You should disregard this bit when evaluating BM message status. The Unterminated Bus Bit in the interrupt status word is no longer used. Earlier versions of the 1553 firmware tried to determine if there was an unterminated bus by checking the characteristic of the 1553 signal. This capability was removed from the firmware. The MIL-STD-1553 Universal Core Architecture Manual contains the current description of the interrupt status/enable word for the BC, RT, and BM function.