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[ The PC Guide | Procedure Guide | Configuration Procedures ]

Motherboard and Case Connection Procedure

This procedure describes how to make the internal electrical and signal connections between the motherboard and the system case. This includes providing power to the motherboard, and hooking up the LEDs and switches on the case. This is an important part of the assembly of a new system or installing a new motherboard. This procedure does not deal with the physical installation of the motherboard, only with making these necessary connections. See this procedure instead for help with the installation itself.

Procedure Overview:

  • Difficulty Level: 3 (Moderate). Some of the connections can be tricky to make in some circumstances.
  • Risk Factor: 3 (Moderate). You can destroy your AT-style motherboard if you plug the power cords into it backwards. The only reason I don't rank the risk factor even higher is that I took such exceptional steps to warn about this prominently below. :^)
  • Hardware Required: A flashlight is often very helpful for this step, as it can be hard to see the minute printing on the motherboard after it has been installed into the system case.
  • Software Required: None.
  • Time to Perform: 10 to 15 minutes.
  • Preparation / Warnings:
    • If you have not already done so, please read the section on general installation and assembly tips. Pay particular attention to the notes about connecting cables.
    • For reference, you may want to read this section describing the cables and connectors coming from the power supply, and/or this section describing the connections on the motherboard.
    • Motherboards and system cases vary. Your motherboard may not have all the items I mention here, and the same holds true for your case. Furthermore, you may find that your case and motherboard don't match in every situation; for example, some cases have a turbo button and some do not, and some motherboards have a connection for a turbo button and some don't.
    • Watch out for "off by one" errors when attaching cables to pin headers. In particular, some motherboards combine several pin connectors into a larger block. The individual connections are the same, the motherboard just physically groups the pins together into a larger matrix. Be careful when working with these as making a mistake is much easier to do. Refer to the manual.
    • If the system is in a tower case, it is much easier to perform this procedure with the case resting on its side.
    • LEDs have two wires and are unidirectional, so they will not work if attached backwards. You need to connect the positive lead from the case to the positive pin on the motherboard, and the same with the negative. Unfortunately, the case connector almost never has the positive and negative labeled. Fortunately, attaching them backwards will usually not cause any damage; the LED just won't work. One tip you can use: most cases employ for each LED one colored wire (green, yellow, red, orange, blue) and another wire that is either black or white. When this is the case, usually the colored wire is positive (signal) and the black or white wire is negative (ground). This isn't always true, but it's better than a random guess in most cases.

Procedure Steps:

  1. Connect Power to Motherboard: Find the special power connector(s) coming from the power supply and attach them to the motherboard. The details of this step depend on the form factor you are using:
    • AT Form Factor: There should be a pair of six-wire cables coming from the power supply. These go to the 12-pin power connector on the motherboard. The connector has 12 pins in a long row. Now this is important: the four black wires go together in the middle and the colored wires go on either side. (You may be wondering: why did they do this, make two six-wire connectors that can be so easily mixed up? Good question, but it's been this way since the earliest PCs!) The connectors can be a bit tricky to get onto the pins due to the funny plastic tabs that some of them have. Be patient.
    • ATX Form Factor: There should be a single cable with up to 20 wires in a 2 by 10 rectangular configuration. Somebody wised up here and keyed this connector, so you cannot insert it incorrectly.

Warning: If you are careful about nothing else in this procedure, be careful about this step if you are working in an AT system. If you connect these cables backwards, I see a new motherboard purchase (and maybe more) in your immediate future. Seriously.

  1. Connect Power to CPU Fan: Attach the connector coming from your CPU fan to one of the four-wire power cables coming from the power supply. Many of these fans actually have a dual in/out "pass-through" connection that lets you connect the fan and still leave a connector free to attach to another device. Some CPU fans use a two- or three-hole mini-plug that gets power from the motherboard instead of the power supply directly. If your motherboard has a matching connector, you can use this; otherwise, you will need an adapter or a different fan.
  2. Connect Case Power Switch (ATX systems only): On ATX systems the power switch on the case attaches to the motherboard and not the power supply. Find the appropriate two-pin connector and attach the wires coming from the switch to them. Make sure you don't mix this up with the reset switch.
  3. Connect Turbo Switch (if applicable): If your case has a turbo switch and your motherboard has a connector to which to attach it, connect it now if you so desire. Bear in mind that in modern PCs, even if the motherboard has a place to connect this, it doesn't really do anything. If there is no place to connect this or you decide not to bother, then roll up the wires coming from the switch and tuck them away somewhere out of the way within the case. Note that on older PCs, the system may not function properly if the turbo switch is not connected (or it may only function in creaking "non-turbo" mode).
  4. Connect Reset Switch: Find the wires coming from the reset switch and attach them to the appropriate pin connector on the motherboard. This switch has no polarity so it doesn't matter which way you connect it to to the motherboard.
  5. Connect Power LED and/or Keylock Switch: OK, this one is a bit of an oddball that depends entirely on the system case you have. Most motherboards have a five-pin connector that is used for both connecting the power LED and the keylock switch. The pins are normally laid out as follows:

Pin #



Power LED Output (positive)


No Connection (NC)


Power LED Ground (negative)


Keylock Signal (positive)


Keylock Ground (negative)

    The reason that these two seemingly-unrelated functions are together in a single, five-pin header on the motherboard is that older cases had the four signals attached to a single, five-pin connector that matched. You may find one of these in your case, or you may find two separate connectors, one for the power LED and one for the keylock switch. Make sure you connect them properly, and refer to your motherboard manual for details on how to align the connectors. Some cases of course do not have a keylock switch, so you will leave those pins unconnected in that situation.

  1. Connect Turbo LED: Find the wires that come from the turbo LED on the case and connect them to the appropriate place on the motherboard. As with the turbo switch, this is a bit of a leftover from years gone by and may not be present on all systems. In addition, on modern systems even if there is somewhere to connect it, the motherboard won't likely actually be controlling it. I usually connect it anyway to avoid concerns about why the LED isn't lit, but it really is entirely optional on newer systems. You may use this LED for other purposes if you wish; for example, on a system with a SCSI host adapter you can connect the LED to the host adapter and use it as a SCSI activity indicator.
  2. Connect Hard Disk Drive Activity LED: Find the appropriate pins on the motherboard and connect the wires coming from the hard disk activity LED to them. Here again, there is some variability among motherboards. Some have two pins for the LED, which makes connection simple (as long as you watch your positive and negative). However some have four pins. In this case, only two of the pins may work, and you have to make sure you get the right pair. On other boards, however, you can use either pins 1 and 2, or pins 3 and 4. Consult your motherboard manual.
  3. Connect Speaker: The system speaker uses two wires to connect to the motherboard. Conventionally, this is done using a four-pin connector and header, with the two wires going on the outside (pins 1 and 4). The motherboard should have a matching 4-pin header. Some cases no longer have the four-pin connector, and instead just have two single-pin connectors. If yours is like this, simply attach to pins 1 and 4 on the motherboard header.
  4. Double-Check Connections: Check over the connections you just made to ensure that they are correct. Make sure you haven't accidentally loosened anything, attached connectors to the wrong pins, or left out any connectors that come from your case. Check to make sure that the wires coming from the case are not interfering mechanically with any of the internal components on the motherboard, especially the CPU fan.

Next: External Peripheral Connection Procedure

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