The above block diagram shows a simple redundant system in which two computers are connected to an internally redundant Interface Unit (IU). Only one of the two computers is required to output a control signal for some mission critical operation. Also, one of the IU buses is required to complete the mission (i.e.: the outputs from MUX_A are on the A-Bus and the outputs from MUX_B are on the B-Bus). The IU receives the two control signals CONTROL1 and CONTROL2 into dual redundant multiplex devices labeled MUX_A and MUX_B. Each mux chip has a bypass capacitor connected across the power supply input to ground.

This looks to be a robust redundant system. However, review of the FMEA shows that there are single point failures lurking in the design…

The FMEA worksheet for this circuit is at WORKSHEET. The process is to hypothesize a failure and determine that failure’s effect on the rest of the system. The failure mode of each device may be determined by the data in FMD-97. FMD-97 is a database compiled by Reliability Analysis Center (RAC). This database contains many electronic devices and the failure modes of the devices. For example, the capacitor used in the circuit shown is a Ceramic 1 uF device.

The failure modes of this generic device are as follows:





The failure mode of the IC in the circuit can also be found from the database. The failure modes Including single point failures (SPFs) are shown in the FMEA worksheet. Go to the WORKSHEET.

There are single point failures in this design. See this link for a fix to the design to remove the Single point failures…FMEA_NO_SPF

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