Appendix A: MIL-STD-1553 Evolution and Revisions

A.1 MIL-STD-1553A vs. MIL-STD-1553B

While MIL-STD-1553A (1975) is technically obsolete for new designs, it is still active and encountered in legacy aircraft systems currently in service (such as early blocks of the F-16 and AH-64A). Engineers maintaining or upgrading these older platforms must be aware of the significant differences, as "A" compliant hardware is not always fully interoperable with "B" systems.

The transition to "B" (1978) tightened electrical specifications to reduce signal distortion and standardized the protocol to ensure interoperability between different manufacturers.

Table 4 - Comparison of Key Differences 1553A vs. 1553B

A.2 The Notices (1 through 4)

Since 1978, the standard has been updated via "Notices." These are cumulative updates that refined the "B" specification.

Notice 1 (1980) - USAF Only

• Scope: Applied primarily to USAF aircraft.
• Section 20: Added "Section 20" to the standard, which defined specific requirements for Air Force applications.
• Broadcast Ban: It famously prohibited the use of Broadcast commands entirely due to the lack of feedback (no Status Word response), which was deemed unreliable for mission-critical operations.

Notice 2 (1986) - The Tri-Service Update

This is the most significant update and is often what people mean when they say "MIL-STD-1553B." It harmonized requirements across the Air Force, Army, and Navy by replacing Section 20 (USAF only) with Section 30 (Tri-Service).

Key specific changes in Notice 2 include:

• Redundancy: All Air Force buses must be, as a minimum, dual standby redundant.
• Addressing: Terminal addresses must be externally selectable.
• Subaddressing: RTs and BCs must support Mode Codes with both Subaddress 0 (00000) and 31 (11111).
• Required Mode Codes: RTs must implement the following Mode Codes:
  • Transmit Status Word
  • Transmitter Shutdown
  • Override Transmitter Shutdown
  • Reset Remote Terminal
• Dynamic Bus Control: The US Air Force is restricted from using Dynamic Bus Control.
• Timing Constraints:
  • Reset Remote Terminal must complete within 5.0 ms.
  • Initiate Self Test must complete within 100.0 ms.
  • RT-to-RT timeout defined as 57 µs ±3 µs.
• Status Word:
  • Must contain valid information on power-up.
  • Only the "Message Error" bit is mandatory (others conditional based on features).
• Broadcast: Re-allowed but restricted to Mode Codes only.
• Testing: Introduced "Data Wrap-Around" capability for testing.
•  Hardware/Electrical:
  • Cable shielding increased to 90% minimum coverage.
  • Coupling shielding increased to 90% minimum coverage.
  • Actual bus impedance defined as 70-85 Ω.
  • Direct coupled stubs prohibited for Army and Air Force applications.
  • Power On/Off noise restrictions added ±250 mV for transformer coupled).

Notice 3 (1993)

• Administrative: Simply reaffirmed that the standard was valid for use in new acquisition. No technical changes.

Notice 4 (1996)

• Administrative: Changed the official title of the document. No technical changes.

A.3 MIL-STD-1553C (2018)

In 2018, the Department of Defense released MIL-STD-1553C. 

• This is a "housekeeping" update. It consolidates the original MIL-STD-1553B text plus the content of Notices 1, 2, 3, and 4 into a single, clean document.
  • Technical Changes: There are no new technical requirements in Revision C compared to Revision B Notice 4.
  • Usage: You can consider 1553B Notice 4 and 1553C to be technically identical for design purposes. New contracts may cite "Revision C" simply to point to the cleanest version of the document.

Appendix B.