With respect to data direction, each segment of the \cbus\ can be in one
of the following three modes
 \begin{itemize}
	\item \parhead{Write mode} The data lines are actively
driven by the distal-side pins of the node closer to the host, and
are sensed by the proximal-side pins of the node closer to the modules.
	\item \parhead{Read mode} The data lines are actively
driven by the proximal-side pins of the node closer to the modules, and 
are sensed by the distal-side pins of the node closer to the host.
	\item \parhead{Recovery mode} The data lines are not
driven at either end. This mode is used for one clock cycle during the
transition from read mode to write mode, to prevent even brief periods of
bus contention between the nodes at the two ends of a data
line.
 \end{itemize}

[discuss in more detail the propagation of mode changes and its
interaction with the readiness lines] 
