Specifies the hardware clock mode for the line.  The values for
 clock-mode are:

 	INTERNAL	For software controllable loopback use of
 			the clock.  This causes the line device
 			to supply a clock signal such that all
 			transmitted messages can be looped back
 			from outside the device.  Note that in
 			order to use this parameter, the operator
 			may have to connect a loopback plug in
 			place of the normal line.

 	EXTERNAL	For normal clock operating mode.  The
 			clock signal is supplied externally to
 			the controller.