Understanding the MIL-STD-1553B bus
Despite being over 50 years old, the MIL-STD-1553B bus is still widely used, which is probably because it is really good at its job. If you're new to the standard, read on to learn more about its features, the terminology used and how to get the most out of the protocol.
By Duane Teachout, Product Manager, Cinch Connectivity Solutions
One reason why it is still popular is because it is so reliable. That's important and significant, given that the environments where the bus is used have become more and more challenging over those five decades. We can attribute that reliability to many things, but in particular the data rate, the protocol and the use of isolation to increase its robustness.
Data rate: The data rate for the MIL-STD-1553B serial bus is 1 Mbps, or one million bits per second. This data rate is more than sufficient considering the bus is really only intended to be used for command and control, and not for streaming high volumes of multimedia data.
Protocol: The bus is basically a serial link between devices (see below for types of devices), which carries packets of data. The packets are separated by a time of no less than 4μs, and each packet is made up of 16 bits (word) of data, encoded using a self-clocking scheme called Manchester Code. Each bit has a width of 0.5μs and the first two pulses each last 1.5μs, which sets them apart as the packet's 'header'. A parity bit is tacked on the end. This simple but robust protocol has proven itself over many years to be sturdy enough for the most demanding applications, such as military, defence and aerospace, which is where the MIL-STD-1553B is still most commonly found.
Isolation: Although the standard does allow direct coupling, it also states that it should be avoided if possible, and the preferred approach is to use transformer coupling. This adds DC isolation to each part of the bus, or Stub (see below for more terminology and definitions). Transformers are used inside the couplers to provide isolation because it introduces fault tolerance and common mode rejection. Direct coupling, on the other hand, provides no such protection. This is important, because a fault anywhere in the system could cause the entire bus to fail. This can be avoided using isolation.
Although relatively simple, a system based on the MIL-STD-1553B standard will comprise several distinct pieces of equipment, each of which are very carefully defined. Here are some of the terms you may come across when looking into the technology and the suppliers offering them.
Terminal: Any piece of equipment that attaches to the bus is effectively called a Terminal. Several types of terminal are defined by the standard, including a the Bus Controller (which controls the flow of data on the system), Bus Monitor (which is any device intended to receive data from the bus) and Remote Terminal (any other device not classified as either a Bus Controller or Bus Monitor). The rest of the system provides the interconnection between terminals.
Stub: A stub is the way in which other parts of the system, including Remote Terminals, Bus Controllers and Bus Monitors are interconnected. The stub is essentially a wired connection between two devices, one of which will be a Bus Coupler (see below).
Bus couplers: The bus coupler is a critical part of any MIL-STD-1553B system, as it enables terminals to be added/removed. All the various parts of the system will be interconnected using bus couplers, so this is typically where the transformer isolation will also be found. Bus couplers come in various sizes, depending on the number of stubs/connections they provide. Each bus coupler essentially provides a way of tapping into the bus, so as minimum it must include an input and output to pass the bus. In addition, it may include a number of stub connections. Bel offers a range of isolated bus couplers that feature 1, 2, 3, 4, 5 or 8 stub connections.
Cables: A terminal will connect to the bus through a stub using a cable. The type of cable used will dictate the system's overall performance, essentially due to the attenuation each cable span introduces. While the standard sets no upper limit on the length or size of a system, the attenuation will eventually cause the serial bus to fall outside the limits for the signal's rise-time, which is set by the standard. it is important, therefore, to choose cables that offer the best performance, in terms of resistance and termination.
Connectors: A connector is needed wherever the bus is accessed. The standard doesn't define the type of connectors used but it does define the level of shielding they must provide. Bel provides an extensive portfolio of connectors for use in MIL-STD-1553B systems, as well as complete MIL-STD-1553B cable assemblies.
Adapters: Building a complete system requires all of the above components, impedance matched to provide the best performance over the defined bus length. As part of the standard's attraction is its ability to expand to add new devices as they are needed, adapters provide a way of stubs and cable assemblies together that use different connector types. For example, the range of adapters offered by Bel include triax-to-twinax adapters with combinations of connector genders on each side.
Terminators: Each end of the bus must be terminated using a terminator with a resistance that matches the cables used. The standard allows some flexibility here, so for this reason sourcing all of the components needed for a MIL-STD-1553B bus system from the same supplier will provide a level of assurance, that all components will be impedance matched.
The MIL-STD-1553B bus remains popular in applications that demand high reliability because it is based on good engineering practices and implemented using the best technologies. When looking at the ecosystem that has evolved around the MIL-STD-1553B standard, it is important to consider what the standard does and does not specify; it offers a considerable amount of flexibility for a standard so critical to military, defence and aerospace applications. Choosing a supplier that understands this and offers a complete portfolio of solutions will give manufacturers a competitive advantage when bringing new systems to market.