MIL-STD-1553 is a military standard published by the United States Department of Defense that defines the mechanical, electrical, and functional characteristics of a serial data bus. It was originally designed as an avionic data bus for use with military avionics, but has also become commonly used in spacecraft on-board data handling (OBDH) subsystems, both military and civil, including use on the James Webb space telescope. It features multiple (commonly dual) redundant balanced line physical layers, a (differential) network interface, time-division multiplexing, half-duplex command/response protocol, and can handle up to 31 Remote Terminals (devices); 32 is typically designated for broadcast messages. A version of MIL-STD-1553 using optical cabling in place of electrical is known as MIL-STD-1773. MIL-STD-1553 was first published as a U.S. Air Force standard in 1973, and first was used on the F-16 Falcon fighter aircraft. Other aircraft designs quickly followed, including the F/A-18 Hornet, AH-64 Apache, P-3C Orion, F-15 Eagle and F-20 Tigershark. It is now widely used by all branches of the U.S. military and by NASA. Outside of the US it has been adopted by NATO as STANAG 3838 AVS. STANAG 3838, in the form of UK MoD Def-Stan 00-18 Part 2, is used on the Panavia Tornado; BAE Systems Hawk (Mk 100 and later); and extensively, together with STANAG 3910 "EFABus", on the Eurofighter Typhoon. Saab JAS 39 Gripen uses MIL-STD-1553B. The Russian made MiG-35 also uses MIL-STD-1553. MIL-STD-1553 is being replaced on some newer U.S. designs by IEEE 1394. Revisions MIL-STD-1553B, which superseded the earlier 1975 specification MIL-STD-1553A, was published in 1978. The basic difference between the 1553A and 1553B revisions is that in the latter, the options are defined rather than being left for the user to define as required. It was found that when the standard did not define an item, there was no coordination in its use. Hardware and software had to be redesigned for each new application. The primary goal of the 1553B was to provide flexibility without creating new designs for each new user. This was accomplished by specifying the electrical interfaces explicitly so that electrical compatibility between designs by different manufacturers could be assured. Six change notices to the standard have been published since 1978. For example, change notice 2 in 1986 changed the title of the document from "Aircraft internal time division command/response multiplex data bus" to "Digital time division command/response multiplex data bus". MIL-STD-1553C is the