Space Station Multiplexer De-Multiplexer - MDM

Honeywell, Space Systems Group; Glendale, AZ

SIMSCRIPT II.5 with SIMGRAPHICS is being used to model the performance of the Multiplexer De-Multiplexer (MDM) being designed for NASA's Space Station Freedom. Various MDM architectures are being modeled to provide an understanding of their merits.

The MDM transmits and receives data or commands from the Space Station's main computers, processes signals from station sensors (inputs), sends signals to station effectors (outputs), and provides computing power to Space Station software hosted on the MDM.

Systems engineers at Honeywell, Satellite Systems Operation Division, are using SIMSCRIPT II.5 to simulate the constantly changing data and traffic rates to analyze critical timing relationships within the MDM.

SIMGRAPHICS provides an immediate visual display of the model and data that is being investigated. It translates the simulation's output into comprehensible displays, which can change in simulation time due to the varying data rates the MDM experiences. These displays include timing diagrams (histograms), a clock (in microseconds), and iterative bar graphs (level meters). Design and data discrepancies are observed quickly and clearly, and corrections are immediately implemented.

The hardware and software of the MDM is modeled together as independent functions within the same model. The independence of the hardware and software portions of the model helps to spot performance and interoperability difficulties resulting from design and timing changes from each area.

The hardware part of the model accurately provides several data buses (some shared), several intelligent asynchronous devices, and a CPU for the software. The simulated CPU properly models the behavior of the actual CPU according to the traffic the MDM is experiencing. This includes providing prioritized preemptive interrupts (by clocks, serial buses, external devices, etc.), DMAs, wait states, and by passing actual MDM data to the software.

The software executes under this environment and responds in kind to the data it receives. The data and the MDM's timing parameters are passed into the model via a text file.

Future activities include using an independent SIMGRAPHICS program to build and time the rather complex data records used by the MDM. The MDM's timing is based off these records (commands) some of which stay resident in the system and invoke themselves periodically. Therefore, this tool must also provide the user with the ability to predict the model's behavior.

Together, these tools will assist Honeywell in analyzing the MDM's behavior before the MDM actually exists: during its design cycle, during integration and test, and ultimately during the deployment cycle. They will provide information on the characteristics of the MDM beyond its designed operational envelope to a detail not feasible or cost effective to measure.

Alternatives Considered: NETWORK II.5, GPSS, and SLAM

Benefits of SIMSCRIPT II.5: Excellent support, fast prototyping

Customer Quote: "SIMSCRIPT II.5 helps to identify and prevent otherwise potential implementation hazardous and critical performance problems. CACI's quick response to our sudden request for support was most helpful in providing a well received exhibition of our simulation."