More about com ports

I can't be satisified without rambling on further about serial port settings. I know this can all be very confusing and frustrating, so I am going to go in to a fair amount of detail here for your sake and for mine. (I have to write this down or I will go nuts trying to remember it all.)

This will probabaly all be somewhat confusing and out of a logical order. The idea is that you can take the information you need and deduce what you need from it. It is basically a log of my settings and what resulted with each.

First is what computers I had and what settings they had, then a record of my experiences along with an attempt to explain why two devices cannot share serial ports.

If...then...

I like to use my modem from time to time and I like to game pretty often against a friend. Therefore I want for my modem to work and for my Com port to work for gaming. At times I can scratch my head and wonder why I can't get the two to coexist. Sometimes I can't get a null-modem game to work at all without pulling out my modem, and other times I seem to get it all dialed in perfect and it all works. It always seems that once I figure all of this stuff out it changes. I think (again) that I am getting a better understanding of all of this now.

Below I will report some settings that I have tried and what happened with them. This won't be that spectacular since I have only logged the most recent events, which is only a few.

If on the Cyrix computer I
• Enable serial port 2 with settings 2E8/IRQ3
• Leave the modem in the computer jumpered for Com2, IRQ3
• Use com4 in the game
  Then the connection works, and my modem works in Windows 95 with the software settings at Com2.

If on the Cyrix computer I
• Enable serial port 2 with settings 2E8/IRQ3
• Leave the modem in the computer jumpered for Com4, IRQ3
• Use Com2 in the game
  Then the connection works and the modem works in Windows 95 with software settings at Com4.

If on the AMD computer I
• Enable serial port 2 with settings COM2/3F8
• Have no modem present
• Use Com2 in the game
  Then the connection works.

What is the difference between Com2 and Com4?

Here goes another one of my pathetic attempts to explain something with an analogy...

In a game you have the option of using Com1, Com2, Com3, or Com4. If you look on the back of your computer, or in your BIOS, you probably don't see any such thing as Com3 or Com4. So what is up with that?

Your mouse, provided that it is a serial mouse and not a PS/2 mouse, plugs into a Com port in the back of your computer. It probably plugs into Com1. That plug in the back of your computer has a bundle of wires coming off of it that goes in and plugs into your motherboard. In this case, your mouse is physically using that part of the motherboard right there where it plugs in and you can see that place with your eyes.

Your modem plugs into a slot, probably and ISA slot, and you set it up to use a Com port. Often they use Com2, but not always. (Your modem can probably use Com1-4) The way you set that is with the modem jumpers. Your modem has to use an IRQ and a Com port. Some modems, (the good ones) have a seperate jumper for the Com port and the IRQ setting. On others there may be only one jumper to set and it selects an IRQ that is either fixed with the port setting or it is software configurable.

I set up a modem to use Com2. Unlike the mouse, the modem does not use that place on the motherboard labeled "com2" where you can see it with your eyes like the mouse uses. This is where things get a little hazy to me, but I think I've got it. That place on your board where you can actually see uses an IRQ. For example, my mouse uses IRQ 4. How do I know that, I didn't set any jumpers on it. That decision was made by me in my BIOS. (Cyrix computer) I set it up to be IRQ 4 and the DMA as 3F8. So I suppose the mouse uses what is given to it.

I cannot expect my modem to work on Com 1 because if I did that, it would share the same IRQ as my mouse. The result is a conflict where more than one component is asking for processor time at the same time. Like two different people having reservations at the same hotel in the same room on the same day. (An IRQ is a window of time during a cycle that is allotted for a device to request processor time. For example, suppose my boss is a very busy guy but he knows I need to talk to him. To remedy this, every 15 minutes he will stick his head out the door and listen for any requests. If he hears one, he will work on that, if not, the window of time passes and he goes back to work. There are 16 IRQ's on a computer ranging from 0-15. Therefore your cpu "listens" 16 different times during a cycle.)

So how does Com 4 get away with using the same IRQ as Com2? Interesting question. I will try to answer it in this way... Suppose Mr. Modem lives in a house and uses a telephone. The name of his house is Com2 and the name of his telephone service is the IRQ3&T company. (get it? AT&T. nevermind) New to the neighborhood is Mr. Null Modem game port man. He needs to move into town but the only other house is already lived in my Mr. Mouse. (In this example, the house is the DMA. It is where the device resides in memory. It must live somewhere whether or not it is in use.) There is obviously a dilemma here. So the neighborhood manufacturers made the two houses into duplex houses. Now there is room for Mr. Game man in the Com4 house, but remember that though Com2 and Com4 are seperate houses, it is truly only one dwelling. This duplex house uses only one phone line with two telephones. One phone is used by Mr. Modem, the other is used by Mr. Game man. Therefore, the phone service used by the Com2 house is IRQ3 and the phone service used by the Com4 house IRQ3, the same phone service. Both Mr. Modem and Mr. Game man are telecommutters, they never leave home. That is why they do all of thier business from the phone. If Mr. Modem is at work and Mr. Game man tries to go to work, he can't. The two guys will just sit there dazed and confused. Sometimes that may even make the neighborhood freeze and make you have to reboot. As long as Mr. Modem is asleep or relaxing, Mr. Game man can go to work. This works out well most of the time because these two guys rarely need to go to work at the same time. Think about it, how often do you use your modem at the same time you play a serial game or run a direct cable connection? Probably never. You may, however, use your mouse at the same time you play a game or when you are using your modem. That is why Mr. Mouse and Mr. Modem don't live in the same duplex.

I don't know enough to explain everything about serial ports and I know that it isn't all covered here but I hope that this is helpful where needed.