Character synchronisation is performed only once when a link is started or re-started following a link disconnection. This is illustrated in Figure 34.

Figure 34 SpaceWire Link Start-up and Character Synchronisation

One end of the link (bottom trace, End B) starts to send Nulls, a specific sequence of 8 data bits. The other end detects this sequence, synchronises its receiver and starts to send Nulls back (top trace, End A). When End B receives these Nulls it synchronises its receiver. Further handshaking is done between the two ends of the link which is described later in the section “Exchange Level“. The SpaceWire interfaces initially start operating at a line data signalling rate of 10 Mbits/s. Once the connection has been made the two ends of the link can switch to higher speed operation if required. This is clearly visible in Figure 34.

Before a link sends its first Null both data and strobe lines are set low as shown in Figure 35 SpaceWire Link Start-up.

Figure 35 SpaceWire Link Start-up Bit Sequence

Unfortunately the bit sequence for a Null is not unique in a SpaceWire bit sequence so it cannot be reliably used for resynchronisation of the signals without first stopping the SpaceWire link and then restarting and re-synchronising it.