US Patent 7124655 Two-wire sensor for measuring a physical parameter

To measure a physical parameter, a two-wire sensor produces pulse-width modulated signals (PWM) whose pulse width is preferably modulated as a function of the physical parameter to be measured. To indicate an error or a malfunction, the two-wire sensor provides an error signal with a specifiable pulse-width ratio that is preferably 1:1, while asymmetric pulse-width ratios are provided for the measurement signals (PWM). To measure a physical parameter that assumes only one of two states or values, the two-wire sensor produces a first measurement signal with a specifiable asymmetric pulse-width ratio and a second measurement signal formed by inversion of the first measurement signal. A plurality of two-wire sensors can for example be connected to a common two-wire line and operated in time-multiplex mode.