Hello,
Both 1428 and 6534 have a Real Time Signal Interface (RTSI) connectors on them. Using this bus you can route signals (timing, triggering, etc.) between boards directly instead of going through the PCI bus. This will simplify your coding and improve reliability and performance of your application. You'll need a RTSI ribbon cable to connect two (or more) boards together.
Hope this helps!
Best regards,
Yusuf C.
Applications Engineering
National Instruments