Tech Talk (4/97) - Plug & Play

Sarasota PC Monitor

Tech Talk (4/97)

Plug & Play

by Brian K. Lewis, Ph.D.
Member of the Sarasota Personal Computer Users Group, Inc.

The idea of Plug & Play was that the computer user would be able to quickly, and easily, add new peripherals to a computer system. Gone would be the days of hassling over DMA's, IRQ's, base addresses and other exotic items understood by only the most technically oriented. As the Macintosh publications exclaimed, now the PC can do what the MAC has done since 1984. All you would need to do would be to plug in the new item and restart your computer. Admittedly, you might have to install driver software for your new component. However, the computer BIOS and the operating system would handle the setup for the new hardware. Unfortunately, reality is somewhat different from the PR hype in both cases. The basic problem of interfacing the new and existing hardware isn't always as simple as the Plug & Play documentation seems to suggest.

Let's look at some examples of the problems I have encountered and the solutions they required. First, one of the most common of the Plug & Play devices is a modem. Almost every manufacturer has Plug & Play modems on the market. If you buy a new computer with the modem installed, your problems are solved for you at that point. However, if you are considering adding a faster modem to your system and you are running Windows 95 you might encounter a problem. Almost every 486 or Pentium system ships with two built-in communications ports, COM 1 and COM 2. These ports use interrupts 4 and 3 respectively. However, the other two communication ports, COM 3 and COM 4, use these same interrupts. COM 1 & 3 share IRQ 4, while COM 2 & 4 share IRQ 3. In Windows 3.x this was not a problem. These systems generally had the mouse on COM 1. Since the computer is constantly polling the mouse, IRQ 4 could not be shared. Most modems could then be set to COM 4 and IRQ 3. Windows 3.x didn't kick up a fuss about two COM ports sharing an IRQ. If the mouse was attached to a dedicated mouse port, then IRQ 4 was available since the mouse would generally be using either IRQ 10 or 11. In a system with a mouse port and two COM ports available, installation of the modem should be simple since it could share either of the COM port IRQs. Unfortunately, Windows 95 doesn't like to have shared IRQ's. That is considered an error. So installing a Plug & Play modem in a Windows 95 system generally results in assignment of the first unused COM port. In this case it would be COM 3. The IRQ would something other than 3 or 4, the first choice seems to be 5. So far this isn't a problem even if a mouse is on COM 1. Providing, that the communications software permits Windows to handle the IRQ while it looks only for the COM port. Some older software insists on knowing the IRQ and refuses to accept any IRQ other than 3 or 4. In that case, you have to upgrade the software to a version compatible with Windows 95. Anyway, we now have a working modem.

The next problem that might arise, is the user decides to add a sound card, speakers and CD-ROM drive to the system. Plug & Play sound cards are available in kits complete with the CD-ROM and speakers. So all that is necessary is to take off the cover, plug in the sound card (no jumpers to set). Then install the CD-ROM and connect it to the sound card. Fire up the computer and let Windows 95 finish the installation. You may not even have to add any software for the sound card, just a driver for the CD-ROM. Sounds great doesn't it? Plug in the speakers and listen to your computer. Windows 95 even includes some music in the Media folder for you to test the sound system. Now back to send some e-mail and tell the kids all about how easy it was to install this great sound system. Whoops! What's this error message? No modem installed! But it's still there!

The problem is that most sound cards are automatically installed on IRQ 5. Remember, that was the IRQ Windows assigned to the modem. So the modem was reassigned to a different IRQ, probably 10. At least that has been the case in my experience with these set ups. So how do I find out this information. First, right click the "My Computer icon" and select "properties" on the pop-up menu. This brings up the tabbed pages, the second of which is the device manager. Open the device manager page and you will probably see the modem item has a yellow circle with an "I" in the center. This shows that there is a problem just as you suspected. Highlight the modem line and then click the properties button. This tells you that the modem is not properly installed and is not working. Somehow you knew that already.

To make a long installation shorter, let's cut to the bottom line. The conflict in IRQ's is caused by the CD-ROM connected to the sound card. The default here is usually IRQ 10. The solution that works, not necessarily the best solution, is to force the sound card to use IRQ 10 and the CD-ROM to use IRQ 11. This can be done in the properties for these devices. Then delete the modem and restart the computer. This forces Windows to reinstall the modem and it should select IRQ 5 as the first available IRQ. If it does, then everything will work and you are back in business.

If your mouse is on a dedicated mouse port, then you have to work out a different solution to avoid the IRQ it is using. Another problem can occur if you have two printer ports. The second port, LPT2, is usually assigned to IRQ 5. So again, you need to refer to the device manager to find the free IRQ's to solve the problem. Plug & Play does not always work correctly in these situations. Instead, you have to force Windows 95 to accept changes to the IRQ setup.

Now maybe you can begin to understand my comment in a previous article about "Plug & Pray." Certainly, Plug & Play may have simplified the adding of some peripherals, but it has also introduced its own complications. Windows device manager, if properly used, can help solve most of the conflicts that may arise with Plug & Play devices.

Dr. Lewis, a former university and medical school professor, is currently a computer consultant and computer instructor for Sarasota County Technical Institute. He is available, on a fee basis, as an on-site consultant to help you with your home or business computer problems. He can be reached via e-mail at bklew@ix.netcom.com or voice mail at (941) 925-3047.

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Copyright 1997. This article is from the April 1997 issue of the Sarasota PC Monitor, the official monthly publication of the Sarasota Personal Computer Users Group, Inc., P.O. Box 15889, Sarasota, FL 34277-1889. Permission to reprint is granted only to other non-profit computer user groups, provided proper credit is given to the author and our publication. We would appreciate receiving a copy of the publication the reprint appears in, please send to above address, Attn: Editor. For further information about our group, email: spcug@netline.net Web: http://www.spcug.org/

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