The packet level of SpaceWire simply defines the fields in a SpaceWire packet: destination address, cargo and end of packet marker, as illustrated in Figure 44.

Figure 44 SpaceWire Packet Format

The “Destination Address” is the first part of the packet to be sent and is a list of data characters that represents either the identity of the destination node or the path that the packet has to take through a SpaceWire network to reach to the destination node. In the case of a point-to-point link directly between two nodes (no routers in between) the destination address is not necessary.

The “Cargo” is the data to be transferred from source to destination. Any number of data bytes can be transferred in the cargo of a SpaceWire packet.

The “End_of_Packet” (EOP) is used to indicate the end of a packet. The data character following an End_of_Packet is the start of the next packet. There is no limit on the size of a SpaceWire packet.

In addition to the normal end of packet marker (EOP), there is another end of packet marker, the Error End of Packet marker (EEP), which indicates that the packet has been terminated prematurely because of an error that occurred as the packet traversed a SpaceWire link or network. A packet terminated by an EEP is illustrated in Figure 45.

Figure 45 SpaceWire Packet Terminated by EEP

When a packet is terminated by an EEP it is likely that the Cargo is incomplete and may contain some erroneous data characters. It is also possible that an error occurs at the start of the packet, while sending the destination address. In this case, there is no cargo, just an incomplete destination address followed by an EEP. If this incomplete destination address is a path address, as the packet terminated by the EEP moves across the SpaceWire network, each router will forward the packet and delete the leading character of the packet. Eventually all the destination address characters will be deleted before the packet arrives at its destination, since it was an incomplete destination address. All that is left is an EEP. Since a router cannot know where to forward an EEP on its own it will simply delete the EEP. The prematurely terminated packet will then have disappeared completely.