# What's the difference between a power rail and a signal line?

I've been educating myself on the fundamentals of integrated circuit design, and I've come across the terms power rail and signal line.

It seems that both of these things are moving energy around the circuit, making me think they're two names for the same thing.

Is that correct or are they logically different?

• Your signal line link takes you to "floating node" Commented Mar 1, 2023 at 11:02
• @Andyaka That's where I came across the terminology! Do you think I should change it? Commented Mar 1, 2023 at 12:02
• It's irrelevant to the definition of a signal line. Commented Mar 1, 2023 at 12:08
• @Andyaka I can't find a good reference that describes a signal line. If you know of one I'll update the link! Commented Mar 1, 2023 at 12:15
• About the only system I can think of where the two are equivalent is so called "Dallas 1-Wire" (which has a second wire for ground). That system signals it's data by loading or unloading it's power line for serial communication. Commented Mar 1, 2023 at 20:44

## 4 Answers

Yes, both of them are wires, and provide paths for electrons to flow, so they are the same thing. But you may be missing a subtlety: the purpose of a power rail is to carry power (= energy), and the purpose of a signal line is to carry information. That means when designing a power rail we are more concerned with things like voltage drop and heat dissipation; when designing a signal line we are more concerned with things like interference and reflections.

In the most basic cases, there is nothing to worry about and either role can be fulfilled by any old wire.

If you have a wire carrying lots of current, you may need to make it as big as possible. My computer motherboard has a power rail that is about 5cm wide. Yes - the rail is as wide as the CPU. The ground plane on the other side is just as big. But the shape is not so important. They are big copper squares.

If you have an important signal you may need to carefully design the shape of the wire to make its properties consistent for the electrical signals to reduce signal distortion. If you look at a circuit board with, for example, PCI Express or USB 3 or 10G Ethernet, you may see pairs of tracks with no sharp corners, only smooth curves, and a big gap away from any other tracks. Those tracks carry very delicate very-high-speed data signals. Computer memory busses provide a slightly different example: the tracks do not necessarily follow smooth curves, because the wiring is very short and there is not much room for interference to happen in the first place, but they follow zigzag patterns because it is important that it takes the current exactly the same amount of time to go through each wire.

In radio broadcasting they have wiring that is both high-current and reasonably sensitive, to get 100kW of radio power from the transmitter amplifier up to the antenna. For this they have to use very special types of wiring, such as a metal pipe with another pipe maintained in the center with spacers and the empty space filled with nitrogen gas. 99% of people won't ever encounter these in an electronics career.

Note that the relevant difference is "power" and "signal". "Rail" vs "line" is not important, although it may convey some vague information anyway, just like "lane" vs "road". "Power line" is often used and obvious, but I've never heard of a "signal rail", just like I've heard of neighbourhood streets but never heard of interstate bike lanes.

This answer is written about PCBs. You are working on chips where the constraints are different (e.g. not so important to avoid signal reflections, because the wires are so damn small that the reflections are so fast they don't matter).

• @Connor When a certain part of the wire has different resistance/capacitance/inductance than the part before it, this reflects part of the signal Commented Mar 1, 2023 at 12:03
• @Connor it usually ruins the signal e.g. by mixing up bits with other bits Commented Mar 1, 2023 at 12:12
• @Connor circuits have two sides as current has to balance out; generally, minimizing the area in between the signal and the other side of that circuit will minimize the amount of electromagnetic fields it picks up and minimize the amount it emits. That means running two wires with opposite signals close together (called a differential pair) or one wire above a ground plane. That's separate from the impedance discontinuity thing, which is not about interference from outside, but is basically the signal creating echoes that interfere with itself Commented Mar 1, 2023 at 17:13
• I think part of the reason we have power "rails" is because in industrial hardware, there's often an actual energised rail that every piece of equipment sources its power from. Not to mention, it's not uncommon to see circuits designed with a power line that runs across the top in a reasonably straight line, with offshoots to each component hanging off it. It sort of looks like a rail with things dangling. Commented Mar 2, 2023 at 21:22
• @ScottishTapWater possibly, like a shower curtain rail. Commented Mar 3, 2023 at 10:19

One way to look at it is that a power rail optimises for delivering current (and sometimes high voltage), while a signal optimises for delivering high frequency (since the higher frequencies a wire can carry faithfully, the more information it can convey). Obviously they are interlinked, but these are the designer's primary goals.

Although good engineering design says that you only need to optimise it sufficient to do the job required of it - most signals aren't carrying gigabits of information, so they don't need anything like the kind of special design needed for that, but they may have to fit in a tight space or use a cheap kind of wiring and so that takes priority.

Similarly most chips take milliamps not amps, so they don't need a very complicated power supply design (although sometimes chips take nanoseconds-short but large gulps of current, and the power supply must be designed for that). So we focus on designing something sufficient to do the job required of it.

Of course there are edge cases - electric cars and trains need careful power engineering because there are hundreds or thousands of amps, and good design is the difference between working and an explosion. But most designs aren't quite as critical.

A power supply rail is a DC voltage (think of the terminals of a battery) where the DC voltage remains constant over time or, droops (or rises) very slowly. It carries no important information but can supply sufficient current to the connected circuit to permit it to work effectively. It also has a very low impedance usually.

A signal line carries information in the form of a digital or analogue voltage or current (think of a microphone as producing a signal or a digital communications line transmitting data). It might have a medium to high impedance and, it cannot/won't usually supply very much signal current to the circuits that connect to it. It might be a pure AC voltage/current or an AC voltage/current superimposed with a DC level. Both signal amplitude and frequency-spectral content may be important. It needs handling with care and it can be easily interfered with from noise sources (unlike a power supply rail).

It seems that both of these things are moving energy around the circuit, making me think they're two names for the same thing.

Yes both move energy but, a power rail supplies energy to make the circuit function whereas a signal line's energy is incidental in most cases because it is usually only the voltage or current that is used to carry the information.

Broadly, power rails carry more current.

• It's not just that -- in the example of a radio transmitter, you could have a power rail that's carrying less than a watt (to a preamp or a microprocessor), sitting less than a foot away from an output line that's carefully impedance controlled to 50 ohms and carrying hundreds or thousands of watts. Commented Mar 1, 2023 at 23:54
• Thanks, Tim, and are you saying that output line, carefully impedance-controlled to 50 ohms and carrying hundreds or thousands of watts, is a signal line? Commented Mar 2, 2023 at 21:43
• One could argue either way -- it's certainly not a power rail. Commented Mar 2, 2023 at 22:15