ISO-CMOS

MT8981D

2-23

If bit 6 of the Control Register is 0, then bits 2 and 0

of each Connection Memory High location function

normally (see Fig. 5). If bit 2 is 1, the associated ST-

BUS output channel is in Message Mode; i.e., the

byte in the corresponding Connection Memory Low

location is transmitted on the stream at that channel.

Otherwise, one of the bytes received on the serial

inputs is transmitted and

the contents of

the

Connection Memory Low define the ST-BUS input

stream and channel where the byte is to be found

(see Fig. 6).

If the ODE pin is low, then all serial outputs are high-

impedance.

If it is high and bit 6 in the Control

Register is 1, then all outputs are active. If the ODE

pin is high and bit 6 in the Control Register is 0, then

the bit 0 in the Connection Memory High location

enables the output drivers for the corresponding

individual ST-BUS output stream and channel.

Bit

0=1 enables the driver and bit 0=0 disables it (see

Fig. 5).

Applications

Use in a Simple Digital Switching System

Fig. 7 and 8 show how MT8981s can be used with

MT8964s to form a simple digital switching system.

Fig. 7 shows the interface between the MT8981s and

the filter/codecs. Fig. 8 shows the position of these

components in an example architecture.

The MT8964 filter/codec in Fig. 7 receives and

transmits digitised voice signals on the ST-BUS input

These

signals are routed to the ST-BUS inputs and outputs

on the top MT8981, which is used as a digital speech

switch.

The MT8964 is controlled by the ST-BUS input D

C

originating

from

the

bottom

MT8981,

which

generates the appropriate signals from an output

channel in

Message Mode.

This architecture

optimises the messaging capability of the line circuit

by building signalling logic, e.g., for on-off hook

detection, which communicates on an ST-BUS

output. This signalling ST-BUS output is monitored

by a microprocessor (not shown) through an ST-BUS

input on the bottom MT8981.

Fig. 8 shows how a simple digital switching system

may be designed using the ST-BUS architecture.

This is a private telephone network with 128

extensions which uses a single MT8981 as a speech

switch and a second MT8981 for communication with

the line interface circuits.

A larger digital switching system may be designed by

cascading a number of MT8981s. Fig. 9 shows how

four MT8981s may be arranged in a

non-blocking

configuration which can switch any channel on any

of the ST-BUS inputs to any channel on the ST-BUS

outputs.

Figure 7 - Example of Typical Interface between 8981s and 8964s for Simple Digital Switching System

8981 used

as

speech

switch

MT8981

STo0

STi0

STo0

STi0

MT8981

8981 used

in message

mode for

control and

signalling

MT8964

Filter/Codec

Signalling

Logic

Line Driver

and

2- to 4-

Wire

Converter

Line Interface Circuit with 8964 Filter/Codec