Step 10:
In/Out Connector Installation

This step does not apply to the ATX form factor type of motherboard, because I/O connectors are pre-installed. So, if you have this mobo, continue on to the next step:
Step 11 - Connecting the Motherboard.

For AT & Baby AT Motherboards

For those with AT and Baby AT form factor mobos, the following are the first steps to connect the motherboard to the components of the computer.

Do the following:

Ground yourself.
AT motherboards usually come with slot inserts with parallel and serial ports in them. These are screwed into expansion slot bays in the back of the case.

Since AT computers are older, you may wish to install expansion cards. However, you will notice your expansion slot bays are occupied. The answer is to move those slot inserts. Many cases will have vacant port holes that are covered with metal caps. These are located above the regular card slot bays. Simply remove the metal caps and use the holes as dedicated parallel and serial ports.
If you install the I/O ports into dedicated slots on the case, make sure you select those that fit your I/O ports, such as 9-pin or 25-pin. Unscrew and remove the cover from these slots.
Connect the I/O ports to the motherboard. The motherboard manual will describe the method appropriate for your motherboard, including the identification of the correct connectors for the separate ports.

Always be careful to line up the red side of the ribbon cable with pin 1 on the connectors.

In general, the port connectors are usually labeled "PRINT" for the printer or LPT1, then COM1 and COM2. The serial mouse's 9-pin connector usually connects to COM1.

If you are using a PS/2 mouse, the connector is attached in the same manner. Install the insert near the PS/2 connector on the motherboard. Then connect the mouse cable to the connector, which is usually a few pins sticking straight up from the board.