The pros and cons of PS/2 to USB adapters and converters

Now that most PCs and laptops ship with only USB ports, you'll need an adapter for older PS/2 devices. Here are some things to look out for when picking an adapter or converter.

Many of us are familiar with a host of devices that allow a PS/2 peripheral -- typically a mouse or a keyboard

-- to be attached to a USB port. These devices are doubly useful now that most PCs ship without PS/2 ports and attach all of their peripherals via USB.

You may want to attach legacy PS/2 devices to those machines, whether out of technical necessity or user preference. In some cases, though, it's not possible to directly substitute a PS/2 device with a USB replacement:

  • The device itself is unique in some respect. Some tablets or other pointing devices are like this.

  • The user of the device prefers that device to the exclusion of others. I've known a few people -- myself included -- who were very picky about their choice of keyboard and have grown attached to a given model (typically an older PS/2 model with a specific kind of force feedback in the keys).

However, PS/2 to USB devices are not without their problems. In an ideal world, they would simply provide you with a consistent way to use legacy hardware with modern systems. This being a less-than-ideal world, there are a number of ways these devices can give you less-than-ideal results.

Remember that an adapter is not a converter. This is one of the most common mistakes made by people trying to use PS/2 hardware on USB-only systems. Many simple PS/2-to-USB plugs or adapters are not designed to do anything more than perform a wire-to-wire conversion, and they are mostly intended for adapting a USB device to a PS/2 connector, not the other way around. A more sophisticated set electronic -- a converter -- is required to connect a PS/2 device to a USB port.

PS/2-to-USB converters are not all the same.
There are a slew of PS/2-to-USB adapters on the market, and each one of them behaves slightly differently. These differences are enough to make them irritating to work with at best and useless at worst. Here are some common problems you might encounter with these devices, as originally compiled on the GeekHack.org wiki (a good source of information for which PS/2-to-USB converters do the job better than others):

Incomplete scan code support. A scan code is the signal sent from the keyboard to the computer representing a given key press. USB keyboards have their own set of scan codes that are distinct from the older PC XT/AT/3270 scan codes, which have to be mapped to the newer ones by the converter hardware. Some converters don't have every single scan code mapped, and so some exotic keystroke combinations may not work. GeekHack mentions the Alt-PrintScreen combo, which is useful in Windows for taking a screenshot of the current window and saving it to the clipboard, as being one possible omission. This can be worked around -- for example, you could use a screenshot-taking program to perform this function or map it to another keystroke -- but it's still a sign of how these devices are not always implemented carefully.

Incomplete ISO layout support. The International Standards Organization (ISO) keyboard layout -- a standard known as ISO IEC 9995 -- provides for a 101-key layout for U.S. users and a 102-key layout for other countries. Some PS/2 converters do not support 102-key layouts and as a result, can't support "dead key" functions like a compositing key for typing many non-ASCII characters. This might not be a problem for most environments, but it could be for those that use 102-key layouts and legacy keyboards with newer, USB-only systems. (At one point, I owned a Japanese-language keyboard that had this problem.)

Older keyboards that draw more current. IBM's classic Model M keyboard draws more power than can be supplied by most conventional USB adapters, which supplies a maximum of 500 mA at 5 volts (or 2.5 watts). Such keyboards may not work properly with these adapters, or they may work only when plugged into a powered USB socket.

Not all PS/2 devices are properly supported. Most keyboards tend to work with converters (barring the exceptions discussed above), however, some other PS/2 devices, such as touch pads, trackball mice, or some other varieties of pointing devices, may not work at all.

In the long run, the best solution is to completely replace any legacy PS/2 devices -- but in the few instances where this simply isn't possible, it's wise to do some research.

ABOUT THE AUTHOR
Serdar Yegulalp has been writing about personal computing and IT for over 15 years for a variety of publications, including (among others) Windows Magazine, InformationWeek and the TechTarget family of sites.


This was first published in October 2010

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