Valid versions of NI-DAQ for LabVIEW RT 5.1.2 are 6.7, 6.8, 6.8.1, and 6.9. See NI's KnowledgeBase Entry 1XUFIOV2. Part of it says:
"If you have LabVIEW RT 5.1, then you may use either NI-DAQ 6.5.2 or NI-DAQ 6.6. If you have LabVIEW RT 5.1.2, then you may use NI-DAQ 6.7, 6.8, 6.8.1, or 6.9 (NI-DAQ 6.9 is recommended). The upgrade from LabVIEW RT 5.1 to 5.1.2 is FREE (see the National Instruments Upgrade Advisor: ni.com/upgrade)."
When do you get the "Waiting for RT Engine to respond" error? Are you using an RT Series PXI Controller or an RT Series DAQ device (PCI-7030/6XXX)? If you are a PXI controller, make sure that the software versions on both the controller and the host are the same (using Remote System Explorer). If you are using a PCI-7030/6XXX, are you re
setting it after performing the software upgrade?
BTW, RT DAQ posts would probably have a better chance of getting answered if you post them to Developer Exchange: Measurement Devices: Real-Time DAQ.