  We address the impact of intra-site spin-orbit (SO) coupling and associated
inter-component Rabi coupling on the modulational instability (MI) of
plane-wave states in two-component discrete Bose-Einstein condensates (BECs).
Conditions for the onset of the MI and the respective instability are found
analytically. SO coupling allows us to produce the MI even for a small initial
wavenumber (q < {\pi}/2) for miscible states. In particular, SO coupling
introduces MI even in the absence of hopping coefficient, a concept which may
have wider ramifications in heavy atomic BECs. Our investigations predict that
the impact of Rabi coupling is more pronounced compared to the other system
parameters. We have also shown how our results of the linear stability analysis
can be corroborated numerically. The fact that we have brought out the
stability criteria in different domains of system parameters means that our
model is tailor made experiments.
