  It is very controversial whether radio--X-ray correlation as defined in LH
state of XRBs can extend to quiescent state (e.g., X-ray luminosity less than a
critical value of $L_{\rm X,c} \sim10^{-5.5}L_{\rm Edd}$) or not. In this work,
we collect a sample of XRBs and low luminosity active galactic nuclei (LLAGNs)
with wide distribution of Eddington ratios to reexplore the fundamental plane
between 5 GHz radio luminosity, $L_{\rm R}$, 2-10 keV X-ray luminosity, $L_{\rm
X}$, and black hole (BH) mass, $M_{\rm BH}$, namely $\log L_{\rm R}=\xi_{\rm X}
\log L_{\rm X}+\xi_{\rm M}\log M_{\rm BH}+\rm constant$. For the whole sample,
we confirm the former fundamental plane of Merloni et al. and Falcke et al.
that $\xi_{\rm X}\sim 0.6$ and $\xi_{\rm M}\sim 0.8$ even after including more
quiescent BHs. The quiescent BHs follow the fundamental plane very well, and,
however, FR I radio galaxies follow a steeper track comparing other BH sources.
After excluding FR Is, we investigate the fundamental plane for BHs in
quiescent state with $L_{\rm X}< L_{\rm X,c}$ and sub-Eddington BHs with
$L_{\rm X}> L_{\rm X,c}$ respectively, and both subsamples have a similar
slope, $\xi_{\rm X}\sim0.6$, which support that quiescent BHs may behave
similar to those in low-hard state. We further select two subsamples of AGNs
with BH mass in a narrow range (FR Is with $M_{\rm BH}=10^{8.8\pm0.4}$ and
other LLAGNs with $M_{\rm BH}=10^{8.0\pm0.4}$) to simulate the behavior of a
single supermassive BH evolving from sub-Eddington to quiescent state. We find
that the highly sub-Eddington sources with $L_{\rm X}/L_{\rm
Edd}\sim10^{-6}-10^{-9}$ still roughly stay on the extension of radio--X-ray
correlation as defined by other sub-Eddington BHs. Our results are consistent
with several recent observations in XRBs that the radio--X-ray correlation as
defined in low-hard state can extend to highly sub-Eddington quiescent state.
