Am79R251

The Intelligent Access™voice chipsets integrate all functions of the subscriber line. Two chip types are used to implement the linecard; an Am79R251 device and an ISLAC device. These provide the following basic functions:
1. The Am79R251: A high voltage, bipolar device that drives the subscriber line, maintains longitudinal balance and senses line conditions.
2. The ISLAC device: A low voltage CMOS IC that provides conversion, control and DSP functions for the Am79R251.

Complete schematics of linecards using the Intelligent Access voice chipsets for internal and external ringing are shown in Figure 3 and Figure 4.

The Am79R251 uses reliable, bipolar technology to provide the power necessary to drive a wide variety of subscriber lines. It can be programmed by the ISLAC device to operate in eight different modes that control power consumption and signaling. This enables it to have full control over the subscriber loop. The Am79R251 is designed to be used exclusively with the ISLAC devices. The Am79R251 requires only +5 V power and the battery supplies for its operation.

The Am79R251 implements a linear loop-current feeding method with the enhancement of intelligent Thermal Management. This limits the amount of power dissipated on the Am79R251 chip by dissipating power in external resistors in a controlled manner.

Each ISLAC device contains high-performance circuits that provide A/D and D/A conversion for the voice (codec), DC-feed and supervision signals. The ISLAC device contains a DSP core that handles signaling, DC-feed, supervision and line diagnostics for all channels.

The DSP core selectively interfaces with three types of backplanes:
• Standard PCM/MPI
• Standard GCI
• Modified GCI with a single analog line per GCI channel

The Intelligent Access voice chipset provides a complete software configurable solution to the BORSCHT functions as well as complete programmable control over subscriber line DC-feed characteristics, such as current limit and feed resistance. In addition, these chipsets provide system level solutions for the loop supervisory functions and metering. In total, they provide a programmable solution that can satisfy worldwide linecard requirements by software configuration.

Software programmed filter coefficients, DC-feed data and supervision data are easily calculated with the WinSLAC™software. This PC software is provided free of charge. It allows the designer to enter a description of system requirements. WinSLAC then computes the necessary coefficients and plots the predicted system results.

The Am79R251 interface unit inside the ISLAC device processes information regarding the line voltages, loop currents and battery voltage levels. These inputs allow the ISLAC device to place several key Am79R251 performance parameters under software control.

The main functions that can be observed and/or controlled through the ISLAC backplane interface are:
• DC-feed characteristics
• Ground-key detection
• Off-hook detection
• Metering signal
• Longitudinal operating point
• Subscriber line voltage and currents
• Ring-trip detection
• Abrupt and smooth battery reversal