Characters in SpaceWire are used for three different functions: link control, sending packets and sending time-codes.

The characters and control codes used for link control are the Null and FCT, which are known as L-Chars or Link characters. They are used in the Exchange level and not passed up to any higher level.

The characters and control codes used for sending packets are the data characters and end of packet markers (EOP and EEP), which are known as N-Chars or Normal characters. These characters are passed up to the packet layer.

The time-codes are used for sending time and synchronization information and are passed to the time-code handler of a node or router.

A SpaceWire link interface interleaves L-Chars, N-Chars and time-codes. N-Chars from one packet are not interleaved with N-Chars from another packet. A received character should have its parity checked before it is acted upon.