Library /sys$common/syshlp/ncphelp.hlb SET, LINE, CLOCK clock-mode  *Conan The Librarian (sorry for the slow response - running on an old VAX)

 Applies only to synchronous DDCMP lines.  Specifies the hardware clock
 mode for the line.  The values for clock-mode are:

     INTERNAL    For use of the clock in test mode.  Setting this value
                 causes the line device to supply a clock signal that
                 will allow all transmitted messages to 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.