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Old 04-25-2004, 08:28 PM   #1 (permalink)
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Default com ports and 9 and 25 connectors

Hi
I read some place that MS-DOS originally had four com ports.
Are these COM ports UARTs.
i read that initally windows supported just 2, and now they support 4.
do these four com ports physically come out of the computer. i can see only see two 9 pin conenctors. i can see a 25 pin connector, i guess that is the LPT port.
where are the other two COM ports?
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Old 04-25-2004, 09:00 PM   #2 (permalink)
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I am not sure of the limitations of COM ports. I know for sure that it is no where near 4 though. typically, these are needed for external modems, terminal emulation and other devices but the need for a personal computer to have these is slowly diminishing. The COM ports are the the 9-pin connectors. You may only have 2 on your particular motherboard. Every computer doesn't have to have 4 installed. But, I've used devices developed by Digi and Comtrol that house multi-communication port adapters. You install a card in one of your available PCI or ISA slots in your PC and then a cable connects to this card and has more available ports extending off of it. Sometimes they are called "octopus cables". They allow more COM port connections off of the cables. I've used these for serial interfacing with other computers. Typically, these COM ports off of the "octopus cable" would start with the last COM port on the machine. These ports would be labelled "P" and then a number. If there were 8 ports on the "octopus cable". Then they would be labelled "P1", "P2", "P3" and so on....
If you had 2 COM ports on the back of your computer (COM1 and COM2), then the port labelled "P1" would be COM3, "P2" would be COM4, "P3" would be COM5 and so on.....
I've personally seen and used a PC that had over 100 of these COM ports in hospitals.

Here is a picture of a "octopus cable"

Also, these can come in a range of sizes too. COM ports are just serial communication so they don't have to be the typical 9-pin fashion. They can be RJ-11 (Phone Line), RJ-45 (Ethernet), DB-25 (Looks like a Parallel port or LPT port....but it's serial), etc.

-Mike
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Old 04-27-2004, 05:54 AM   #3 (permalink)
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thanks for the reply, and thanks for the pic. a picure speaks a thousand words.
do all these com ports run the rs232 or 422 protocols?
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Old 04-27-2004, 03:40 PM   #4 (permalink)
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These run the RS 232, but you can get ones that commun icate through 422.

-Mike
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