Real-time spacewire instrument simulation in a day

The testing of spacecraft onboard data-handling equipment, like a mass-memory unit, requires test equipment that can simulate the instruments or other onboard equipment that connects to the unit under test. These simulations have to respond like the real instruments, or other equipment, to commands and send out representative data. For some instruments the data may be a fairly high data-rate. When designing test equipment that is to simulate a unit with a SpaceWire interface the typical approach is to connect a SpaceWire interface card to a computer running a real- Time operating system and to write complex software that mimics the real- Time behaviour of the instrument. This is often a difficult and time consuming task. The SpaceWire EGSE unit has been designed to rapidly simulate many SpaceWire instruments in real- Time with microsecond accuracy. A typical simulation takes about one day to develop. A special SpaceWire simulation language has been written that describes the SpaceWire packets to be sent, the link speed, actual data-rate, and timing of a sequence of packets, and the internal state of the instrument. For example in a "stand-by" state the instrument may be waiting to start sending packets. Every ten seconds it may enter an automatic housekeeping "keep- Alive" indication state, where it sends a short SpaceWire packet to a central controller to indicate that the instrument is still alive. When a command is received via SpaceWire or via some other means the instrument enters an "active" state sending SpaceWire packets according to a predefined schedule. The SpaceWire EGSE language can be used to rapidly describe instrument behaviour including the schedule of packets to be sent and the detailed contents of those packets. Once the description has been written it is compiled and downloaded into the EGSE hardware which executes the required behaviour in real- Time. The EGSE hardware can interact with other simulation software using software events that are passed via an API to the EGSE unit. The software event can cause a state transition in the EGSE unit changing the perceived behaviour of the simulated instrument. External hardware triggers can be used in a similar way. This paper introduces the SpaceWire EGSE language, and describes a couple of instrument simulations designed for a major spacecraft application. It also briefly describes the simulation of the on-board data-handling system for Solar Orbiter and feedback from this test case that is helping to enhance the capabilities of the SpaceWire EGSE unit.