My intel tells me this is the motherboard for an eMachines T3642 Desktop PC,http://www.ebay.com/itm/GATEWAY-EMACHINES-W3644-MCP61SM2MA-4006232R-MOTHERBOARD-T5246-T3646-T3642-S3649-/330737918167?pt=Motherboards&hash=item4d018420d7
If this holds true,http://support.gateway.com/s/MOTHERBD/Shared/4006232R/4006232Rnv.shtml
In the list click on the blue - Product Views
The Ebay link shows you where the letters, of the above Gateway link, are on the motherboard.
1) B - Auxiliary 12 Volt ATX connector.
This is where the 4-pin ATX +12 Volt power cable plugs in,
Example of power cable from Power Supply,http://www.playtool.com/pages/psuconnectors/connectors.html#atx12v4
Power for the Processor (CPU) Do NOT forget it.
2) E - Main Power Connectorhttp://www.playtool.com/pages/psuconnectors/connectors.html#atxmain24
Note* Color of connectors for 1) above, and 2), do NOT matter.
What the connector is, Does.
Note also that there is a pivoting Lock, on both power cable connectors.
The matching motherboard connector has a Lock TAB.
When the power cables are deemed to be plugged in tightly, and correctly; the Lock's hooked end will be over the Lock Tab.
[Squeeze the top of the Lock in, to release. Release when removing.
The Lock works on the same action as a seesaw on a playground ]
3) F- Floppy Disk Drive Connector
A 34 wire thin ribbon IDE cable, connects here on the motherboard, and plugs into the back of the Floppy Drive. The cable is the Data cable.
The power cable that goes to it is a 4-pin Small peripheral power cable. Looks like this example,http://www.playtool.com/pages/psuconnectors/connectors.html#floppy
IF a Memory Card Reader, (Media Card Reader. Same/same) is used instead; it will use the SAME power cable.
The data cables are different though. They connect to a Front USB header on the motherboard, and are a rounded cable.
ONE of the Front USB headers on the motherboard, will be used for the Card Reader.
Click on S - USB 2.0 Connectors
[NOTE* In EACH USB header are provisions for attaching to TWO USB ports. Front OR back of computer.
One will be used for the TWO data cables, going to the Card Reader. Which one? Pick one.
BIOS will set up a new Interrupt Request for it. (IRQ) ]
G - Parallel ATA IDE Connector
40 wire flat ribbon data cable goes here. (ONLY 39 wires are actually used, though)http://en.wikipedia.org/wiki/File:PATA-cable.jpghttp://en.wikipedia.org/wiki/File:ATA_cables.jpg
Now I'm going to get a little lengthy, and detailed;
In the first link immediately above, PATA-cable.jpg
See the two small 'ribs' on the Top/Middle of the connector?
(May just be one solid block, also)
That is a Locating Lug. When you plug the connector into the motherboard, it lines up with a Cutout in the motherboard connector.
IF there is NO Locating Lug;
Look at the side of the cable that has the faint Red stripe.
This Red stripe is on the SAME side as Number 1 wire.
When plugging into the motherboard connector, the side of the cable that has the Red stripe, goes DOWN. Down towards the Bottom of the motherboard.
Number 1 pin in the motherboard connector, is at the Bottom of the motherboard connector.
If no Locating Lug when plugging into a device; Harddrive or Optical Drive, (CD/DVD drive);
The side of the flat data ribbon cable that has the Red stripe, goes towards the POWER CABLE pins, on the device.
Let's look at an IDE (PATA) harddrive, and the 4-pin Peripheral power cable that plugs into it;http://www.playtool.com/pages/psuconnectors/connectors.html#peripheral
See where the power cable plugs in? The Red stripe on the IDE flat ribbon cable, will be next to the power cable.
Also can be said as, when the IDE flat ribbon cable is plugged into the Harddrive, the Red stripe will be on the Right.
Same with an Optical Drive.
You MAY have an optical drive's flat data cable plugged in here.
IF it is an IDE (PATA) optical drive.
MORE than likely though, your Harddrive is a SATA unit, and plugs into a SATA connector on the motherboard.
U - SATA Connectors
Note there are two. SATA1 and SATA2.
The Harddrive is supposed to plug into SATA1.
The beauty of SATA is though, that you can plug the Harddrive into SATA1, OR SATA 2.
BIOS will find the device.
This is an example of a SATA data cable, and it's connector,http://en.wikipedia.org/wiki/File:SATA_ports.jpg
(Your SATA connector on the motherboard, is different than the example shown)
Between a SATA data cable's connector, and a SATA power cable's connector; a SATA power cable connector is the Larger of the two.
A SATA data cable connector has 7-PINS.
A SATA power cable connector has 15-PINS.
Note also the L-shaped opening. This lines up with an L-shape on the Harddrive connector, or an Optical Drive connector. (Data cable connector)
Note also that a SATA power cable has this same L-shaped opening.
More to follow in a Comment.