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User Interfaces and Interface Transmission Technologies to Assistive Devices

When making products usable for all users, including elderly users and those with disabilities the Design for All concept should be followed. If this is not achievable, one solution that may be chosen is to offer alternative interfaces that permit the use of assistive devices to fill the gap between the user interface of the original device and the abilities of the user.

What is a user interface?

The user interface (or Human Computer Interface) is the means by which people - the users, interact with the system - a particular machine or device. The user interface provides means of:

  • input - allowing the users to manipulate a system
  • output - allowing the system to produce the effects of the users' manipulation

A braille displayProducts, known as assistive devices, have been specifically designed for people who have disabilities. Assistive devices need to be used in cases when the input and output facilities of ICT devices are unable to match the abilities of specific users. It is therefore necessary to characterise the input media and their possible replacements in assistive devices in order to define the data that need to be transferred between ICT device and assistive device. Table 1 (below) describes standard input media and their possible replacements while Table 2 (below) lists the respective output media.

Table 1. ICT input media

ICT input media
Replacement assistive device / media
Keys (keyboards, keypads) Alternative keys with different characteristics (larger, softer, more space between keys)
QWERTY keypad
Voice input
Speech to text
Alternative keyboards
Voice input (microphone) Text to speech
Pointing device
Pointing devices Text input
Speech input
Alternative pointing device (e.g. for use with tongue)
Cursor keys

Table 2. ICT output media

ICT output media
Replacement assistive device / media
Visual display Larger display
Display with colour conversion
Braille pad / Braille display
Speech / audio (screen reader)
Audio output Vibratory
Amplifier / attentuators
Speech to text (possibly with Braille extension)

ICT device input requirements

The input required by ICT devices can be characterized by the way the information is entered into the device:

  • Text and/or control commands typed on a keyboard, the keys either being used for character input or as function keys. On mobile phones one of the standard ways of text input is by multiple key clicks on a number input keypad.
  • Text and/or command input using voice and a microphone.
  • Voice or video input to be transmitted to another ICT device without interpretation in the ICT device.
  • Position information being input by means of a pointing device (mouse, keypad, touchscreen). This information is either being input as a pair of numbers representing the x- and y-coordinates of a position on a screen or, alternatively, as a list of pairs of x- and y-coordinates representing a curve being input with the pointing device.

The minimum Man Machine Interface (MMI) necessary for input to an ICT device consists of a number of keys for text and command input as well as either a pointing device or a set of cursor keys to control the focus of the input.

ICT device output capabilities

The standard media for representing output on an ICT device are:

  • display-screens for the display of text, graphics and/or video information;
  • loudspeakers for the presentation of audio communication, spoken text or state information;
  • light emitting diodes for the presentation of state information like e.g. on/off or caps-lock on a keyboard;
  • tactile output using a vibra system (e.g. for signalling of incoming calls in mobile phones).

The minimum MMI necessary for the output from most ICT devices must be capable of presenting textual, graphical and state information using audio and visual output media

Who would benefit from standardised user interfaces?

A person with any disability will require assistive technology to use ICT equipment whenever they cannot operate the controls, obtain information from the device or understand how the device operates.

Elderly and disabled users would benefit from standardisation of the interfaces to ICT so that one
assistive device, e.g. a display for the presentation of information in large letters, could be used for the widest possible range of products from different manufacturers.

The manufacturers themselves would benefit by complying with European and international regulations if they offer a compatible interface, even if they leave the production of the
assistive devices to third party manufacturers.

Problems encountered by disabled people and the ageing population using non-standardised user interfaces

Blind and Partially Sighted

A blind or partially sighted person will encounter problems with tasks on an ICT device that requires the use of sight, especially if provision has not been made for screen text size and colour configurations.

Hearing impaired

A person with hearing loss will have difficulty in hearing speech commands or audible signals given on an ICT device.

Physically impaired

People with physical impairments may have difficulty in operating controls on a device, for example, a person with an impaired sense of touch may find it difficult to use a touch pad on a computer.

Cognitively impaired

Some people who have a cognitive impairment may find that they do not understand how the device operates, or understand icons, abbreviations and long sentences or instructions.

Ageing population

Some elderly people suffer from a failing memory which can affect their ability to recall and learn things and may lead to confusion, therefore, devices that require significant programming or re-programming may cause problems.

Interface transmission technologies

Information to be exchanged

In most cases the requirement for an assistive device is either to produce output from the ICT system in a different modality (e.g. to convert text on a screen to speech output) or to produce an enhanced version of the same modality (e.g. larger characters on a high contrast display). In other cases the requirement may be for the modification of characteristics of signals which are not usually under user control (e.g. requesting more time for crossing at pedestrian controlled traffic lights).

The table below indicates the types of information to be exchanged between an ICT system and an assistive device.

Assistive device/service
From ICT system to the assistive device
To the ICT system from the assistive device
Control & status
Control & status
Braille display
Tactual graphics display
Synthetic speech display
Enhanced visual display
Keyboard / pointer
Speech recognition
Hearing aid
Tactile hearing aid
Alarm/monitor system
Smart house
Navigation system

Wired technologies

An RS-232 interfaceRS-232

RS-232 is an interface designed in the 1960s for communication between a dumb ASCII terminal and a modem. For many years it was the standard interface to interconnect computers, terminals, printers, scanners, mice, etc.

RS-232 is very suitable for connecting assistive data devices. However, it is not suitable for voice. Simplicity is the big advantage of RS-232.

Plain analogue audio

The simplest way of connecting audio devices is to do it with a plain analogue one-directional 2-wire interface constituting out of signal and ground. A bi-directional interface would be two one directional interfaces, where the ground wire could be common.


Depending on the requirements, the choices for video transmission are:

  • The interfaces used for connecting video cameras and recorders to TV sets. These interfaces are limited in resolution. In Europe 625 lines per frame/50 half-frames per second is used. North America and Japan 525 lines per frame/60 half-frames per second is used.
  • Also on the physical level there are differences. It is possible to transmit red, green and blue over separate wire pairs. It is also possible to transmit luminance and modulate the colour in the luminance signal. Three different colour modulation schemes are used in Europe (PAL, SECAM-France and SECAM-East) and one other in North America and Japan (NTSC).
  • The interfaces used for connecting monitors to PCs. These interfaces provide for a high resolution.
  • Streaming video. Video is transmitted as MPEG streams over an Internet connection. A computer is necessary for decoding it.


USB is an interface used for connecting up-to 127 (slave) devices to a single host (master). The physical connection is a tiered star topology. The logical connection is point-to-point.

A typical USB configuration is a PC acting as a host (master). The host initiates all data transfers. Typically each device has a corresponding device driver at the host, which communicates with the device driver for the USB host adapter, which in turn communicates with the USB physical layer.

Wireless technologies

The two most dominant wireless technologies for local area connections are WiFi and Bluetooth.

WiFi / IEEE 802.11 family

WiFi is the trade name for the popular wireless technology used in home networks, mobile phones, video games and other electronic devices that require some form of wireless networking capability. In particular, it covers the various IEEE 802.11 technologies.

IEEE 802.11 refers to a family of specifications developed by the IEEE for wireless LAN technology. IEEE 802.11 specifies an over-the-air interface between a wireless client and a base station or between two wireless clients.

The purpose of WiFi is to provide wireless access to digital content. This content may include applications, audio and visual media, Internet connectivity, or other data. WiFi generally makes access to information easier, as it can eliminate some of the physical restraints of wiring; this can be especially true for mobile devices.


Bluetooth is a standard for wireless accessory connections. With the current specification, up to 7 slave devices can be set to communicate with a master device. The maximum range is 10m but the range can be extended to 100m with additional amplifiers. The primary application areas have been considered to be interconnecting mobile phones, computers and PDAs. Secondary applications include smart housing and CANs (car area networks).

DECT (Digital Enhanced Cordless Telecommunications )

A DECT cordless telephoneDECT is an ETSI standard for cordless telephony, developed in the 1990s. The standard is defined in the various parts of EN 300 175-1. The typical application for DECT is a base station connected to the public or a private telephony network. The base station communicates with a handset, after configuration and set-up.

DECT aims to provide speech transmission at a quality comparable to the wired telephony service. In addition to voice calls it supports some data services like SMS, circuit switched and packet switched data. The DECT standard guarantees interoperability between similar devices from different manufacturers.


The Infrared Data Association (IrDA) defines physical specifications communications protocol standards for the short-range exchange of data over infrared light, for uses such as personal area networks (PANs).

The infra-red spectrum is unregulated world-wide which overcomes one problem faced by the radio-based systems, but has the disadvantage that it is directional and requires both ends to be in the same room. Another significant advantage is that it is available now and it is inexpensive. A problem is that the standards are not uniquely defined so equipment from one manufacturer is not always compatible with that of another.

Checklist for user interfaces



  • Text size should be configurable
  • Text colour and background colour ahould be configurable
  • Provision should be made to permit a Braille display or screen reader to be fitted
  • Publicly accessible equipment should offer alternative display modalities
  • The text and background colour combination should have high contrast
  • A clear open typeface, should be used for text
  • Text should not be placed over a background image or over a patterned background
  • White or yellow type on black or a dark colour is more legible, provided that the typeface (font), weight and size are suitable. Small type and very bold type tend to blur for some people, reducing legibility
  • Avoid shades of blue, green and violet for conveying information since they are problematic for older users
  • Speech output of instructions, as an addition to (and not a replacement for), on-screen instructions, is recommended
  • There should be no noticeable flicker on the screen
  • Products should be designed to avoid causing the screen to flicker with a frequency greater than 2 Hz and lower than 55 Hz
  • Use no more than five colours when coding information
  • Adhere to existing colour conventions e.g. red for stop
  • Structure the visual display layout so that the user can predict where to find required information and how to use it


  • The text and background colour combination should have high contrast
  • Avoid shades of blue, green and violet for conveying information since they are problematic for older users
  • There should be no noticeable flicker on the screen
  • Structure the visual display layout so that the user can predict where to find required information and how to use it

According to Namahn (2000), touch sensitive areas or keys for users with no impairments, should be of a:

  • "minimal size: 22mm across"

However, Colle & Hiszem (2004) recommend that:

  • a "key size no smaller than 20mm...should be used if sufficient space is available"

Key size should be varied according to the size and use of the screen and the impairment type. For example, for one-handed thumb use of mobile handheld devices equipped with a touch-sensitive screen Parhi, Karlson and Bederson (2006) state that:

  • "target size of 9.2mm for discrete tasks and targets of 9.6mm for serial tasks should be sufficiently large...without degrading performance and preference"
  • There should be high contrast between touch areas, text and background colour
  • Text or controls should not be placed over a background image or over a patterned background
  • White or yellow type on black or a dark colour is more legible, provided that the typeface, weight and size are suitable
  • Controls are labelled in a large high contrast font

Audio output

  • Inductive coupling facilities should be made available in public terminals and on fixed telephones
  • Provision should be made to permit a device, such as a flashing light ringer, to be connected
  • Standard 3.5mm connectors should be used
  • The user must be able to increase the volume of the audio signal (preferably to at least 90 dB SPL)
  • Provide text versions of audio prompts that are synchronised with the audio so that the timing is the same
  • There should be audio cutoff when an external listening device is connected
  • Audio output is supplemented by captioning or subtitles
  • Signals from the basic system, such as alarms, warnings, status lamps and error messages, should have the following characteristics:
    • Alternative forms - auditory, visual or tactile, allowing both visually and hearing impaired persons to adapt the signalling to their perceptive characteristics
    • The volume, and if possible also the pitch and frequency, of auditory output should be adjustable
    • Visual signals should be placed where they are easily perceived
  • There is an option to reset the volume to a default level
  • Warnings and similar alert messages must remain stable for a sufficiently long time to be discovered by the user. A way of avoiding problems is to let the message remain until dismissed by the user

Icons / Graphics

  • Symbols should match the medium
  • Symbols should create the illusion of manipulatable objects, e.g., it should be clear that they can be selected, how to select them and be obvious when they are selected
  • Graphical symbols should be constructed with as few graphical components as possible (usually not more than 2 or 3 components)
  • Symbols should always be presented upright
  • The symbol stands out from the background
  • Ensure that each icon is distinct from, and clearly visible within, a surrounding group of symbols
  • Try to make each symbol as distinct from the others as possible
  • Symbols should be designed according to a grid, or a basic pattern as human perception is sensitive to optical weight. Thus symbols sharing the same pattern are more easily recognized, and provide a feeling of unity and of consistency
  • Angles smaller than 30° as well as filled areas are avoided

Keyboards / keypads

Basic ergonomic features
  • All commands and options can be accessed by using the keyboard. This could be provided by operating the arrow keys or by keyboard equivalents for all menu items
  • The keyboard should be detachable and have a sufficiently long cable, approximately 1.5 metres (Nordic Cooperation on Disability, 1998), so that it can be placed according to the user's needs
  • According to the Nordic Cooperation on Disability (1998) the keyboard should be of a low-profile type, i.e. the height of the key-row beginning with A-S should be not greater than 30mm
  • There should be good friction between the keyboard and the desktop
  • The colour of the keytops should be matt
  • The surface of the keytops should be concave
  • The user should receive a tactile and audible feedback when pressing a key. It should be possible to adjust and switch off the audible feedback
Localisation of keys
  • Groups of keys (alphanumeric, numeric, function keys) should be separated by distinct spaces, with a distance of at least half a key (This requirement is not applicable on laptops.)
  • Groups of keys should be distinguished by different colours on the key tops, but in a way that colour-blind persons may discern the colours.
  • The F and J keys on the alphanumeric keyboard, and the 5 key on the numeric keyboard, should be marked with a tactile identification, preferably in the form of a ridge on the keytop edge nearest to the user.
  • Frequently used keys, such as Enter, Shift, Escape, Ctrl, Backspace etc., should be placed and have a shape that differ from other keys so that they are easy to find.
Identification of keys
  • The contrast between the colours of the characters and the background of the keytop should be the best possible
  • The height of the characters of the alphanumeric and numeric keys should, according to the Nordic Cooperation on Disability (1998), not be less than 4mm.
  • The height of the characters of the other keys should not be less than 4mm, if there is available space.
  • The text on the keys should be printed in sans-serif font
  • Text should not be printed in the colours red or green
Activating keys
  • The user should be allowed to customise the keyboard with respect to features such as repeat rate, key activation delay, delay between acceptance of two consecutive key presses, minimum time for pressing a key before the key repeat begins, and serial instead of multiple simultaneous keystrokes etc.
  • If key repeat is supported, the delay before repeat should be adjustable to at least 2 seconds per character (IBM, 2008)
  • If sufficient space is available, shift keys (upper and lower case, ctrl, alt etc.) should be duplicated, one on each side of the keyboard, and be placed symmetrically
  • The keyboard should be designed to provide sufficient space to allow the user to mount a keyguard
  • The system should allow the connection of two keyboards, which could be used simultaneously, for instruction purposes

According to the Nordic Cooperation on Disability (1998) the power to press a key should be:

  • "between 0.3 and 0.6 Newton"

Pointing devices

  • The user should be allowed to execute pointing functions from the keyboard
  • Provide the user with the facility for modifying cursor speed, distance and double-click interval
  • The operation of a pointing device should not require two simultaneous hand movements
  • The power needed to operate a pointing device should be between 0.3 and 0.6 Newton (Nordic Cooperation on Disability, 1998)


  • EIA RS-232-C (1969) Interface between data terminal equipment and data communication equipment employing serial data interchange
  • ETSI EN 300 175-1 (V1.4.2) (2008) Digital Enhanced Cordless Telecommunications (DECT);
    Common Interface (CI) - Part 1: Overview
  • ETSI ES 202 076 - Human Factors (HF); User Interfaces; Generic spoken command vocabulary for ICT devices and services
  • ETSI ETS 300 679 Terminal equipment (TE); Telephony for the hearing impaired; Electrical coupling of telephone sets to hearing aids
  • ETSI TR 102 068 (2002) Human Factors (HF): Requirements for assistive technology devices in ICT
  • ETSI TS 102 511 (2007) Human Factors (HF): AT commands for assistive mobile device interfaces
  • IEEE 802.11 IEEE standard for Information Technology; Telecommunications and information: Exchange between systems; local and metropolitan area network; specific requirements - Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specification
  • ISO 13406-1 (1999) Ergonomic requirements for work with visual displays based on flat panels. Part 1 - Introduction.
  • ISO 13406-2 (2001) Ergonomic requirements for work with visual displays based on flat panels. Part 2 - Ergonomic requirements for flat panel displays
  • IEC 80416-1 (2001) Basic principles for graphical symbols for use on equipment - Part 1: Creation of symbol originals
  • ISO 80416-2 (2002) Basic principles for graphical symbols for use on equipment - Part 2: Form and use of arrows
  • IEC 80416-3 (2002) Basic principles for graphical symbols for use on equipment - Part 3: Guidelines for the application of graphical symbols
  • ISO 80416-4 (2005) Basic principles for graphical symbols for use on equipment. Part 4 - Guidelines for the adaptation of graphical symbols for use on screens and displays.
  • ISO 9241-151 (2008) Ergonomics of human-system interaction - Part 151: Guidance on World Wide Web user interfaces
  • ISO 9355-1 (1999) Ergonomic requirements for the design of displays and control actuators. Part 1: Human interactions with displays and control actuators.
  • ISO 9355-2 (1999) Ergonomic requirements for the design of displays and control actuators. Part 2: Displays.
  • ISO 9999 (2007) Assistive products for persons with disability - Classification and terminology
  • ISO/CD 24500 Guidelines for all people, including elderly persons and persons with disabilities - Auditory signals on consumer products
  • ISO/IEC 15411 (1999) Information technology - Segmented keyboard layouts
  • ISO/IEC 15412 (1999) Information technology - Portable keyboard layouts
  • ISO / IEC 24755 (2007) Information technology - Screen icons and symbols for peronal mobile communication devices
  • ISO/IEC CD 24786-1 Information Technology - User interfaces - Accessible user interface for accessibility setting on information devices - Part 1: General and methods to start
  • ISO/IEC TR 15440 (2005) Information Technology - Future keyboards and other associated input devices and related entry methods
  • ISO/IEC TR 19765 (2007) Information technology - Survey of icons and symbols that provide access to functions and facilities to improve the use of IT products by the elderly and persons with disabilities
  • ISO/IEC TR 19766 (2007) Information technology - Guidelines for the design of icons and symbols accessible to all users, including the elderly and persons with disabilities
  • ITU-T E.902 (1995) Interactive services design guidelines
  • ITU-T P.85 (1994) A method for subjective performance assessment of the quality of speech voice output devices
  • Specification of the Bluetooth System: Volume 1 - Core (Revision 1.1) (2001)
  • Specification of the Bluetooth System: Volume 2 - Profiles (Revision 1.1) (2001)
  • USB 2.0 Specification (2000)

Further information


  • Colle, H. A. & Hiszem, K. J. (2004) Standing at a kiosk: effects of key size and spacing on touch screen numeric keypad performance and user preference. Ergonomics, 47(13), 1406-1423, October 22.
  • ETSI TR 102 068 (2002) Human Factors (HF): Requirements for assistive technology devices in ICT
  • IBM (2008) Hardware self contained, closed product accessibility checklist. [accessed 21/02/08].
  • Namahn (2000) Touch screens - the ergonomics: a Namahn brief. [accessed 06/02/08].
  • Nordic Cooperation on Disability (1998) Nordic Guidelines for computer accessibility. [accessed 21/02/08].
  • Parhi, P., Karlson, A. K. & Bederson, B. B. (2006) Target size study for one-handed thumb use on small touchscreen devices. Proceedings of the 8th conference on human-computer interaction with mobile devices and services, Helsinki, 12-15 September 2006, 203-210. New York: ACM.
  • Wikipedia (2008) Bluetooth. [accessed 24/10/08].
  • Wikipedia (2008) HomeRF. [accessed 27/10/08].
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  • Wikipedia (2008) RS-232. [accessed 24/10/08].
  • Wikipedia (2008) User interface. [accessed 23/10/08].
  • Wikipedia (2008) Wi-Fi. [accessed 05/11/08].