Introduction

The BMXSDO0802 safety digital output module provides the following diagnostics using its T_U_DIS_SIS_OUT_8 device DDT elements:

  • output diagnostics

  • internal error detection

  • channel wiring diagnostics

  • overvoltage and undervoltage diagnostics

Output Diagnostics

Each output channel is tested for operational effectiveness at the start of each cycle (or scan). The test consists of toggling the output contact states (from ON to OFF, or OFF to ON) for a time too short to cause an actuator response (less than 1 ms). If the channel does not successfully toggle between energized and de-energized, the CH_HEALTH bit in the T_U_DIS_SIS_CH_OUT DDDT structure is set to 0, indicating it is not operational.

Internal Error Detection

The module processes the output value using two separate, identical circuits. Each circuit reads the midpoint voltage on the channel. The two values are compared, and if the values are not the expected values, an internal detected error is flagged by setting the IC bit in the T_U_DIS_SIS_CH_OUT DDDT structure to 1, indicating it is not operational.

Refer to the architecture diagram for the BMXSDO0802 safety digital output module for a visual presentation of this process.

Channel Wiring Diagnostics

The wiring of the actuator to the output channel can be continuously diagnosed for any of the following conditions:

  • cut wire (open circuit)

  • short circuit to 24 Vdc

  • short circuit to 0 Vdc

  • cross circuit between two parallel channels

  • channel overload

    NOTE: Channel overload can be detected only if the output is energized.

The availability of these diagnostics depends on the diagnostic function being enabled in the configuration page of the module.

If one of these conditions is detected, the T_U_DIS_SIS_CH_OUT DDDT structure sets the associated bit value to 1, as follows:

  • the OC bit is set to 1 if an open (cut) wire condition is detected.

  • the SC bit is set to 1 if a short circuit to the 24 Vdc source or cross circuit between two channels is detected.

  • the OL bit is set to 1 if a short circuit to the 0 Vdc ground or a channel overload condition is detected.

Overvoltage and Undervoltage Diagnostics

The module continuously tests for an overvoltage and an undervoltage condition. The following threshold values apply:

  • Undervoltage threshold = 18 Vdc

  • Overvoltage threshold = 31.8 Vdc

If either condition is detected, the module sets the PP_STS bit in the T_U_DIS_SIS_OUT_8 device DDT to 0.