Why a PCI modem uses COM port?
What does it mean :confused:

I had this question recently but I didn't find my answer.
I read some articles about modem and COM port but they didn't notice this.

A modem can use PCI for it's data transfers. Why and how it uses COM port?

I think this thread is more related to PCI than modem, so I made it here. ;)

From my A+ book.

A serial port is a physical item, a 9- or 15- pin male DB connector in the back of your pc, but a COM port is just the I/O address and IRQ assigned to it.
A parallel port is a 25- pin female DB connector on the back of your pc, but an LPT is just the I/O address and IRQ assigned to it.

COM and LPT ports are nothing more than preset combinations of IRQs and I/O addresses, so when you set COM1 for your modem you assign a IRQ and a I/O address.

Hope it helps.

COM (and LPT for that matter) are enumerations (1,2,3 etc) of specific addresses reserved in fixed locations of the BIOS Data Area of one's RAM.

All modems (not just PCI ones) require such a number and thus have a specific linked memory address. Serial ports are regularly called COM (or communication) ports even though ports are physical things and enumerations are virtual.

In modern systems COM1 is almost always associated with the first Serial Port while on legacy systems COM1 is associated with the first serial device recognised by the BIOS, whichever port it happens to be attached to. PCI and USB modems dont have any other specific external ports and so their enumerations normally start with 3 or above.

I think I understood you, but what does a sound card use(Wich COM port)?

COM numbers relate to serial communication devices (such as serial mice, modems, direct cable networking and UPS devices), their associated ports and to UART chips.

I don't know of a sound card that uses a serial port (http://en.wikipedia.org/wiki/Serial_port) in this way or that is assigned a COM number.

Can someone please tell me how to post my own message, replying to this message was the only thing I could find how to get on the board. At least I hope I can. Also, I just joined today.

Why modem don't work like a sound card?

this is very interesting. Why doesn't have soundcard a COM port? Back in the old DOS times, installing a soundcard required to set a hex adress (ie 220h - probably RAM address, but it's just my guess), DMA and IRQ. Now I think the COM port is something that combines the three numbers under one. I think that soundcard doesn't uses COM ports but just for legacy reasons but I don't know if I am understanding it correctly. Can someone write something comprehensive about this one please.
Thanks

I don't know but my rockwell modem has some jumpers that let me set the COM port and IRQ seperately.

COM is for Communications.

Such serial communications do however involve a two-way exchange down a single channel. The analogy could be trucks using CB radio comminications on a single channel.

Modems and LANs thus require a COM enumeration because the devices at each end need to be able to send and receive without conflict.

Sound output is essentially a one-way process. The analogy would be turning on the radio and lisening to it.

Thus you need to sus out the difference bertween using a port (an entry to a bus such as an ISA or PCI bus) and communicating using that port. Older physical serial ports are on an ISA bus; more modern ones are directly on the PCI bus.

Can someone please tell me how to post my own message
Go to the most appropriate forum for your post at http://www.pcguide.com/vb/index.php? and click on the link to that forum.

The click on the "New Thread" button (one at the top and one at the bottom of the page) to start a new topic or ask a new question.

So what about a PCI video card? It is two-way, isn't it? Or a video capture card.

So what about a PCI video card? It is two-way, isn't it? Or a video capture card.
I think you've missed the point about the use of a COM number and of how ports and buses function. All such devices have i/o demands but don't engage in a COM(munication) conversation with an external COMPUTER.

External COMPUTER?

External COMPUTER?
Yes. Modems generally connect to computers via the internet. DCC can also be used to connect to other standalone computers. The term external computers was used to differentiate between connection to other external devices that are not COMmunication devices - such as mice and video capture.

Hello Sia

First off...I'm not an expert so please correct me if I'm wrong (ANYBODY) But I believe the easiest way to explain things are....it just depends on the job of the specific part of the computer.

I believe that any part of the pc that needs to communicate with something OUTSIDE of that particular pc it's in...needs to be assigned a comm port OR Communications port. This is because the part NEEDS to communicate with outside sources other than the computer they are in.

Devices such as a Video Card however, do NOT need to communicate with another pc other than the one it's plugged into...so I don't believe it would need a comm port to activate. The Video Card just sends the picture to the monitor...it doesn't need to be able to access anything outside the pc it's in...it doesn't need to COMMUNICATE with anything OUTSIDE the pc. The monitor is just part of THAT same pc...it's not considered an outside source..it's still part of the same pc that the Vid Card is in.

Printers need to be plugged into a COMM port again because it is a totally separate device located OUTSIDE the pc that is connected to it of course.

A modem needs a COMM Port because it communicates with other computers...OUTSIDE sources.

That is why Paul used a trucker and a cb radio as an example of a comm port. The Trucker is listening but also communicating back to an outside source. Listening to music on the radio....well...you can't really talk into the radio to communicate with anything...you're ears are just receiving the sounds or in a computers case receiving the data.

I hope I was able to give a simple but accurate explanation of what is being talked about here. If I was wrong in anything please correct me as I'm still learning much myself....after all...I'm only an Apprentice for now.

I think a great deal of confusion is caused by referring to serial ports as COM ports - however understandable that may be. COM values or references or whatever else you want to call them are simply numerical and are used as IDs for that communication channel.

What is a port - ? - well I see it as a physical entry point to a bus. Leaving aside the older ISA bus and concentrating on the PCI bus a modern serial port on the outside of the PC allows connection to the PCI bus. Serial modems thus get onto the bus at this point. PCI modems fit into a slot and get onto the bus that way - perhaps such slots should also be called ports - but that is another matter.

Whether you use a PCI or a Serial Modem, all modems will require a COM number. Other "devices" sharing data transmission on the bus (but not engaging in a two way "conversation" with another computer) dont require COM enumeration.

Printers need to be plugged into a COMM port There are such things as serial printers that would attach to a serial port - but dont use a COM number; instead printers are referenced by LPT numbers and are most commonly attached to parallel ports.

I have for the sake of simplicity deliberately left USB out of this topic - but USB modems will once again require a COM number.

Thanks a lot for making it more clear. Sorry about the mistake...I just tried to understand everything the best I could in order to translate it...lol

However, I did think that the LPT port was another type of comm port simply made for printing and other related devices.

In any case, thanks a lot for the information.

Why devices that are engaged in a two way communication with an external computer need COM ports but otherwise they don't?

Since you continue to refer to a COM port I will restate that:-
I think a great deal of confusion is caused by referring to serial ports as COM ports - however understandable that may be. The COM number is a type of ID; think of it sort of like a preset radio channel if you want to extend the CB radio analogy. It needs the number to know which channel to communicate on and to find and use a UART chip. The UART chip has FIFO (first in first out) buffers that allow two modems to talk to one another in a comprehensible manner.

Its not good if two modems talk to each other at different speeds or at the same time. The correctly identified UART chip (which is at the heart of COM) is the main controller of this in both the computers participating in a "conversation".

The COM number is assigned by the BIOS during start-up but it is not always "written in stone" and can relate to different serial ports and to different motherboard cards.

Serial ports may be polled by the BIOS in the order that devices are found on them. This means that if there are two ports and just one attached device then that modem's details might always get assigned to the COM1 data area. This effect of the BIOS, known as address packing, can cause conflicts with Windows, which always expects COM1 to be linked to Serial Port1 and to find its "details" stored in that specific associated memory address in the RAM's BDA (BIOS Data Area).

I've said enough on this already so I'll let someone else jump in if I haven't been able to get the concept across.

Why an internal modem don't use main memory (RAM) as the buffer?
Why UART chip is needed? I thought it is only for external devices like external modems that are async and/or have a slow speed, why a sync internal PCI modem
should also use that chip?

A modern internal modem (that comes on a card to plug inside the machine) will usually contain its own good UART.
from http://pclt.cis.yale.edu/pclt/COMISDN/16550.HTM

You mean it doesn't use mobo's UART? If yes, again, why should it use a COM?

I should repeat my last questions because I myself look only the last reply!!!!

Why an internal modem don't use main memory (RAM) as the buffer?
Why UART chip is needed? I thought it is only for external devices like external modems that are async and/or have a slow speed, why a sync internal PCI modem
should also use that chip(UART)?