I am trying to hook an HP keyboard to an old Mac desktop but the ADB end of the keyboard does not match to the female end on the Mac. Is there an adapter I can get to make it fit?
The term "ADB" dates back to the mid-1980s and stands for Apple Desktop Bus. The ADB connector is a round, 4-pin connector which is the same type of connector used for S-video.
This is an obsolete connection system used on older Macintosh computers for connecting low speed devices such as keyboards and mice. ADB was used from 1986 through 1999 and has since been replaced by USB.
Before exploring ADB adapter options, let's make sure that the HP keyboard connection isn't a PS/2 connector which looks similar to the ADB connector but has 6 pins instead of 4.
Macintosh Keyboard Input Jacks
Now, let's look at the keyboard input on the Macintosh computer. If the female input has 4 pins, it is an ADB connection and should pair nicely with a 4-pin ADB keyboard connector. If the input looks more like a phone jack, then you'll need a compatible keyboard as those systems are not compatible with other systems. If your Macintosh sports a USB port, you should be able to find an adapter for either ADB or PS/2 keyboards.
If your keyboard is a PS/2 keyboard (with 6 pins) and the Macintosh has an ADB connector, you will need a PS/2 to ADB adapter. While I did find a PS/2 to ADB adapter on GeeThree.com, apparently it only works with mice, not keyboards. IOGear makes a product called MiniView Mac Adapter (GCV160) which allows you to connect a PS/2 mouse, PS/2 keyboard, and VGA, SVGA, or Multisync monitor to an ADB Macintosh. This is a legacy product and may or may not be available. You could check eBay or Craigslist as a possible source as well.
If your Macintosh has a USB port, connecting either an ADB or PS/2 keyboard is much simpler as both PS/2 to USB and ADB to USB adapters are readily available.