"As noted in section 3.5.2, the CPU module on units 4167 and 4122 did not
incorporate EDAC, nor was it required by the aircraft manufacturer’s specification"
It was apparently added in a later HW revision
"The LTN-101 ADIRU’s CPU module was later redesigned to reduce costs and to
include error detection and correction (EDAC). EDAC is used for detecting and
correcting single-bit errors in RAM chips to give protection from single event
effects (SEEs, see section 3.6.6). This change was a significant redesign and
resulted in a new CPU module part number (466871-01). The EDAC was
performed by a new ASIC, and all of the RAM chips used on the CPU module were
replaced with a different chip.13"
It was apparently added in a later HW revision
"The LTN-101 ADIRU’s CPU module was later redesigned to reduce costs and to include error detection and correction (EDAC). EDAC is used for detecting and correcting single-bit errors in RAM chips to give protection from single event effects (SEEs, see section 3.6.6). This change was a significant redesign and resulted in a new CPU module part number (466871-01). The EDAC was performed by a new ASIC, and all of the RAM chips used on the CPU module were replaced with a different chip.13"
https://www.atsb.gov.au/sites/default/files/media/3532398/ao...