Connecting to station telephones

This topic describes how to connect station telephones or other devices to a CX 2000C board:

• Cabling considerations

• Cable connections

• Developer's cable kit

Cabling considerations

When cabling your stations (especially off-premises stations), consider the following issues:

• As the cable length increases, the DC resistance increases. Most telephones operate correctly if the loop current is at least 20 mA. To maintain this minimum current, the total cable resistance (the resistance on the tip wire plus the resistance on the ring wire) must be less than 1500 Ohms over the expected temperature range. When Telcordia guidelines are followed, the cable length cannot exceed 18,000 feet. If these guidelines are exceeded, the attached device may not operate properly.

• As the cable length increases, the cable presents an impedance to the audio path. This attenuates the audio signals in both directions and creates an echo path. Attempting to compensate for the loss by increasing gain will increase the echo and other noise. The 18,000 foot cable limit also sets a limit of acceptable audio quality for traditional telephony services. To improve the frequency response of the loop, many telephone networks add a device called loading coils when subscriber loops approach or exceed 18,000 feet. This can increase the echo.

• Telephone networks offer services commonly called foreign exchange circuits. These should be considered for applications requiring longer loops.

• If any section of the wiring between the board and connected local telephone lines runs outdoors or between buildings (buried or above ground), be sure to provide adequate lightning protection.

NMS recommends 24 AWG (0.6 mm) twisted pair cable for distances over 50 feet (15 meters). The following table lists the recommended cable types and maximum distances for each:

Cable connections

As shown in the following illustration, the station interface connectors are located on the CX 2000C rear I/O transition board and bracket. They are RJ-21, 25-pair interfaces.

The connectors are each designed to accommodate a 25-pair cable. This cable is commonly wired to a punch-down block or break-out box. The telephones or other station devices are connected to the block or box through standard telephone wiring as shown in the following illustration:

The RJ-21 connector on the cable must be the 180-degree design. The common 90 degree RJ-21 connector is not compatible with the CX 2000C board.

The following illustration shows the pin locations for the RJ-21 connectors on a CX 2000C board:

Connector pinout

The following table describes the pinouts for the top RJ-21 connector on a CX 2000C (the one labeled STATIONS 1 TO 24):

Note: Pins 25 and 50 are not used.

The following table describes the pinouts for the bottom RJ-21 connector on a CX 2000C (the one labeled STATIONS 25 TO 48):

Note: Pins 25 and 50 are not used.

Developer's cable kit

NMS provides an optional developer's cable kit. The kit contains two 10 foot RJ-21 cables and two breakout boxes. Each breakout box connects one RJ-21 to 24 standard RJ-11 (POTS) jacks for individual telephones. Use the cables to connect to the breakout boxes or to standard 66 or 110 blocks.

All components of the developer's cable kit sold by NMS are also commercially available from telephone product distributors such as Graybar and Anixter. These distributors can provide variations in cable lengths.