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

 Specifies the controller mode for the line. There are two possible
 modes:

 LOOPBACK    Internal device loopback mode.  Note that not all devices
             support this mode.

 NORMAL      Normal operating mode, which is the default.