USB Keyboard Limitations
Be careful about choosing a USB-based keyboard to hack. The USB keyboard interface is designed to limit the number of simultaneous possible keystrokes to six. This isnt a problem when used for games such as Pac-Man or Donkey Kong, which are unlikely to need more than two or three keystrokes at the same time. However, modern games such as fighting games can easily require many more than six simultaneous keystrokes, particularly if you are building a four-player control panel. PS/2 keyboard encoders often have a maximum simultaneous keypress limit as well, but its typically between 10 to 20 or so, and unlikely to cause you issues unless you are building a four-player panel. One final drawback of USB keyboards is that DOS support for them is limited. Newer computers will probably work with USB keyboards in DOS, but older computers probably will not without finding and installing special drivers that are not guaranteed to work. Keyboard hack recommendationsMultiple Keyboard Connections You may find there are times when you will want more than one keyboard device functional at the same time. Why would you want that? Take as an example a situation where youve created a keyboard hack for your arcade controls but still need to be able to operate your computer for non-gaming functions. You could simply swap plugs, but you should not do that while the computer is on, and rebooting every time you want to change programs would grow tiresome quickly. Wouldnt it be better to have two keyboard devices plugged in at the same time? Your keyboard-hack based arcade controls would be ready to use whenever you wanted, and your un-hacked keyboard could sit inside the cabinet ready to be used as needed. You have a few options if you want to use more than one keyboard device at the same time. All of the solutions are good ones and use different approaches to the problem. Tip When I refer to using more than one keyboard device at a time, I mean two keyboard encoders with whatever is connected to them. Presumably it would be a keyboard-based set of arcade controls as one device, and a keyboard as the second. It could just as easily mean two sets of keyboard-based arcade controls. Just remember that the phrase "keyboard device" doesnt necessarily mean a keyboard. Keyboard splitters A keyboard splitter is not someone who has broken apart their keyboard in order to hack it. A keyboard splitter is a device that converts a single PS/2 keyboard port into two ports. You can find a circuit diagram to build your own on the Internet, if you are so inclined. If you are not electronically inclined, purchasing a splitter is a better choice. P.I. Engineering sells the Y-key key Dual Keyboard Adapter shown in Figure 8-10. This retails for around $50 and is plug-and-play. Simply plug in the adapter, plug in both keyboard devices, and turn on the computer. Both keyboard devices are fully functional and the Y-key key is daisy chainable allowing three or four keyboard devices to be connected simultaneously.
A very easy way to have a second keyboard available on your arcade cabinet is to use a USB keyboard. Your computer has to have a USB port, of course, but almost every computer made in the last few years has them. Some of the first computers with USB ports had problems running a USB keyboard until the operating system loaded, making configuring the BIOS settings in your computer a problem. However, this is also an unlikely scenario today. Just make sure if you use a USB keyboard that it is your actual keyboard and not used for a hack, because of the six keystroke and operating system limitations discussed in the USB keyboard limitations section.
Probably the most elegant solution to the multiple keyboard question is available with many of the commercial keyboard encoders in the next section. They include a keyboard pass-thru connector which allows a keyboard to be plugged into the back of the keyboard encoder device. Not every commercial encoder comes with a pass-thru, so you need to read the details on the ones you might be considering.
A keyboard hack is not terribly difficult, but it is fairly time-consuming. You should ask yourself what your time is worth to you. There is some sense of satisfaction for having accomplished a keyboard hack and having spent very little money on it, but with solderless commercial alternatives available starting at $34, a keyboard hack is probably not worth the effort. I do not recommend spending the time on one unless you are on an extremely tight budget. If you are going to pursue a keyboard hack, there is a good article on the Internet that I suggest consulting as an addendum to the material presented here. Marshall Brooks has created an excellent document expanding on the issues presented here, as well as providing matrix maps for several keyboard models. Keyboard hacks are recommended only for those on a tight budget building two-player panels.