Honeywell engineers enhance Space Station computers

By John McHale

CLEARWATER, Fla. - Engineers at Honeywell Space Systems in Clearwater, Fla., enhanced the flight Multiplexer/Demultiplexer (MDM) computers previously used on NASA`s fleet of space shuttles, for the use on International Space Station.

The main difference between the Shuttle and Space Station MDMs is the central processor. The Space Station devices use a commercial 386SX processor from Intel Corp. in Santa Clara, Calif., says Bob Davis, senior staff engineer for Honeywell`s Data Control Products department.

Supportability for the 386 will not be a problem, Davis says, because Intel supplied Honeywell with spare parts, and if the spare parts run out, Intel will sell the 386 die, Davis explains. "Intel`s software also is backward compatible and will work with the 386," he says

The 386 came in a high-reliability, hermetically sealed package and passed Honeywell`s radiation testing, Davis adds.

The technology evolved to include distributed processing capabilities to enable each remote-terminal MDM to execute commands independently of a central computer, Davis says.

Under the $240 million development contract with the Boeing Co. in Seattle, engineers from the Honeywell Space Systems Satellite Systems Operation in Phoenix built and delivered 51 MDM flight hardware units; 74 MDM ground hardware units; 66 Multiplexer/ Demultiplexer Application Test Environment (MATE) units; and various test units and miscellaneous spares.

The MDMs, which have a modular computer architecture with a high-speed interface, will control core systems - power, environmental control, guidance, and navigation - as well as sensors, and effectors. They will also manage payload operations and handle the station`s most critical commands from ground control.

"The modular architecture adds flexibility," Davis says. "Any channel is configurable."

The Space Station uses two types of Honeywell MDMs: the Space Station Multiplexer/Demultiplexer (SSMDM) and the Enhanced Space Station Multiplexer/ Demultiplexer (ESSMDM).

The SSMDM collects signals from sensors and effectors, astronauts, or subsystems. It processes the information and transmits signals and commands such as steering the solar arrays and channeling the power necessary to operate scientific experiments.

An SSMDM consists of a chassis, power supply, I/O control unit, I/O backplane, SX backplane, and I/O circuit cards. Astronauts can replace circuit cards on orbit, which simplifies maintenance and reduces downtime and operating costs, Davis says.

The I/O control unit includes the processor, memories, I/O circuit card control logic, and analog-to-digital converter.

Communication is via independent dual-redundant MIL-STD-1553B data bus channels, which enables the device to operate as a bus controller, bus monitor, or in remote terminal mode. Also, the SSMDM`s built-in hardware and software self-test capabilities can detect and report 94 percent of all possible failure modes, Davis says.

The enhanced version

The enhanced MDM is the command-and-control resource, and serves as the Space Station`s master bus controller of all SSMDMs, Davis explains.

In addition to the SSMDM, the enhanced version uses a 2.2-million-instructions-per-second central processor, a math coprocessor, and expanded RAM, he says. It also has enhanced computing speed and a high-rate data link to communicate and control the mass storage unit and scientific payloads. It is software and hardware self-diagnostic.

The high-rate data link circuit cards communicate with the Mass Storage Unit - a ruggedized military re-writable hard disk - and the Space Station`s scientific payloads for 10 megabit- per-second fiber optic data transfer and receipt.


The MATEs enable engineers involved in the Space Station program to develop and test their application software in preparation for in-flight operation. MATE performs verification and debug testing for Space Station flight software.

The MATE system includes Digital VAX and Sun Sparc 5 development workstations, dual tape drives, a real-time software tool set from Integrated Systems Inc. of Sunnyvale, Calif., internal and external Ethernet, utilities platform (bus analyzer), and a real-time input/output platform.

Honeywell engineers also developed mechanisms such as a Power Data Transfer Assembly (PDTA), which transfers the energy from the solar panels to the spacecraft`s payload. In addition to the PDTAs, they will deliver two bearing assemblies, two utility transfer assemblies, and seven beta gimbals for the solar arrays.

The International Space Station is an international orbiting laboratory to be used for the advancement of a wide range of scientific and technical research. The 16 nations involved are: the United States, Brazil, Canada, Italy, Belgium, Netherlands, Denmark, Norway, France, Spain, Germany, the United Kingdom, Sweden, Switzerland, Japan, and Russia. The first element of the International Space Station, the Russian-built Functional Cargo Block, will be launched in June 1998 and the first U.S.-built hardware will be launched in July 1998.

Click here to enlarge image

NASA officials say supporting the Intel 386-based space station computers, pictured above, will not present a problem.

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