ATX motherboards probably always locate the COM1 serial port connector on the motherboard ATX rear panel, but COM2 often comes unwired from the motherboard. To use COM2, you add an internal serial cable from the motherboard to a bracket in a PC rear panel PCI slot. This slot bracket is not shown here, but you surely need one with a bracket too. The motherboard end of the internal serial cable has a 10 pin IDC connector (or perhaps IDS, both fit the same motherboard header socket).
It is very important to know that there are two common but incompatible ways of wiring internal serial cables. My PC recently needed one to enable COM2 on an Asus AV8 Deluxe motherboard. I discovered that what I needed, and what I had, were both rather hard to determine. This page is intended to at least show you what you have. If you don't know what you need, then for a couple of dollars each, just order one of each type, and one of them should work. You are way ahead by just knowing that there are two types.
What is available on the retail store shelves is usually the Cross-wired type. However those retail store cables are usually not marked as to what they are, so you really don't know. You have to remove the hood and actually check the wiring to know what you have. The two types are very distinctive then. The other test is that one will work, and one will not.
Knowing what you need is harder to accomplish. Perhaps your motherboard manual shows the functional wiring of the header for the COM2 connector, but Asus doesn't show these pin numbers, at least not in my A8V manual.
However, Asus wants the Straight-through type internal serial cable.
Straight-through wiring for Asus. It is straight-through because the ribbon cable wires go to the same pin number on the DB9 connector. The DB9 pins are numbered (in this view) across the bottom side from 1 (red wire at top end), 2, 3, 4, and 5. Then 6 on the top side (in this view) runs back to the original end, then 7, 8 and 9. The straight-through wiring has this distinct look of four wires routed back to the other side. Straight-through is sometimes called Everex wiring.
DB9, male pins, numbered from solder side here
Ribbon DB9 - Straight-through
1 ---------- 1 DCD
2 ---------- 2 RX
3 ---------- 3 TX
4 ---------- 4 DTR
5 ---------- 5 GND
6 ---------- 6 DSR
7 ---------- 7 RTS
8 ---------- 8 CTS
9 ---------- 9 RI
One source for a Straight-through cable
Crossed wiring. Regardless of the name, this cross-wiring appears very orderly, alternating top and bottom rows with each next sequential ribbon wire. It may appear orderly, but the cable is cross-wired due to DB9 pin numbering. Cross-wired is sometimes called Intel, or DTK wiring type, however it is not impossible to find conflicting wrong names. The best terminology to know is the Crossed or Straight-through wiring (seeing a wiring diagram is best). The wire with the red stripe is pin 1.
DB9, male pins, numbered from solder side here
Ribbon------- DB9 - Cross-wired
1 ---------- 1 DCD
2 ----------------- 6 DSR
3 ---------- 2 RX
4 ----------------- 7 RTS
5 ---------- 3 TX
6 ----------------- 8 CTS
7 ---------- 4 DTR
8 ----------------- 9 RI
9 ---------- 5 GND
One source for a Cross-wired cable
