An industry perspective from GE Intelligent Platforms
Enhancing MIL-STD-1553's bit rate
Continued budget restrictions are causing many aging military aircraft and ground vehicles to be deployed with little prospect of replacement. However, these platforms must be sustained and upgraded, preferably with as little disruption to wiring harnesses as possible. Hence, standards like MIL-STD-1553B that were installed when these platforms were built continue to have long lifespans. New types of capabilities such as miniature smart bombs and rockets require all the beneficial characteristics of 1553 such as determinism, low latency, redundancy, and noise immunity, plus widespread developer familiarity, but at higher bit rates and lower cost per connection.
MIL-STD-1553B runs at 1 Mbps using a multidrop redundant bus architecture with a single Bus Controller (BC) and multiple Remote Terminals (RTs). A Bus Monitor (BM) might be included to record and time-tag all bus activity. 1553 uses a deterministic command/response protocol; the BC is typically programmed to operate at a 30 Hz frame rate to interrogate RTs and transfer data to or from attached subsystems. The frame rate and low bit rate determine the system architecture. For example, a typical system might have a mission computer attached via MIL-STD-1553B to many subsystems such as sensors, communications, electronic warfare, stores management, and so on. This is generally referred to as federated architecture, wherein each subsystem needs a great deal of processing and data reduction before transmission to the mission computer.
Federated architecture determines upgrade strategy. If only a few subsystems need to be upgraded or replaced, it is more cost-effective to keep the existing cabling, frame rates, and data transmission concept. It is equally difficult to upgrade a mission system to a faster version of 1553 unless all the subsystems are also revised simultaneously. In this case, it might be beneficial to move to other fabrics such as Fibre Channel and a more integrated avionics architecture.
It might not be cost-effective to replace an entire 1553 backbone, but subsystem upgrades will introduce greater performance, capability, and complexity. External interfaces can be MIL-STD-1553B, for example, as part of a MIL-STD-1760 stores interface.
The introduction of new, highly capable smart munitions identified the need for a faster, cheaper interface than 1553B, prompting development of the 10 Mbps Miniature Munitions Stores Interface (MMSI), also known as SAE-AS5652 Enhanced Bit Rate (EBR) 1553. The greater performance of EBR-1553 allows rapid reprogramming of smart munitions and supports the development of more generic fire control/stores management computers that can handle a broader mix of smart weapons.
EBR-1553 is connected in a star topology radiating from the BC to each RT using RS-485 differential signaling for high levels of noise immunity. Cabling uses regular 1553 Twinax, with each star connection representing a unique RT address. Notwithstanding the revised topology, from a user’s perspective, EBR-1553 retains the deterministic command/response characteristics of MIL-STD-1553B, enabling the development of frame patterns and data flows to suit the application in the same manner as 1553. The performance potential of 10 Mbps and its 1553-like operation make EBR-1553 suitable not only for stores management applications, but also for many other ground and airborne upgrade projects. More complex subsystems or clusters of subsystems that require faster frame rates or greater data transfer capability than the platform’s 1553 bus are possible, plus gateways to new functions or capability can be created.
MIL-STD-1553B is perhaps the most widely deployed standard for military and aerospace applications. With thousands of instances in the field, the interface has been embedded into systems and subsystems using discrete devices and IP cores in FPGAs, integrated onto SBCs, and added as mezzanine modules. Similar flexibility exists for implementation of EBR-1553 (Figure 1). Individual RT implementations as typically found in small, high-volume smart munitions can be very low cost, requiring only RS-485 transmit and receive components plus an FPGA or integrated processor device. A PMC offers an ideal form factor for more complex implementations within a CompactPCI, VMEbus, or VPX (VITA 46) embedded computing subsystem. The PMC-MMSI from GE Intelligent Platforms, for example, supports all EBR-1553 types and up to 12 terminal lines.
MIL-STD-1553B will continue to be deployed for as long as the platforms that have it installed remain in active service, many of them with decades of useful life remaining. EBR-1553 offers additional performance and capability that can be easily integrated into new and upgraded subsystems without disturbing the platform’s system architecture and cable infrastructure.