Understanding COM Ports, Parallel Ports, and IRQ Channels First of all, a Com port (communications) is the same as a serial port. The names are used interchangably. The serial port is normally used for a mouse, modem, special printers such as label printers, and some older scanners. Printer ports are called LPT ports and are used primarily for printers. Sometimes, external tape drives and scanners also use a printer port. The difference between the information that comes out of a serial port as apposed to a parallel port is that a serial port passes a bit after a bit after a bit, etc. The parallel port passes a byte after a byte after a byte, etc. A byte is made up of 8 bits. Serial has gone through several generations including major improvements in speed. Parallel has also gone through speed improvements as well as adding bi-directional communications with connected devices. When IBM designed the first computers, specs called for only one serial port externally and a reserved address for an internal serial port such as on a modem. The second generation computers called "AT"'s, had 2 external serial ports. Com 1 used 3F8 and IRQ4, Com 2 used 2F* and IRQ 3. Later, Com 3 used 3E8 and reused IRQ4, Com 4 used 2E8 and reused IRQ3. In order for most devices of a computer to operate with it, information is passed over a specific address channel with in the computer. In addition, Their is also an IRQ or interupt channel which is used to get the attention of the processor and through this IRQ channel, the processor knows what device is trying to communicate with it. IRQ channels in IBM's first computer were reserved as follows: IRQ 0 System Timer 1 Keyboard 2 3 Com port 2 4 Com port 1 5 Hard Drive 6 Floppy Drive 7 Parallel port 1 Technology in the 386 computer allowed for more IRQ channels, among other things: IRQ 0 System Timer 1 Keyboard 2 Cascade from second controller (IRQ 8-15) 3 Com port 2 4 Com port 1 5 Sound Card (this can use other IRQ's on newer cards) 6 Floppy Drive 7 Parallel port 1 8 9 10 11 12 PS/2 type mouse (if used) 13 Math Co-processor 14 Hard Drive Controller 15 Hard Drive Controller Other devices needing IRQ's include most VGA display cards, USB ports, SCSI controllers, second printer ports if used, 3rd serial device, if used and compatible, and others. They would use any remaining IRQ (8,9, 10, and 11). It is possible and happens many times to actually run out of these "resources" (IRQ's) and you may not be able to add the device you want to add. Mouse There are 3 basic types of mouse (mice, if you will let me). Serial, PS/2, and Bus. IBM came up with the PS/2 mouse and newer computers use them freeing up a serial port in many cases. The PS/2 mouse is a serial device, but wired specially for the PS/2 port. The bus mouse is also a serial device meant for a special add in card. The bus mouse has all but disappeared. It required a special card and did have choices of some IRQ's in the lower area such as 2,3, 4, and 5 (very limited). Connectors of the PS/2 and Bus mouse are quite different and not meant to be interchanged. The mouse is a digital device as apposed to a joystick that is an analog device. Modems Early modems were limited to COM 1 or Com 2. Next generation modems allowed Com 3 or Com 4, but limited to share an interupt with Com 1 or Com 2. Some modems allowed using IRQ2, 3, 4, and 5. At this time, much software could not use any modem that wasn't on Com 1 - 4 with IRQ 3 or 4. In other words, modems on Com 2 or 5 couldn't be used with some software. Software did improve in time to pick up these new IRQ's. Another generation of modems came out with the advent of plug and play. Usually, these modems could go to almost any address and IRQ. Another generation also came out, eliminating the processor on the modem. This was called a "win Modem". It used the computers CPU to do its work, making for a much cheaper and inefficient modem. Most people want "cheap", so manufacturers were able to get away with it. With more powerful CPU's, the pitch was "you would't miss or be able to notice the lost performance". Plug and Play as well as Win Modems did open up modems to more IRQ channels and flexibility. A "win modem" can not operate at the DOS level. It requires code in Windows to tell it how to work with the processor. Sharing an IRQ is usually a difficult task. A mouse on Com 1 with a modem on Com 3 using the shared IRQ didn't work because as soon as the modem was called into action, the mouse locked up. You could use Com 1 and 3 with these devices as long as the modem could find a different IRQ that wasn't used. Most computers come with 2 Com ports and adding an internal modem used a third. Disabling the 2nd Com port was one choice and putting the modem on its address and IRQ was common and worked until the computer user needed the 2nd port. Also, it is common to place a modem on Com 4, sharing the IRQ with the un-used Com 2. Those who had "all in one" office machines (printer, fax, scanner) found that there were complications using the scanner because there were no available Com port IRQ channels because of a serial mouse and internal modem. These users also were usually the same people that used 2 printer ports taking up an extra IRQ channel. Some times, these people had to disable their sound cards to do office work. Internal modems are cheaper than external modems and also more popular. External modems offer flexibility including a light display that can indicate what is happening with a connection and if data is actually being transferred. Printer Ports The first printer port was co-located on the display card. It used an address of 3BC and IRQ 7. When 2nd printer ports came out, they used an address of 378. The third port used 278. In most new computers, the first printer port is now 378, the second 278. It can be confusing to add a printer port because of the way manufacturers label ports on the add in cards. 378 is sometimes still called LPT 2 and adding the 2nd port might land the 2nd port on top of the first. Be careful! USB The answer to lack of interupts and more connections was USB or universal serial bus. Most computers of Pentium class have these ports. USB operates at 12 mhz, very fast, and uses only one IRQ channel, and can have up to 127 devices in theory if you use a hub. A newer version of USB is scheduled to appear in the future, capable of even faster communications. USB requires Windows 98 or later (exception: Windows 95C). I believe USB is quite useful, but also believe there are certain devices that don't belong on it like the keyboard and mouse. It is great for an external modem, second printer, label printer, camera, and scanner. The biggest problem is the software writters who write lousy drivers that conflict with other connected devices. One major manufacturer of CD Writers is know to have drivers that kill other devices and you should be aware that a USB installation may cause major conflicts and kill your Windows installation. The USB port can not be used at the DOS level. Firewire Firewire was developed for Apple for video editing. It came over to the PC side in the last years for the same purpose. It is faster that USB 2.0. Hard drives are appearing that work of the Firewire port. There are reasons that I believe this is an injustice to computer users because of the way a computer is backed up. This also applied to USB devices. See my Backing Up article for details.