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Moving your legacy-based PC into a legacy-free world How to configure your system to make best use of USB and IEEE-1394-based peripherals

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Moving your legacy-based PC into a legacy-free world
How to configure your system to make best use of USB and IEEE-1394-based peripherals

Goodbye to Legacy Devices?

Although your PC probably still has one or two serial ports, a parallel port, and a PS/2 mouse port, the odds are increasingly good that you dont need to use them anymore. The peripherals which formerly connected to the serial port (mice, pointing devices, PDA cradles), the parallel port (scanners, printers, webcams and external drives), the PS/2 mouse port (mice and pointing devices) and the PS/2 keyboard port (keyboards) can connect to the USB port these days. Its increasingly difficult even to purchase devices that plug solely into legacy ports, although many devices include adapter plugs that convert them from USB to PS/2 legacy devices. Similarly, the IEEE-1394a (FireWire 400) port has replaced the SCSI port for high-performance scanners and also handles DV camcorders.

You can learn more about these ports and devices in my book Upgrading and Repairing PCs, 15th Anniversary Edition.

Why Switch to Legacy-Free?

If youre still using serial, parallel, and PS/2 mouse ports for some devices, youre missing out on some significant benefits:

  • Plug-and-Play (PnP) recognition of new devices when installed
  • Hot-plugging and hot-swapping of devices so you dont need to shut down your system to add or remove devices
  • IRQ sharing for more efficient use of hardware resources

If you have devices that can be used in legacy or legacy-free modes, its time to make the switch. Even if you are using Linux, youll find that many recent Linux distributions (such as the new Mandrake Linux 9.2) work well with many USB and IEEE-1394a devices. Naturally, Windows 98 Second Edition and newer versions make working with USB and IEEE-1394a devices very simple.

Making the Switch

The process of switching a device from legacy ports to USB or IEEE-1394a ports is a bit more involved than unplugging the old cable (or adapter) and plugging in the new cable. Table 1 reviews the basic process for devices, which are often supplied with both legacy and USB or IEEE-1394 connections.

Table 1 Moving a Device from Legacy to USB or IEEE-1394 Ports

Device Type

Old Port Type

New Port Type





Reinstall vendor driver, specifying USB; download updated drivers from vendor website

Mouse or pointing device



Enable USB Legacy support for all devices in BIOS (if option available). Install vendor drivers. Windows will install HID (Human Interface Device) support during process


Parallel or SCSI

USB or IEEE-1394a

Reinstall vendor driver, specifying USB or IEEE-1394a port.




Enable USB Legacy support in BIOS while old keyboard is connected (see Figure 1). After attaching keyboard to USB port and restarting, Windows will recognize USB keyboard and install HID support. Install vendors drivers to support multimedia or other proprietary keys.

Although USB devices are PnP devices, this feature requires that device-specific support be present in Windows. Because of the wide variety of USB devices, you should plan to install (or reinstall) the latest version of vendor-supplied drivers for the device before you attach it to the USB port (refer to your device's documentation to be sure). Windows XP supports a wider range of USB devices than older versions of Windows, but its support for printers is often rudimentary. For the greatest range of options, use the vendor-supplied driver if possible.

Even if you use a vendor-supplied driver, you might also need your Windows CD, particularly if you are installing a USB keyboard or mouse (pointing device), because Windows needs to install HID support for these devices.

Freeing Up Resources Used by Legacy Devices

Starting with late versions of Windows 95, Windows has supported a feature known as IRQ Steering, which enables two or more PCI and AGP devices to share a single IRQ. If your system is running Windows 95 OSR 2.0 or later, Windows 98, or Windows Me, its very likely that the IRQ sharing is not as efficient as it could be if serial, parallel, and PS/2 mouse ports are still enabled in the system BIOS. Systems shipped with these versions of Windows typically use IRQs from 9 to 11 for IRQ sharing among add-on card and motherboard-based PCI devices by default. Some devices dont perform well under these versions of Windows if theyre forced to share IRQs.

To provide more elbow room for IRQ configuration, I recommend a two-step approach:

1. Disable legacy ports you dont use.

2. Make the IRQs used by legacy ports available for use by PCI/PnP devices (including USB and IEEE-1394a ports as well as audio and other PCI or AGP devices).

Disabling Legacy Ports

To disable legacy ports, restart your system and run the BIOS setup utility (typically, you press the DEL key on most recent systems; check your system or motherboard manual for details). Open the menu for onboard ports. This might be called the Integrated Peripherals or I/O Devices menu (see Figure 1).

Figure 1. A typical system after the serial (COM) ports have been disabled (A) and USB Legacy mode has been enabled (B).
(Click to enlarge)

Figure 1. A typical system after the serial (COM) ports have been disabled (A) and USB Legacy mode has been enabled (B).

The All Devices setting for USB Legacy mode enables USB mice and keyboards to work at a command prompt or within a BIOS setup menu as well as within the Windows GUI.

Follow your system BIOSs instructions to disable the ports you dont use as shown in Figure 1. Note that your system might not offer an option to disable the PS/2 mouse port.

Enabling Former Legacy IRQs to Be Used by PnP/PCI Devices

To make the IRQs formerly used by legacy ports available for PnP devices, locate the PnP/PCI Resource Exclusion or PnP/PCI Configuration menu. This menu (not present on all systems) is used to control which IRQs are reserved for legacy devices, and which are available for use by PnP/PCI devices (including add-on cards and motherboard-based devices).

To use this feature effectively, you need to know which IRQ is used by a legacy port you have disabled. Table 2 lists this information.

Table 2 Legacy Port IRQ Usage

Legacy Port








PS/2 Mouse


Figure 2 shows a typical example of this menu after IRQ 3 and IRQ 4 have been switched from their default setting (reserved=Yes, often called ISA or Legacy mode on other systems) to the reserved=No mode, also called PnP mode.

Figure 2. This system can now use IRQ 3 and IRQ 4 for PnP/PCI devices (C).
(Click to enlarge)

Figure 2. This system can now use IRQ 3 and IRQ 4 for PnP/PCI devices (C).

Note that disabling legacy ports is not sufficient to make the IRQs formerly used by these devices available; you must also explicitly tell the system of these changes through the BIOS setup program.

After you make these changes, save the changes to the CMOS and restart the system. Windows and the BIOS are free to use the additional IRQs with your PCI and PnP devices.

What about Windows 2000 and Windows XP? These versions of Windows provide more efficient IRQ sharing, especially when ACPI power management is enabled. Thus, it is usually not necessary to disable legacy ports on systems running these versions of Windows.

Understanding the Limitations of Legacy to USB Adapters

As a way of easing the transition from legacy to USB devices, you can also purchase adapter cables or devices which enable a legacy device not made for USB to plug into a USB port. I dont recommend these adapters for two reasons:

  • The adapters are quite expensive
  • They can restrict the capabilities of your hardware

For example, you can buy a universal port replicator which plugs into a USB port from several vendors. This device, intended primarily for notebook computer users, has a couple of serial ports and a parallel port as well as a couple of additional USB 1.1 ports. The parallel port supports printing, but on the models Ive seen, bidirectional signaling (used to report low ink or toner) isnt supported, making it a poor bet for use with recent laser and inkjet printers. If you use a parallel printer along with another parallel device, such as a scanner or Zip drive, daisy-chaining isnt supported either.

As long as you depend upon devices which must plug into legacy ports, keep your legacy ports around. But, as youve learned in this article, there are benefits to moving dual-mode devices to your USB ports and disabling legacy ports if you no longer use them.

Copyright©2004 Pearson Education. All rights reserved.

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