The COM4 module is installed in the dedicated position via J1 of the 408M/E. The COM4 interfaces four loop-start CO lines. Since one COM4 may be installed in KSU1 and one COM4 may be installed in KSU2, you can expand the DX-80 system CO line capacity to support up to 16 CO lines. 

Five RJ11 jacks are provided along the bottom edge of the COM4 module for connection of one power failure telephone and the four CO lines. CO lines are terminated at the site by the telephone company at a point of interface called the Demarcation (Demarc). When ordering CO lines for the site, request termination on RJ11 type jacks. Doing so allows you to easily extend the CO lines to the COM4 RJ11 connector (one CO line per jack and connector).

Each CO line circuit incorporates over-voltage protection, ring detector, loop detector, loop/pulse-dial relay, current sink circuit, coupling/isolation transformer (impedance 600:600), hybrid circuit, CODEC & filter, polarity guard circuit, and Radio Frequency noise filter. The fourth CO line port is equipped with CNG Fax Tone Detection circuitry. When programmed as a “FAX” line, this circuit will automatically engage the FAX tone detector. If FAX tone is detected, the system routes the call to the analog port designated as the destination for fax calls.