Mode B vs. Mode A: What's up with that?

Question: “I hear a lot of stuff about Mode A and Mode B when people are talking about Power over Ethernet. What’s up with that? Does it matter, and is one better than the other?”

Yes, it matters which Mode you use. And no, one isn’t better than the other.

The longer answer requires some explanation.

The ABCs of  Mode A and Mode B

Before we get too far into this, we should define Mode A and Mode B. In PoE, Mode A refers to power being injected onto the Ethernet cable on pins 1,2,3, and 6. Mode B means injecting power onto pins 4,5,7, and 8.

Here you can see Mode A, with power on pins 1, 2, 3, and 6….

And here is Mode B, with power on pins 4, 5, 7, and 8…

Did that make perfect sense to you? If so, you can skip over the next section where we discuss pins. If not, don’t worry, we’ll explain…

On Needles and Pinouts (wait, what are pins?)

If the section defining Mode A and B was no help to you at all, then it might help if we add to your vocabulary. Here are a few terms you should know:.

• Pin – An electrical connector. The eight metal stripes you see when you look at the end ends of your Ethernet cable are pins.
• Pinout – The diagram that defines which pins need to connect to which other pins in order to function properly. Mode A and Mode B are two different pinouts.
• RJ45 – The connector type usually found at either end of an Ethernet cable.
• Ethernet cable – The cable commonly used to carry data between devices in a computer network.

To Mode B or Not To Mode B…

That is the question!

99% of the time, you can determine which Mode to use by looking at your device’s required PoE voltage. Devices that are powered with 24 volt PoE always use Mode B. That’s just the way it is.

Devices that are powered with 48 or 56 volt PoE will almost always power up in Mode A or Mode B, but for the sake of simplicity you can think of Mode A as the default for these. Every now and then, a company manufactures a 48 volt PoE device that only works on Mode A or B. This is contrary to the PoE standard set by The Institute of Electrical and Electronics Engineers (aka the IEEE), but it turns out that they don’t have the power to arrest people for violating their standards.

At any rate, it’s easy to resolve this by switching to the other pinout.

What was the question again?

Oh, right. “Does it matter which pinout you use, and is one better than the other?”

It matters when powering a 24 volt PoE device, because it will require Mode B. When powering a 48 volt device, it shouldn’t matter but sometimes it does. So go ahead and use Mode A with 48 volt devices like IP cameras and VOIP phones.

Is one better than the other? No. Both Mode A and Mode B perform their jobs well, as long as they’re the right mode for the job.   

Now how do I find a Mode A or B device?

Now you know what to look for, just check on the product listings and datasheets to see what kind of device you’re looking at. As an example from our PoE Texas product listings, you’ll see something like this for a 24 volt Mode B product:

You can also look through our product listings organized by type for the right ports and voltage.