If you have ever assembled a PC or have seen one inside, you will know that the motherboard needs two connections from the power supply to work, divided as a general rule into two connectors : the 20 + 4-pin ATX and the 4+ EPS. 4 pins . In this article we are going to address why the power supply of the motherboards is divided into several connectors, and why they are not unified in one to make things easier for users.
As you know, the power supply has many different connectors to service the different hardware components, including motherboard, storage devices, graphics card, and so on. Specifically regarding the motherboard, there are two connectors that go directly to it, the EPS that can be 4 or 8 pins (that is why it is called 4 + 4 pins), and you should know that some high-end motherboards require two of these connectors, and the 20 + 4-pin ATX, which continues with that split design despite the fact that only some low-power models require 20 pins, the rest the full 24.
What are the ATX and EPS connectors on the motherboard for?
The 20 + 4-pin ATX connector is in charge of supplying power to practically all the components of the motherboard with the exception of the processor, and that is why we have the 4 + 4-pin EPS connector that, in many sources, comes directly labeled as CPU, as it serves to exclusively provide power to the processor. The 20 + 4-pin ATX is then for everything else, which includes USB ports, PCI-Express sockets, RAM, etc.
In the diagram above you can see the pin distribution of the 20 and 24-pin connector of the power supply. It can be seen that, among the connectors, we have the three voltages used by the power supply to service the PC: + 12V, + 5V, + 3.3V and even the already disused -5V.
For its part, the EPS connector has the following pin distribution:
The difference is more than evident between the two connectors, since the ATX supplies different types of voltage while the EPS only supplies 12V, exclusively for the processor and its voltage controller (the famous VRMs on the motherboard that are the which ultimately filter and convert the voltage that reaches it to supply what the processor needs).
Why aren’t the two connectors unified into one?
Now that we know what each of the two connectors that power the motherboard does, it is inevitable to ask the question: if the ATX connector already supplies + 12V, why do we need the EPS?
The answer to this question lies in how the power supply works, but also in how both the motherboard and the processor work; the power supply converts the alternating current that reaches it into 12V direct current, and then internally converts that voltage back on the 5 and 3.3 volt rails that are the ones that supply -among others- to the ATX connector. The goal of doing all of these conversions is to provide the motherboard with voltages that are closest to what it needs, so that the motherboard’s voltage conversion effort takes minimal work.
Thus, if for example the motherboard needs 1.35V to service the RAM memory, it will use the + 3.3V rail since it is the closest, but when we talk about the USB ports for example, then it will use the rail of + 5V without the need to convert anything. This makes everything more confusing if possible because if the processors work in ranges that barely exceed 1 volt of current voltage, why then do they give 12V?
The answer is simply by margins and by control. Motherboards and especially high-end and overclocking-oriented motherboards have a complex conversion and filtering circuit in their VRM (Voltage Regulator Module) to fine-tune the voltage that is supplied to the processor. Since the operation and speed of the processor is voltage dependent, the processor is supplied to the nearest thousandths (sometimes even ten thousandths), which cannot be guaranteed by the voltage converter in the power supply itself, as it provides power from a coarser and not so fine way.
All this surely makes a lot of sense for you and justifies the fact that different cables are used, since they go through different circuits, but why then use 12V if you could use the + 3.3V rail? The answer is by margins. A processor, despite operating with a voltage of around one volt, actually works with a fairly high current intensity (Amps) which triggers consumption. For this reason, the + 12V rail is provided, which is the highest that a standard PC power supply can give, so that the VRMs on the board have all the margin in the world to be able to allocate resources to the processor.
So as a summary, it was decided to use two different cables from the power supply to service the motherboard to leave one exclusively for that fine adjustment that processors need. They could really be unified into a single connector if they wanted, but it would be practically like making a 32-pin (24 + 8) connector to simply put together all the pins that are needed for operation, no more, no less.
