Abstract: In a parallel motion finger probe, the parallel motion arm is rigidly connected to a shaft which is supported in a linear ball sleeve bearing, instead of by a parallel motion linkage as in prior finger probes. The parallel motion arm can be biassed in either of its two opposite directions of movement by a leaf spring, whose biassing force and direction can be adjusted by a control wheel having an eccentric groove co-operating with a pin on a spring control arm to which the spring is connected. The leaf spring also serves to prevent rotation of the parallel motion arm about the axis of the bearing. Displacement of the parallel motion arm is sensed by an inductive displacement transducer of the LVDT type.

Abstract: Departure of a logic signal from the ranges of signal values corresponding to defined logic levels is monitored by comparing the logic signal with the boundary values of these ranges. Provided the logic signal between transitions is below the lower threshold (14) or above the upper threshold (12), a NOR gate (16) and a D-type bistable circuit (18) open an AND gate (22) to enable a counter (24) to count 10 MHz clock pulses. If the logic signal is between the thresholds, the AND gate disables the counter. The four most significant digits of the total number of clock pulses actually counted after 1,000,000 have been supplied is displayed, giving an indication of the percentage of time for which the logic signal has a value corresponding to a defined logic level. As an alternative, the output signal of the NOR gate (16) can be inverted (26) and supplied to a moving-coil or digital voltmeter (30).

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.