Abstract: A keyboard has tactile-feel keys (64-70) each with a grounded terminal (78) under an insulated touchable terminal (88) supplied with a square wave signal.Touching a key increases the capacitive delay of the square wave signal on that key, so it is retarded relative to first delayed clock pulse which then resets a bistable circuit (142) to stop a counter (152), which controls scanning of the keys, at a value indentifying the touched key. This enables selection of the touched key to be verified, for example on an associated video display, whereafter the key can be pressed to actuate it, thereby establishing contact between its terminals. The resulting grounding of the touchable terminal suppresses the square wave signal, resetting another bistable circuit (172) in response to another, more delayed, clock pulse to generate a signal indicating that the desired key function is to be implemented.

Abstract: Digital simulation apparatus, e.g. a digital coastline generator, for simulating a terrain-representative display of the kind produced by a movable scanning-type display device, e.g. a radar display device, comprises a main store for storing digital data representative, in Cartesian co-ordinate form, of terrain features such as contours and buildings in an area, known as the playing area, typically 256 nautical miles by 256 nautical miles. The data is divided into groups relating to contiguous zones each typically 2 nautical miles by 2 nautical miles. The apparatus further comprises means for producing a signal representative of the current position of the display device in the playing area, means for producing successive signals representative of successive segments of scan of the display device, and means responsive to these signals to select from the main store those groups of data corresponding to zones intersected by the current scan segment.

Abstract: A printed circuit mother board, for interconnecting the respective edge connectors of a plurality of other printed circuit boards having digital circuitry thereon, has a set of generally straight, approximately parallel tracks on each side. Each set of tracks comprises alternate signal tracks and ground tracks, and the ground tracks of each set are interconnected with each other to approximate to a ground plane. Additionally, the tracks on one side of the board are slightly laterally displaced with respect to the tracks on the other side, so that each signal track on either side is opposite a ground track on the other side. The possibility of interference between signals carried by the signal tracks is thus reduced, and the board has transmission line characteristics.