ISO1228 monitors the following fault conditions: Power Loss Detection on the field side, Over Temperature Detection and Thermal Shut Down on the field side, Wire-Break detection on any input channel, and CRC failure in serial communication across the isolation barrier. Upon detection of any fault, the corresponding bit in the SPI register is made high. Similarly, any fault detection is flagged on the nFAULT pin (active low). Ignore the nFAULT pin and the data in the SPI registers till 25ms after power up. The fault status in the SPI register is maintained till the fault register is read, provided the underlying fault condition is resolved. nFAULT pin is asserted for at least 9µs or until the SPI fault register is read, whichever comes first, provided the underlying fault condition is resolved. If Field Power Loss or CRC bit is set, other bits in the Fault register (02h) do not care.

For more information see How to Dynamically Switch Between Serial and Parallel Modes Using ISO1228.