Downtown Doug Brown

When I wrote Parallel Port Tester, I discovered that detecting parallel ports and their I/O addresses in Windows is a pain in the butt. I have received several inquiries asking about how I did it, so it’s obvious that others agree with me. Hopefully, this post will help everyone learn how to automatically detect their parallel port addresses. I certainly don’t claim to be an expert, and there may be better ways to do it, but this method seems to work for me.

Don’t use Win32_ParallelPort

One of the strategies I’ve seen recommended is to do a WMI query on the Win32_ParallelPort class. This is originally how I started implementing Parallel Port Tester, but I quickly discovered that it sucks! Sometimes it gets the I/O address for the wrong port, and sometimes it doesn’t even find your parallel port at all. It seems to depend on the version of Windows and the chipset manufacturer. In particular, I found that it would not detect a parallel port implemented by a Moschip chipset in Windows XP. It also was returning incorrect I/O addresses on a user’s system running Vista. Anyway, I’d highly recommend that you avoid this class. I could not get it to work correctly. Maybe I was doing something wrong…but I’m pretty sure it just sucks. It sucks so bad that I disabled this method of discovering parallel ports in version 1.0.0.1 because of a report that it was returning erroneous information.

Instead, find objects belonging to the Ports class manually

For more reliable results, what I do is search for all devices that are ports (which also include COM ports). The GUID for this class is {4D36E978-E325-11CE-BFC1-08002BE10318}. I only pay attention to returned ports that are LPT ports. On each found LPT port, I request all of the port resources to determine the I/O addresses.

How do you do that? Here’s some sample C# code using .NET. Note that I’m only showing relevant bits and pieces, so you will have to fit them into a class and function as appropriate.

using System.Management;

private static readonly ushort SPPIOLength = 8;
private static readonly ushort ECPIOLength = 4;

SelectQuery portsQuery = new SelectQuery("Win32_PnPEntity",
    "ClassGUID=\"{4D36E978-E325-11CE-BFC1-08002BE10318}\"");
ManagementObjectSearcher portSearcher = new ManagementObjectSearcher(portsQuery);
foreach (ManagementObject port in portSearcher.Get())
{
    // Grab its name. This will be in the form "Blah Blah Parallel Port (LPT1)".
    // We're going to get ugly now...
    string name = (string)port.Properties["Name"].Value;

    // Skip ports that are not parallel ports. This is UGLY...I'd much rather
    // do this a cleaner way, but it seems that different parallel ports do
    // things differently.
    if (!name.Contains("(LPT")) { continue; }

    // and extract the parallel port name by looking for everything inside
    // the parentheses. I don't know how else to do it...
    int beginLoc = name.IndexOf("(LPT") + 1;
    int endLoc = name.IndexOf(')', beginLoc);
    name = name.Substring(beginLoc, endLoc - beginLoc);

    // Grab the device ID so we can find all the associated port resources.
    string deviceID = (string)port.Properties["DeviceID"].Value;
    // Replace single backslashes with double backslashes to be suitable
    // to use in a query. Note that I also had to escape it myself in
    // code...so \\ represents a single backslash.
    // TODO: Any other escaping necessary?
    deviceID = deviceID.Replace("\\", "\\\\");

    // Now search for I/O ranges of this port.
    ObjectQuery resourceQuery =
        new ObjectQuery("ASSOCIATORS OF {Win32_PnPEntity.DeviceID=\"" +
        deviceID + "\"} WHERE ResultClass = Win32_PortResource");
    ManagementObjectSearcher resourceSearcher =
        new ManagementObjectSearcher(resourceQuery);

    // Find the SPP and ECP base addresses
    ushort SPPBase = 0xFFFF;
    ushort ECPBase = 0xFFFF;
    foreach (ManagementObject resource in resourceSearcher.Get())
    {
        // Grab starting & ending addresses
        ushort startAddress =
            (ushort)(UInt64)resource.Properties["StartingAddress"].Value;
        ushort endAddress =
            (ushort)(UInt64)resource.Properties["EndingAddress"].Value;

        ushort rangeLen = (ushort)(endAddress - startAddress + 1);
        // If we haven't yet found the SPP base address, and this range
        // is big enough, use it as the SPP base address.
        if ((SPPBase == 0xFFFF) && (rangeLen >= SPPIOLength))
        {
            SPPBase = startAddress;
        }
        // If we haven't yet found the ECP base address, and this range
        // is big enough, use it as the ECP base address
        else if ((ECPBase == 0xFFFF) && (rangeLen >= ECPIOLength))
        {
            ECPBase = startAddress;
        }
    }
    // Although I didn't include the code here, if I fail to detect an
    // SPP address with the rules above, I grab the first returned
    // address and use it as the SPP base address. Then I set the ECP
    // address to 0xFFFF (which I treat as meaning "unable to find")

    // TODO: If SPPBase != 0xFFFF, add it to the list of discovered
    // ports -- the name of the port is stored in the variable "name"
}

I hope this code is helpful to someone out there! From this point, you can use something like Inpout32.dll to access the port registers directly.

I’m not a huge fan of the hack where I extract the “LPT#” name from the long name by searching for the “(LPT” string, but I couldn’t find a better way to do it. Suggestions would be appreciated! There also may be a better way to use parameters to do the ASSOCIATORS query instead of ugly escaping and string concatenation, but it worked and that was good enough for me on this particular project. I couldn’t find an easy way to set it up for use with parameters or else I would have done it that way.