Reversing Polarity: It's Probably Not the Cable Assembly

[Kaleb_Kohlhase]

We have received a number of inquiries regarding cable assemblies claiming that they are “reversing polarity” but there is absolutely no way the wires are crossing. I am writing to try to clear up misconceptions that seems to be somewhat common. There are cases where a rectangular cable assembly is built to reverse the polarity, but these are not the default design.

Label Misconception

When you learn about electronics for the first time and come across a schematic, you are going to come across pinout numbers and labels. Connector manufacturers often do assign arbitrary labels to their connectors, but these are not always going to “coincide” with the same order as the schematic. You may not even see numbers on these holes, they may only exist in the drawing. The labels are only there to act as a potential wiring guide to add to wire labels. These do often end up being useful as part of instructions for an assembler putting together wiring harnesses.

Physically Printed Labels Can’t Swap to Match

Here is something important to remember, labels cannot physically change when you rotate a part. Here is a useful diagram to help establish this concept:

Even if they don’t have physical labels on the part for pin order, the drawing is only using these as a suggestion. The only way you can match the printed pin order is to flip the other side upside down.

Connection Order Mis-Match Doesn’t Swap Polarity

I have also seen cases where reversed label order must mean that the polarity swapped, see the diagram below:

Mating Design Misconception

The other problem that I think causes confusion is that it is assumed that the pinout must be rotated 180 degrees on the other end. Generally speaking, most devices assume a basic pinout order. If you did rotate a mating connection 180 degrees, this is the outcome:

The orange section is the harness in the middle and the blue connectors are the mating sides. Notice that I gave specific shapes to the mating connector positive and negative sides. The harness would be wrong in this instance as polarity did swap…from the pinout perspective.

Instead of rotating 180 degrees, most interfaces are mirrored across something called the “mating face.” Here is a top-down visual:

Caveat

You have to be really careful when making this assumption though. There are rare cases where pinout is not what is considered “typical.” This is why harness design is important. If you aren’t seeing the mirrored pinout, you’d have to modify the harness accordingly.

Industry Standard Order

Naturally, if there are pin orders on connectors, flipping 180 degrees will cause the other side to be in reverse order. This is generally the “correct” way of doing things.

99% of the time the signal order is going to be the same on both sides regarding the actual board pinout.