There are three HUB modes in EBR-1553:

SPEC mode – RTs have an RT address, 0 to 30, and each has its port on the hub.

SWITCH mode – For a four RT system, the first RT will support RT numbers 0, 4, 8… The second supports 1, 5, 9… and so on.

LINK mode – all RTs are address 0 (!) and the BC sends all messages to RT0, but the RT number addressed depends on its port number in the hub.

From all EBR-1553 HUB modes, Link mode is the only one actually used.

Link mode implementation requires additional modifications to the legacy-1553 digital chip. There are very few companies that implement EBR-1553 BC HUB in Link mode in the digital chip, others provide work-arounds.