The sync header contains two bits that are always opposite of each other (either 01 or 10). The JESD204C receiver can find the block boundaries by looking for a 66-bit boundary that always contains a 0 to 1 or 1 to 0 transition. Although 0 to 1 and 1 to 0 transitions occur at other locations in a block, it is impossible for the sequence to appear at a fixed location, other than the proper sync header location, in successive blocks for a long period of time. The sync header indicates the start of a block and can be used for block alignment monitoring. If a 00 or a 11 bit sequence is seen at the assumed sync header location of a block, then block alignment may have been lost. Multiple occurrences of incorrect sync header bits should trigger a search for the sync header after sending SYSREF to all devices to reset LEMC alignment.

A sync header ([0:1]) of 01 corresponds to transmission of a 1 while a sync header of 10 corresponds to a transmission of a 0. The transmitted bit from the sync header of each block of a multiblock are combined into a 32-bit word called the sync header stream. The sync header stream is used to transmit data in parallel with the user data in order to synchronize the link by marking the borders of multiblocks and extended multiblocks. In addition, the sync header stream provides one of either CRC, FEC or a command channel. ADC09QJ1300 supports CRC-12 and FEC and does not support CRC-3 or the command channel.

The 32-bit sync header stream always ends with a 00001 bit sequence, called the end-of-multiblock (EoMB) signal, that indicates the end of a multiblock. For CRC and command channel modes, a 00001 sequence never occur in any other location in the sync header stream. For FEC mode, it is possible for a 00001 sequence to appear in another location within the sync header stream, however it is improbable to see the 00001 sequence in the same location within a sequence of multiple multiblocks. Therefore, in FEC mode it may take more than one multiblock to find the end of a multiblock. The end of an extended multiblock is found for all modes by monitoring bit 22 of the sync header stream, the EoEMB bit, which indicates the end of an extended multiblock when set to a 1. The EoMB (00001) and EoEMB signals, as well as fixed 1s in the sync header stream for CRC and command channel modes, form the pilot signal of the sync header stream.

The defined format for each form of the sync header stream are defined in the following sections.