A power port is a schematic symbol with some special attributes. More on this in a moment.
This page may, before 10 November 2011, have had some errors in it due to confusion in my brain between the roles of pin names and pin and pad numbers. Those errors are now, I hope, fixed. I have a page with more on the pin/pad name/number confusion if you are really curious.
The page you are reading is my most sketchy attempt to talk about power ports.
I've also written a tutorial focusing on power ports, where you can find more extensive help. And there's my tutorial about pin and pad numbers and names/ net names. This also talks about power ports along the way. It is a more theoretical essay, with information on the underlying mechanisms of KiCad in general. It isn't one of the "click this to do that" tutorials.
(I'll remind you of these tutorials at the bottom of this page.)
I am by no means a "KiCad Expert"… I'm just the one-eyed king hoping to help any blind people out there. Power ports, which are valuable and important, give me a headache still. You might also want to read section at the xtronics wiki.
As it is such an important element in most designs, there is a special button on the eeSchema right hand toolbar for going into the "place power port" mode. (Quicklaunch key: "P")
Just before we go on… I have spent the better part of a day editing these pages, changing MANY references to "component" to "schematic symbol". (It is one of the two central terms in any KiCad manual!) The latter is "the new" component. And it is a better name. But! The Nice People at KiCad, who have conducted a similar, but much more taxing, exercise, had, at version 4.0.4 one more place to make the change. In Eeschema, where it should say "Place schematic symbol", it still uses the old term, "Place component". So don't let the references to component in the next paragraph bother you! Think "schematic symbol"… even though you will see "component".
The "Place power port" mode duplicates the function of the "Place component" (Quicklaunch "A") but with a frill… If you invoke "Place power port", you will get something that is actually simply the "Place component" mode, but with the power.lib library of schematic symbols pre-selected for you.
If you connect a power port (more on this in a moment) to a line on the schematic, then by that you will have defined the line as one of your power rails, or as the ground line.
Not only does this help you keep track of these things, but also there are schematic symbols with invisible pins. These pins have names (not numbers) like "GND", "+5v", "Vcc", "Vdd", etc.
If a line on the schematic is connected to a power port called, say, "+5V", then any invisible pins with the same name are also "connected" to that line. When you move on to PCBnew, after mapping the schematic symbols in the schematic to the pads in the PCB design, you will find all of the connections you explicitly required, plus connections that were created through the "power ports" mechanism. It is quite brilliant, actually!
Big Gotcha… cost me about 2 hours one night.
We've talked about using power ports…. "GND", Vcc", etc.
Besides putting a power port on power lines (which includes the "GND" line), for eeSchematic's electrical rules check, you also need to attach a "power flag" to at least one point on "each power line". If your circuit has, say, Vcc and GND, that means TWO power flags. One for each. Even if you've used the "trick" of having a number of "bits" of the circuit only "connected" because each has a GND power port on it, you only need one power flag on the GND line. (You can connect it to any one of the bits which have a GND port.)
And another thing: The "power flag" thing is added the same way you added the power ports. It is in the same list that held Vcc and GND. You use the same schematic symbol, "power flag" on BOTH the Vcc and GND lines. (Two of them, two flags. One for each line.)
If you have several "separate" lines in the schematic, but each has, say, a "GND" power port connected to it, the lines are "connected"… in the design, if not by ink on the "paper". This helps you create nice, clean, clear schematics.
When you place a power port on your "page", be sure that it is connected to the line you may have merely placed it "on". A junction never hurts, and makes sure.
The "did it connect? did it fail to connect?" rule is quite simple: If you place a power port on the end of a line, or on a "corner" (sharp bend) on the line, then it will probably have connected.
What you see when you go to PCBnew will tell you if something connected. You can also try dragging (not merely moving the power port. If the line you think it is connected to is also dragged, the two are connected. (Escape, or ctrl-z, are ways to undo "damage" done by the "try to drag it" test!)
What makes a schematic symbol a power port?
Power ports are schematic symbols… they are just a sub-class, with certain features, just as a lion is a mammal with certain features.
Part of what makes a power port a power port is the fact that the "Power Symbol" box on the "Options" tab of the schematic symbol's properties is ticked. I believe that power ports usually… always?… consist of a single pin.
The "reference" property of a power port is given a name starting with a hash, "#". The eeSchema manual says "Pin names starting with “#”, are reserved for power port symbols."
While it is usually the number of a pin that links it to a pad on the footprint, when power ports are involved, names come into the picture too.
See also: Tutorials
The page you are reading was a mere brief (!) note about power ports. (Well- they are important tools for making your life easier and your projects' designs more reliable!)
As I said at the top of the page, I've also written a tutorial focusing on power ports, if you're looking for more help. And there's my tutorial about pin and pad numbers and names/ net names. This also talks about power ports along the way. It is a more theoretical essay, with information on the underlying mechanisms of KiCad in general. It isn't one of the "click this to do that" tutorials.
You might also want to read section at the xtronics wiki.