Central Office or PBX Line Circuit

A 400-type card provides access to one Central Office or one PBX line. The card handles the lamp functions for the line, connects the telephone set to the line and initiates audible ringing for the line.

The 400-type KTUs are four inch cards with 18 contacts. The 400G and 400H have LEDs on the card that indicate the KTU's status. These LEDs function differently, depending on the card. On the 400G, the LED is illuminated whenever the line is in use. The LED on the 400H matches the lamps on the key telephone sets (flashing for ringing, winking for hold, steady for in use).

The 400-type cards are configured so that a key telephone set will still have access to the Central Office or PBX line in the event of a power failure. Some functions (lamps, hold, ringing, etc.) will not work without power, but a key telephone set can still place calls.

Options on the 400-type cards allow for two lamp options to indicate that a call is on hold. The default option uses a "winking" lamp to indicate a held line. The other option uses a "steady" lamp -- in this configuration, other stations cannot differentiate between a line that is on hold and a line that is in use. A flashing lamp always indicates a ringing line.

The most common 400-type card is the 400D, followed by the 400G and 400H. With the exception of their compatibility with the music on hold KTUs, they are interchangeable. When a 400H is with music on hold, the 498A music on hold KTU must be used. The 451B KTU can be used to provide music on hold with any 400-type KTU.