I need to add a correction to the answer I posted above. Not only E series boards use the local bus. There are other boards that use it, although for the same purpose, this is, to control the SCXI chassis.
Some boards are capable of sending the controlling signals to the SCXI chassis AND receiving/sending signals from/to the SCXI modules through the local bus. Other boards are only capable of sending the control signals to the SCXI chassis, so it is necessary to read/send signals from/to the SCXI modules by external cabling.
The following document summarizes how NI PXI boards use the PXI Backplane timing and synchronization features:
KnowledgeBase 2W1AOUW
D : How do National Instruments PXI Boards map to the PXI Backplane?Claudia Lorente
Modular Instrumentation Specialty Products Group
Applications Engineering
National Instruments
http://www.ni.com/ask