HDD header hints

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In this article, we’ll look at one of the most misunderstood little connectors in your PC: the HDD header on your motherboard. When we’re done you’ll know everything you need to use this signal in your mod projects.

The HDD header uses standard .100″ pin spacing.

If you need to replace a female connector, it’s almost always easiest to salvage one from a decommissioned case. But in those rare cases where you just have to buy one: the item to get is a “2 Pin .100″ Header Connector.” Unfortunately the pins are sold separately, so don’t forget to order that. And they’re difficult to crimp without the right tool. Futurlec is one of the few sources I’ve found for these connectors.

The HDD signal does not come from your motherboard.

It comes from the drives themselves. The signal is passsed along to the motherboard from the drives. All the header on the motherboard does is provide a convenient connection point for the case LED.

The HDD signal is carried on pin 39 of the IDE/PATA cable.

If you have a need to access the HDD signal from just one drive, it’s available on pin 39. Interrupting that single wire will allow you to access the signal, and won’t interfere with the drive’s operation. Obviously modifying a cable is far preferable to modifying a drive or motherboard, since replacements are inexpensive.

The HDD signal is active LOW.

What does active LOW mean? Most people appreciate that digital signals have two values, 1 and 0. And most people could tell you that 1 means ON and 0 means OFF. But in active LOW logic, the 0 (LOW voltage) means ON. This is precisely how the HDD signal works.

If you look at the header pins on your motherboard, you;ll see one labeled +. You would tend to assume that this is the HDD signal,and the other pin is ground. The real deal is that the pin marked + is really stuck at +5V with respect to ground all the time. This doesn’t change. It’s the other pin that’s connected to the HDD signal. So when the other pin goes LOW, the LED lights.

The HDD signal that appears at the header is the logical OR of all the signals from the hard drives in your system.

The LED comes on if you access any of your IDE drives. This happens because the motherboard combines the signals with a logical OR function. So your mobo isn’t just a dumb passthrough, it’s doing a little bit of processing for you.

The HDD signal does not have a specific voltage.

This may win some kind of prize for most frequently asked question on forums: “What voltage is my HDD header?” And the reason for asking is reasonable–people want to connect other LEDs besides the stock one. But it’s the wrong question to ask.

Not everything in a computer acts like a voltage source. A good voltage source provides the same voltage into a wide variety of loads. An ideal voltage source provides the same voltage to all loads. A well-regulated power supply is a close approximation of an ideal voltage source. But the HDD header on your motherboard is not.

Lots of people have measured the voltage with their meter, and reported numbers back to various forums. This is a flawed experiment in several ways:

  1. They almost never tell you whether there was a load on the header or not. It makes quite a difference. With a blue LED on, I’d expect to measure around 3.2 volts or so. With a red LED, I’d expect closer to 2 volts. But with no LED, the only load on the circuit is the meter, and the effective load of the meter can vary widely, affecting the measurement.
  2. The answer to the question doesn’t matter. Because the header doesn’t act like an ideal voltage source (the voltage you measure depends on the load), it’s a meaningless measurement anyway.
  3. As noted above, the signal is not coming from your motherboard. It’s coming from the drives.
  4. There’s no guarantee that the measurements taken on any motherboard will match the behavior of any other motherboard+drive combination.

So the thing to do if you want to change the LED is to just drop the new one in. The circuit on the drive limits the current to the LED. You don’t need to add resistors to do this mod. I say this with confidence that comes from having replaced dozens of case LEDs with high-brightness LEDs in esoteric colors (and from reading the ATA spec :D).

Driving other loads besides LEDs from this signal is accomplished safely and easily withan optoisolator. For larger loads, a transistor with a higher collector current should be used to switch the load current. TIP31A is one of my favorites. For under a dollar you get the ability to switch a 3A load. TIP32A is the complement in case you need to do complementary switching.

To use a power transistor to drive a load triggered by the optoisolator, place a 1 kohm resistor in the optoisolator’s collector circuit, ground the optoisolator’s emitter, and connect the collector to the base of the power transistor through a 4.7 kohm resistor. Put your load in the collector circuit of the power transistor. (Since we’re switching, we don’t use the base connection on the optoisolator.)

If you’re looking for more technical details, try my cheatsheet developed from the ATA-6 spec: HDD header hardcore.

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