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The proven MIL-STD-1553 databus provides a highly reliable 1Mbyte/sec data channel between the various components of many legacy avionics systems. Although MIL-STD-1553 has been around for over 20 years and its data rate is slow by today's standards, engineers continue to design MIL-STD-1553 based systems for new operational military and commercial applications. Needless to say, the long life displayed by this two-decades old standard is a testimony to its exceptional reliability and versatility.

MIL-STD-1553 components typically operate in very expensive multi-million dollar platforms where the safety of operators and others is a number one concern. Therefore, system designers require absolute reliability from their 1553 hardware and must measure this reliability before the systems are implemented in the field. But measuring reliability requires extensive testing, and testing each component of every system configuration with the actual application platform isn't always a practical solution. Furthermore, using actual 1553 platforms can be very expensive, and these resources cannot always be easily obtained. For these, and a variety of other reasons, a highly capable MIL-STD-1553 simulation and test interface becomes a very valuable tool for MIL-STD-1553 system designers.

To provide value, a MIL-STD-1553 simulation and test interface must be able to perform at least one of the following functions:

• Simulate a Bus Controller (BC). BCs transmit and receive data, as well as coordinate bus data flow.
  
• Simulate a Remote Terminal (RT). RTs perform subaddress data control defined by the BC.
  
• Monitor traffic from BC's and RT's via a Bus Monitor (BM). BMs simply monitor bus transmissions.

Additionally, in order to be a useful tool for simulating MIL-STD-1553 systems and testing MIL-STD-1553 components, a MIL-STD-1553 test and simulation interface should be able to:

• Simulate multiple Remote Terminals (mRT) simultaneously
  
• Act as a chronological BM
  
• Operate in BC, BM and mRT modes simultaneously
  
• Inject errors into bus traffic for test purposes
  
• Interface to high-level bus analysis software for further interpretation of data
  
• Provide maximum flexibility for migration between different platforms

HISTORICAL SOLUTIONS

MIL-STD-1553 designers and customers traditionally implemented 1553 interfaces as bus-based boards. Over time these interfaces have evolved from large, rack-mounted boxes and multi-slot board sets into single-slot solutions with the VME, VXI, PCI, and ISA buses being the most common

Since it is impossible to design specialized boards for every possible customer requirement (including cost), system design engineers have developed a broad approach to meet the majority of customer needs. This approach involved developing "high-level", "mid-level", and "low-level" board designs.

High-level boards are designed to meet every possible need of the MIL-STD-1553 customer. They can operate on multiple channels, operate in BC, BM, and mRT modes simultaneously, inject errors into bus traffic for test purposes, and interface with advanced bus analysis software. These boards include powerful onboard processors, programmable logic, and sizeable onboard memory banks and rely on vendor-specific firmware to perform all 1553A/B functions. (MIL-STD-1553B is the most commonly used protocol. It includes many types of specialized Mode Code commands and allows for RT responses to not exceed 14 microseconds of "dead bus" time. The less frequently used protocol 1553A on-the-other-hand, implements only one type of specialized Mode Code command and allows for RT responses to not exceed 5 microseconds of "dead bus" time.)

Mid-level boards are designed to provide a moderate level of functionality at a moderate price. These are generally hybrid solutions developed from High or Low-level hardware. Some vendors offer de-featured high-level boards in single-channel, single function, and single-channel/function configurations. Other vendors offer up-featured "low-level" boards with multiple channels or specialized RT-only models.

Finally, low-level boards have been designed to provide cost-effective interfaces to users who require less functionality. They operate on a single channel; operate as a BC, single RT, or simple MT; and provide limited ability to interrupt, record, and manipulate data. These boards include small onboard memory banks and rely upon MIL-STD-1553 protocol chips to perform 1553 A/B functions.

RECENT DEVELOPMENTS

A more recent development in the 1553 product evolution is the implementation of MIL-STD-1553 mezzanine boards. Mezzanines offer incredible versatility because they are platform independent. A MIL-STD-1553 mezzanine card can provide a 1553 interface on any platform which can accommodate one or more mezzanine sites. To address the needs of a wide market segment, mezzanine solutions have primarily been based upon the standardized Industry Pack® (IPack) and PCI Mezzanine Card (PMC) mezzanine module formats.

Ipack's small size (approximately 1.8" x 3.9") and modularity provides excellent flexibility. IPack MIL-STD-1553 cards can reside on many commercially available Single Board Computers (SBCs) and dedicated carrier boards for VME, CompactPCI®, PCI, ISA, and other platforms. Current VME6U carrier boards for example are available with up to four or five independent IPacks sites. This provides the flexibility to implement a single MIL-STD-1553 interface and retain several unused IPack sites for other applications or implement several independent 1553 interfaces from a single board slot.

With the increasing popularity of the PMC architecture, manufacturers have also implemented MIL-STD-1553 on PMC cards. Like 1553 IPacks, MIL-STD-1553 PMCs offer designers a great deal of flexibility. MIL-STD-1553 PMCs also can reside on many commercially available SBCs and dedicated carrier boards for VME, CompactPCI, PCI, Multibus, VXI, and other platforms. PMCs are larger than IPacks (approximately 2.9" x 5.9"), and a standard VME6U carrier can accommodate two PMC sites. This provides the flexibility to implement a single MIL-STD-1553 interface and retain an unused PMC site for other applications or implement multiple, independent MIL-STD-1553 interfaces from a single board slot.

MIL-STD-1553 interfaces have been developed to meet a variety of different needs, as illustrated by the variety of MIL-STD-1553 board configurations available. But although available MIL-STD-1553 products meet a wide range of price/performance requirements, they may not meet all the needs of every user. Some may need an extra feature or two that isn't found on existing boards, while others may only need to access functionality that isn't generally available in their preferred format. .

For example, bus-based interfaces can provide high functionality, but users are unable to migrate their applications to different computing platforms. The inability to switch between backplanes may be of no great concern to users with single-platform applications, , short-lived projects or long-lived platforms,but a variety of users may need to migrate between hardware platforms during the course of their application.

• U OUsers who need to exploit the different strengths of multiple computing platforms may need to perform hardware development on a PC and do production testing on a VXI platform.
  
• Still oUsersthers who need to upgrade their systems may need to start development on their existing platform, knowing that their application will eventually migrate to another, as yet undetermined, system.
  
• Users who have limited access to scarce computing resources may need to access these resources now, and continue their application on another platform after these resources are no longer available.
  Clearly, to these users, the ability to easily migrate their applications between different systems would be beneficial, but existing bus-based boards don't support this level of flexibility.

MIL-STD-1553 mezzanine modules solve the flexibility problem by allowing users to easily move their application from one backplane to another. This gives users a level of versatility that is unavailable in bus-based boards. But mezzanines often face their own functional limitations.
A lack of board space dictates that 1553 IPacks are almost always based upon MIL-STD-1553 protocol chips, because these chips require less board real-estate than other design options. MIL-STD-1553 IPacks generally share many of the limitations of low-level bus-based boards. They provide all the 1553 logic necessary to implement a MIL-STD-1553 interface, but they are a limited solution with limited functionality, meaning that 1553 IPacks generally: Operate in a single mode at any time, simulate a single RT, provide a basic BM function and have very limited error detection capabilities.

PMCs provide more board real-estate than IPacks, but board space is still quite limited. This shortage of board space may tempt designers to use MIL-STD-1553 protocol chips as the basis of their 1553 PMC interface, but as with other platforms, this choice severely limits the board's functionality. The PMC platform does provide enough board space to implement mid-level designs or even a high-level design with a single-channel, but many existing boards simply settle for low-level performance.

Simply stated, most existing MIL-STD-1553 mezzanines have low-level performance, based upon their common use of MIL-STD-1553 protocol chips or similar devices. This isn't a problem for users with basic requirements, but other users may find that mezzanine solutions just can't meet their needs. These users may require both the flexibility to migrate between platforms AND the level of performance otherwise available from high-level, bus-based boards.

Another issue facing MIL-STD-1553 system designers is a general lack of software compatibility between different hardware platforms. Many 1553 applications utilize specially developed application software that may require months to develop. When the users of these systems are forced to change hardware platforms, they often have to completely redevelop their application software. This is equally true for users of mezzanine interfaces and bus-based boards. While mezzanine boards can, in effect, create software compatibility by allowing software to migrate in conjunction with the hardware, not all platforms can be accessed by all mezzanine solutions.

CUSTOMER WISH LIST

While bus-based boards and low-level mezzanines have their uses, they do not provide an ideal solution for MIL-STD-1553 system designers and users. An ideal solution would have to combine the best features of current bus-based and mezzanine boards. The package would have high-level functionality and platform-independence, and it would provide a level of software compatibility unavailable with most vendors' MIL-STD-1553 products. In effect, these requirements dictate the use of a mezzanine module with the performance of a high-level MIL-STD-1553 board and a very advanced software package.

THE GOLD+ SOLUTION

Systran Corporation, through its acquisition of Digital Technology Inc. in 1996, is able to draw upon over 18 years of 1553 design experience. Systran's Gold+ series of 1553 boards consists of single and multi-function boards that go beyond the 1553 standard by providing a strong software support package and defining how to simulate 1553 system or subsystem through the use of flexible, DSP-based, microcode. (discussed below).

When developing the GLD+PMC, designers from Systran Corporation attempted to meet the needs of customers who require both high functionality and platform flexibility by combining many of the best features of both the bus-based and mezzanine boards. The GLD+PMC is a PCI Mezzanine Card, as its name implies, allowing flexible migration between different platforms and providing a convenient interface between a host computer and a MIL-STD-1553 data bus. Although it is a mezzanine solution, the GLD+PMC is a powerful MIL-STD-1553 interface. It enables the host to simultaneously operate in real-time as a MIL-STD-1553A or B protocol BC, emulate mRTs, and function as a complete chronological BM. These three operating modes provide extensive bus protocol error detection, such as parity error and no response. This board also has the ability to inject protocol and/or electrical errors into the 1553 bus traffic for test purposes, something that actual 1553 BC and RT devices are unable to do.

Complementing the GLD+PMC board's capabilities is the SIMMPL library/driver software. Featuring an intuitive user interface, SIMMPL streamlines card configuration and startup by handling register accesses and memory management at the library level. With the SIMMPL software, valid board channels are identified and configured automatically. Also, all data structures are automatically initialized with easily modified default values, eliminating the need to pass parameters to software routines. SIMMPL includes a Hardware Confidence Test (HCT) that verifies card memory, registers, and interrupts, as well as 1553 data transfer integrity. This powerful utility also provides direct access to card registers and memory.

The SIMMPL library/driver software also addresses the software compatibility issues that limit the portability of many MIL-STD-1553 applications. SIMMPL stores operating system specific code in a unique area that does not rely on any other part of the interface library, and non operating system specific routines are portable between differing compilers and operating systems. This provides standard features between hardware platforms and operating systems and allows application software to be ported between differing hardware platforms with a minimum of software redevelopment. SIMMPL is currently supplied with Systran's Gold+ series of 1553 Solutions interfaces for PMC, PCI, PC/AT, VME, and VXI platforms, so users have considerable flexibility when porting applications from GLD+PMC hardware to PCI, PC/AT, VME, and VXI machines. SIMMPL currently supports the Windows® 95, Windows NT®, and VxWorks operating systems.

The GLD+PMC uses a powerful Texas Instruments® 32-bit Digital Signal Processor (DSP) to execute the microcode. The microcode can be loaded from on-board PROM or downloaded externally for instantaneous upgrades. It runs virtually independent of all external host accesses allowing for maximum performance. It also allows for simultaneous operation of BC, up to 32* mRT's and BM. The powerful instruction set and 50ns instruction time allow the DSP to execute operations at a performance rate up to 40 MFLOPS and 20 MIPS. Also, parallel ALU operations can be performed in a single 50ns cycle allowing two operations to be executed simultaneously.

SUMMARY

The 1553 standard has been around for over 20 years and general data bus technology has far outpaced the standard's speed performance. However, there is no other current near term replacement that meets the 1553 community's diverse performance and cost requirements. MIL-STD-1553 will continue to be designed into modern avionics systems for the foreseeable future, and legacy MIL-STD-1553 systems will continue to support development. The GLD+PMC board will allow the designers of these systems to protect their investment, by providing all the hardware capabilities and software compatibility of Systran's Gold+ line in a flexible, platform-independent mezzanine format.

Systran Corporation, 4126 Linden Ave., Dayton, OH 45432-3068. Tel: 937-252-5601; Fax: 937-258-2729.