Holding Coil

The Holding Coil Unit, Model 7501-40, is used in conjunction with various Teleline Isolator cards for four-wire DX signalling applications (four-wire simplex DC current), as well as for DC simplex powering requirements (i.e., carrier). This type of signalling, also used by certain PBX tie-lines, is fully described in the installation guide.

Product Number CLEI Code Description
7501-40 NPMIAA01MA Holding Coil


  • 4-wire DX signalling
  • DC simplex powering


  • The unit provides a means of reintroducing DC voltage across the TX and RX pairs of isolated DX signals
  • The unit is bi-directional and, therefore, the DC voltage source can originate from the Station or CO side
  • The unit can be used to power a two-wire circuit from the Tip & Ring, using an external power supply


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