A sensor connecting unit, comprising: - a safety input for connecting with a sensor, or, alternatively, a first sensor connected to a first safety input and a second sensor connected to a second safety input; - a power supply that is controllable through a controllable power switch, or, alternatively, a controllable power supply; - a first I/O processing unit and a second I/O processing unit being connected to each other through a reciprocal comparison communication channel; - the first I/O processing unit and the second I/O processing unit both connected through a measurement circuit (R1, R2, R3, R4, C1) to measure voltage, current or power between a reference input and the safety input and/or the second safety input, and connected to a safety bus (4A, 4B); characterized in that the first I/O processing unit is configured to control the controllable power switch or, alternatively, the controllable power supply wherein the first I/O processing unit has at least two off-time windows, the duration of which are different from each other, during which the power switch or, alternatively, the controllable power supply, is controlled to be switched off; and i) the first I/O processing unit is configured to inform the second I/O processing unit about switching of the power switch or, alternatively, of the controllable power supply, preferably through the reciprocal comparison communication channel or an input line branched from a control line used to control the controllable power switch or alternatively, the controllable power supply; and/or ii) the second I/O processing unit is configured to command the first I/O processing unit to switch the power switch or, alternatively, of the controllable power supply, preferably through the reciprocal comparison communication channel; and/or iii) the second I/O processing unit is configured to measure an output of the controllable power switch or, alternatively, of the controllable power supply to detect switching of the power switch or, alternatively, of the controllable power supply. - a safety input for connecting with a sensor, or, alternatively, a first sensor connected to a first safety input and a second sensor connected to a second safety input; - a power supply that is controllable through a controllable power switch, or, alternatively, a controllable power supply; - a first I/O processing unit and a second I/O processing unit being connected to each other through a reciprocal comparison communication channel; - the first I/O processing unit and the second I/O processing unit both connected through a measurement circuit (R1, R2, R3, R4, C1) to measure voltage, current or power between a reference input and the safety input and/or the second safety input, and connected to a safety bus (4A, 4B); i) the first I/O processing unit is configured to inform the second I/O processing unit about switching of the power switch or, alternatively, of the controllable power supply, preferably through the reciprocal comparison communication channel or an input line branched from a control line used to control the controllable power switch or alternatively, the controllable power supply; and/or ii) the second I/O processing unit is configured to command the first I/O processing unit to switch the power switch or, alternatively, of the controllable power supply, preferably through the reciprocal comparison communication channel; and/or iii) the second I/O processing unit is configured to measure an output of the controllable power switch or, alternatively, of the controllable power supply to detect switching of the power switch or, alternatively, of the controllable power supply.