External Keyboard (Cn17); Atx Power Connector (Cn18); Table 2-3: Serial Port Default Settings - Advantech PCA-6751 series Manual

2.16.2
COM2 is an RS-232/422/485 serial port. The specific port type is
determined by jumper settings (JP1), as detailed in Chapter 1.
The IRQ and address range for both ports are fixed. However, if you
wish to disable the port or change these parameters later, you can do
this in the system BIOS setup. The table below shows the settings for
the PCA-6751/6751V board's ports:
Table 2-3: PCA-6751/6751V serial port default settings
Port
COM1
COM2

2.17 External Keyboard (CN17)

In addition to the PS/2 mouse/keyboard connector, an additional
external keyboard connector is provided.
For external keyboard pin assignments, please see Appendix D.

2.18 ATX Power Connector (CN18)

If the PCA-6751/6751V is used as a stand alone card both the 4-pin
main power connector (CN20) and the ATX power connector (CN18)
must be connected to the power supply. If the PCA-6751/6751V is
used with a passive backplane, the main power connector (CN20)
should not be connected as the card will be powered from the back-
plane.
Use the ATX adapter cable (provided) to connect the
PCA-6751/6751V with the ATX power supply. The ATX adapter
cable has two different connectors at either end. The ATX 20-pin
(female type) connects to the power supply, while the 3-pin (female
type) connects to the ATX power connector (CN18) on the board
itself.
22 PCA-6751/PCA-6751V User's Manual
RS-232/422/485 connection
(COM2-CN15: RS-232; CN14: RS-422/485)
Address
3F8, 3E8
2F8, 2E8
Interrupt Default
IRQ4 3F8
IRQ3 2F8