Type-2 digital input and output signal conditioning circuits for industrial safety applications

In industrial safety-critical applications, 24V digital line signals are often used, for example: Motor control, hydraulic/pneumatic valve control, and device control [1]. In this paper a low probability of failure per hour (PFH) input and output signal-conditioning circuit (24V-→3V, 3V-→24V) has been presented. These signal-conditioning circuits have specifications compatible to Type-2 of the IEC61131 standard [1], including test-signals and status-feedback signals for circuit monitoring. The failure mode and effect analysis have been calculated and the, results show a PFH value of 1-99*1016 for the input circuit and 1*10 11 for the output circuit, which are sufficient to achieve Safety Integrity Level 4 (SIL4). The test signals have been applied by the microcontroller or the control unit to the circuits periodically every 100ms, the feedback signals are read by the microcontroller and compare it with the expected value. A failsafe condition should be implemented in case of detected fault (ex. stopping the machine, sending an alarm signal, or sending an error signal to the main station). Stopping the machine scenario has been implemented in this work for a specific Industrial applications.