|What is plc ?|
Programmable logic controller ?
If so, what machine or device is that on ?
The Com port must be enabled in the mboard's bios Setup.
Some mboards have one physical D-sub male Com (Serial) 9 or 25 pin port (male - you see pins in the port), and another one available by connecting a wiring adapter / port (usually one in a plate that installs in a case card slot space) to a header on the mboard - the female wiring connector for the latter must be installed on the header in the right direction, if it's possible to install it either way
If the ports at either end of the connection are not both male 9 or 25 pin, suitable adapter cables or gender adapters can be used to convert one type to the other type.
( A female 25 pin D-sub port - you see pin holes in the port - on a computer CANNOT be a Com - Serial - port, except on ancient computers made before 1990 -- it's either a parallel printer port or a SCSI drive controller port.)
Both the IRQ and the I/O address of the Com port you connect it to must not be being used by anything else.
E.g. the legacy settings are....
Com 1 normally needs IRQ 4 and 3F8
Com 2 normally needs IRQ 3 and 2F8
Com 3 normally needs IRQ 4 and 3E8
Com 4 normally needs IRQ 3 and 2E8
Those two IRQs and those I/O addresses, and certain other I/O addresses, are only used for Com ports or devices that can use Com port resources, such as a dial-up modem, if the bios is set to default Legacy settings for IRQ 4 and 3, but IRQ 4 and 3 can be used for other devices if settings in the bios Setup are changed so that those IRQs can be used by any PNP device.
(E.g.some network cards and USB controllers can use IRQ 4 or 3).
If the computer has a dial-up modem, it may be already using one or the other or both of those.
Two Com ports can use the same IRQ as long as the devices connected to the ports are not used at the same time.
I have no idea where you would find that info in NT - I have never used NT - but in Win 9x, ME, and 2000 and up that info is in System Information - Hardware Resources , and in Device Manager - Ports - Com port x - Resources
You may need to install software on the computer as well in order to connect to the machine or device.