U.S. Department of Defense
MIL-STD-1553 is maintained by U.S. Department of Defense. The current version is Revision C, dated February 2018. The register lists it as active. The specification is openly available.
A dual-redundant serial data bus that has carried avionics and weapons-system data on military and aerospace platforms since the 1970s — deliberately conservative and extremely well-proven, which is exactly why it's still specified on new defense programs despite far faster commercial alternatives existing.
The full reference entry is for registered members. Registration is free and takes a minute. You come back to this page afterwards.