for the heads up Paul, that makes perfect sense now.

I would certainly adopt the standardised “U” classification, I completely agree it is the way forwards with new work. It would make no real difference if it were only for the schematic but it is of course set to display on the PCB too. I’m working on something I inherited from a long way down the line in years and versions and the people above my head, (non-technical), want things kept with full compatibility. I certainly agree with your point though and will stick to modern standards with new work but I can’t enforce that this time around.

Problem solved and I can now work around this. Many thanks for your help.

I just discovered there is quite an exensive list for:

It lists both U and IC for IC’s. (but says U is recommended). Also deviations from this standard are quite common. I have never seen BR for bridge rectifier, they are usually just a D for diode. and Optocouplers (OP accordint to the table) are usually also the generic U for IC’s.

If you are reverse engineering (or otherwise working on) an old project, then staying with whatever nomenclature is used in that project often makes more sense then converting it to some standard.

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