A digital input circuit for receiving digital input signals of at least one signal generator comprises first and second subcircuits. Each subcircuit includes a digital input to receive a digital input signal and a threshold value element to determine a logic state of the subcircuit. Each subcircuit adopts a first state in response to its digital input signal reaching or falling below a lower threshold value and adopts a second state in response to its digital input signal reaching or exceeding an upper threshold value. The digital input circuit further comprises a combined test and current increasing apparatus to generate a driving signal such that a function test of both the first and second subcircuits is conducted simultaneously with the driving signal and an input current of the digital inputs is increased.