For those severely physically disabled persons who are unable to use keyboards, the computer has to be adapted to operate from the limited signals that can be obtained from controlled movements. These movements need to be identified and appropriate devices provided which can produce suitable signals to operate a computer. To minimize programming effort, it is also necessary to standardize on a particular form of input signal—in our case, one or two on/off contacts -for which the programs are developed. It is desirable for user input devices to be linked to the computer through an Interface Control Unit, which can also be operated from a remote infra-red unit mounted on a wheelchair. Such units are described.