fcd-2 has three versions: one with a v.35 interface, one with a x.21 interface and one with rs-530 interface. the rs-530 interface can be converted to v.36/rs-449/422 via adapter interface cables. the user data connectors of the three fcd-2 versions are different, due to the differences between the interfaces.

    the user data channel interface can be operated either as a dce or as a dte, with the selection being made by the user from the front panel, or from the supervision terminal. as explained below, two dte timing modes are available for the v.35 version.

    the timing mode affects the direction timing information flows, as explained below in the section on system timing.