For engineers who are not involved in military/aerospace designs or similar systems requiring extreme reliability, ruggedness, and redundancy, the MIL-STD-1553B connectivity standard may be an unknown or just a vague recollection. Perhaps they came across the standard many years ago, and if they did, it must be obsolete by now, right?
Yet that’s not the case at all. Yes, this standard may be ancient by today’s standards, but it’s still in widespread use and getting new design-in for use in the harshest of environments, including military vehicles (ground, air, sea), commercial aircraft, and even spacecraft.
This FAQ looks at the key attributes of this standard and some of the available off-the-shelf products that support it. It is not intended as a detailed user guide, as there are many of those available online.
Q: What is MIL-STD-1553B, and when did it begin?
A: MIL-STD-1553 is a military standard that defines mechanical, electrical, and operating characteristics of a serial data communication bus, initially for the U.S. Department of Defense. MIL-STD-1553A was first published as a U.S. Air Force standard in 1973, with first use on the F-16 Falcon fighter aircraft. Other aircraft designs quickly followed, and it continues to be widely used for both military and civilian applications in avionics, aircraft, and spacecraft data handling. It is now widely used by all branches of the U.S. military and NATO, as well as some commercial aircraft.
Q: What are the different revision suffixes?
A: The “B” suffix clarified some inconsistencies and defined some areas that had been left as options for the user to define, leading to possible incompatibilities. The “C” suffix version added additional tables and charts for clarity. Despite these uptakes, the standard is essentially unchanged from its initial release, and the industry always refers to the B version. The standard is maintained by both the U.S. Department of Defense and the Aerospace branch of the SAE (formerly called the Society of Automotive Engineers).
Q: Isn’t such an old standard naturally obsolete?
A: One of the benefits of MIL-STD-1553’s history and legacy has been the high level of interoperability between MIL-STD-1553B-compatible boards, boxes, interfaces, and peripherals from different vendors and periods. Such interoperability is a fundamental requirement for the integration of complex systems, and compliance with MIL-STD-1553 is ensured through a suite of validation and production test plans as published by the SAE.
Overview
Q: What is the basic structure of MIL-STD-1553B?
A: A MIL-STD-1553B system typically uses a dual-redundant, balanced-line physical layer with a differential network interface and time-division multiplexing, half-duplex, command/ response data communication protocol with up to 32 remote terminal devices.
Q: What is its speed?
A: The data rate is just one megabit/sec, which is slow by modern expectations, but adequate for what it is intended to handle. Use of MIL-STD-1553B does not preclude us from using other buses within the vehicle; in fact, many mission-critical systems use MIL-STD-1553B for critical core functions along with higher-speed buses for other functions, which are not as crucial to survival.
Q: What does the general MIL-STD-1553B configuration look like?
A: A typical topology is shown in Figure 1:
It consists of these functions:
- Redundant MIL-STD-1553B buses (at least two, but there can be more)
- A Bus Controller
- A Backup Bus Controller
- A Bus Monitor
- A standalone Remote Terminal with one or more subsystems communicating with it
- A subsystem with an embedded Remote Terminal
Q: What does the Bus Controller (BC) do?
A: There is only one Bus Controller at a time on any MIL-STD-1553 bus. It initiates all message communication over the bus. The 1553B spec dictates that all devices in the system are connected to a redundant pair of buses to provide an alternate data path in the event of damage or failure of the primary bus. Bus messages only travel on one bus at a time, determined by the Bus Controller.
Q: What’s the role of the Backup Bus Controller (BBC)?
A: As its name indicates, the BBC takes over when there is a presumed problem or fault with the primary BC. This might be indicated by an excessively long quiet period on the bus, indicating that the primary active BC has failed. In such cases, the BBC takes over and becomes the primary BC. The switchover may also be initiated as part of a test procedure or switching between on-board and ground-based connections.
Q: What does the Bus Monitor (BM) do?
A: Again, as the name implies, its primary role is to monitor and record bus transactions and do so without interfering with the operation of the Bus Controller or the Remote Terminals (RTs) (their role is explained next). The BM also works with the BBC to initiate switchover if needed.
Q: Finally, what is the function of the Remote Terminal (RT)?
A: The RT is used to interface system sensors, transducers, and effectors with the MIL-STD-1553B system. It provides the physical interface between the MIL-STD-1553B data bus and an attached subsystem, but it can also act as a “bridge” between a MIL-STD-1553B bus and another MIL-STD-1553B bus.
Physical layer connectivity
This level of bus hardware includes cabling, connectors, bus couplers, and terminators.
Q: What does the interconnect cabling look like?
A: In some cases, MIL-STD-1553B systems use twinaxial (twinax) cables with a nominal impedance of 78 ohms, shown in Figure 2. That impedance value is midway between the two tradeoff values of 70 to 85 ohms for best power handling versus minimum attenuation. This tradeoff is similar to the one associated with the use of 50 ohms versus 75 ohms for RF transmission lines and coaxial cables.
The twinax cables are constructed with a two-conductor twisted balanced wire line, surrounded by an outer braided shield with dielectric isolation between the braided shield and the conductors. Twinax cables are designed for balanced, differential signals, so the two inner conductors are symmetrical with respect to ground.
Surprisingly, the standard does not define a single or unique connector. The connector can be a standard body, miniature, or sub-miniature BNC size, as seen in Figure 3. Cabling issues, lengths, and propagation delays are discussed and characterized by the MIL-HDBK-1553A Multiplexing Applications Handbook.
Do not confuse twinaxial cables with triaxial cables. The latter has a center conductor, dielectric, another concentric conductor, then another dielectric layer, an outer shield, and insulation – it is like a coaxial cable, but with an additional conducting layer and dielectric between inner conductor and shield.
Q: Are other connector options supported?
A: Yes, since the twinaxial connector is relatively small and space is often at a premium in an aircraft or other vehicles. In these cases, the MIL-STD-1553B connections can be a multicontact circular connector adhering to MIL-DTL-5015 and MIL-DTL-38999 with contacts for power, data, and various analog and digital signals.
Of course, it is easier to troubleshoot the system if there is a connector dedicated only to the MIL-STD-1553B interface and not for other uses. This is yet another appearance of the common size, power, cost, and troubleshooting connectivity tradeoff seen when choosing a multipurpose, multicontact connector.
Q: How does a unit physically connect to the bus?
A: Connection for each terminal — RT, BC, BBC, or BM — to the bus is done using a stub made of cable of the same type as the bus itself. This stub can be transformer-coupled and thus isolated, or directly connected, as shown in Figure 4:
Q: What’s the difference between the two connections and their relative merits?
A: Direct connect is less expensive and, more importantly, in some cases, it is lighter. However, transformer isolation brings many benefits: DC blocking and isolation, increased common mode rejection, and fault isolation for the entire stub and terminal. In contrast, direct-coupled stubs provide no DC isolation or common mode rejection for the terminal external to its subsystem. Many other networks also use transformer coupling for the same benefits.
There’s another benefit to transformer-coupled stubs. They provide improved protection for MIL-STD-1553 terminals against lightning strikes. This is especially critical for new composite-material aircraft, where the skin of the aircraft no longer provides an inherent Faraday shield as aluminum-skinned aircraft do.
Q: How is a stub connected to the bus?
A: Stubs for RTs, BCs, and BMs are generally connected to the bus using coupling boxes which can provide a single- or multiple-stub connection. These boxes provide shielding and also house the coupling transformers and isolation resistors for transformer-coupled stubs.
Q: What about terminations?
A: As expected, both ends of the bus must be terminated with a resistance having the same characteristic impedance as the cable itself (typically 78 ohms). The reason is to minimize signal reflections that can cause waveform distortion and many other problems.
The next part looks at MIL-STD-1553B formats and protocols, as well as available hardware and development/debug tools.
References
MIL-STD-1553 Tutorial & Reference, United Electronic Industries (UEI)
Review and Rational of MIL-STD-1553 A and B, Data Device Corporation
MIL-STD-1553B, Data Device Corporation
Adding MIL-STD-1553 to any platform made easy, Data Device Corporation
High Performance 1553, Data Device Corporation
MIL-STD-1553 Evolves with the Times, Data Device Corporation
MIL-STD-1553 Goes Commercial, Data Device Corporation
A Practical Approach to Commercial Aircraft Data Buses, Data Device Corporation
MIL-STD-1553, Wikipedia
Manage MIL-STD-1553B bus with just one chip, Gale/Cengage Group
Intel M82553 Protocol Management Unit (PMU), Datasheet Archive
MIL-HDBK-1553A Multiplex Applications Handbook MIL-STD-1553, Test Systems
Selecting RF Cabling for Defense, Space, and Other Harsh Environments: Coax vs Twinax vs Triax, Cinch Connectivity Solutions, Inc.
Twinaxial cabling, Wikipedia
Related EE World content
FAQ on cable impedance: 50 Ω versus 75 Ω
Understanding the MIL-STD-1553B bus, Connector Tips
Cinch Connectivity expands its MIL-STD-1533B bus coupler line, Connector Tips
What are triaxial cables?, Wire and Cable Tips
Understanding the MIL-STD-1553B bus
Making measurements of MIL-STD-1553 connections
The basics of bus analyzers
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