Hi EPC-
The M Series PXI line was recently revised to provide for future compatibility with the new PXI Express architecture. As a result, SCXI control was removed from the PXI-6281 board and several others. If you would like to remain in the M Series line, the PXI-6221, -6229, -6251, -6259, and -6289 boards are available in both varations (i.e. with SCXI control and no PXIe compatibility OR without SCXI control and including PXIe compatibility). Let me know if you need more information about part numbers for a particular DAQ device.
Hopefully this helps-
Tom W
National Instruments