Specifications

52
Schneider Electric
Interfaces (continued)
Telephone companies dispatch pairs of cables to their clients; each pair represents
a different telephone number. Each of these telephone lines is linked to a special
computer called a "switching exchange or PABX" at the central telephone station.
The purpose of this computer is to set up the communications link between the caller
and the number being called.
Computers only process information in binary form, i.e., 1 or 0.
For telephone exchanges they must:
b Decode the numbers dialed.
b Establish the communications link between the two subscribers.
b Convert analogue conversations to digital form (1 and 0).
b Process the digital data.
b Re-modulate the digital data to obtain analogue signals.
Definition:
b PABX: Public Automatic Branch eXchange
b A/D: Analogue to Digital converter
b D/A: Digital to Analogue converter
b CPU: Central Processing Unit
b Caller: User who dials the telephone number
b Person called: User who receives the telephone call
Dedicated lines and modems
A connection made on a telephone line through an electronic exchange (PABX) is
called "temporary" because it represents a simple telephone call that anyone can
make at any time. When the communication is terminated, the line is freed. Specific
software is required to set parameters for the module, open communications on the
telephone line, manage the line, etc.
A dedicated line provides point-to-point connection. This connection is not through
a telephone exchange. It is permanent and requires components other than a
telephone to operate. Such a connection is in service 24 hours a day, 365 days a
year. A dedicated line with a bandwidth of 3 kHZ can carry 19.2 kBps (28.8 kBps
V.FC and V.34 for 3600 Hz).
Local modem links
This type of link is used inside buildings, in private areas, for transmissions over long
distances or in some case in disturbed environments.
ISDN connections
Definition: ISDN (Integrated Services Digital Network) is a telephone network that
uses digital signals up to the user level. Its characteristics make it useful for
transmitting moderate volumes of digital data, over medium distances, between
companies.
In this application, ISDN will progressively replace analogue telephone service. (The
future competitors of ISDN will be DSL technologies and cable.) This type of digital
connection does not use modems; it requires a Terminal Adapter or other specialised
card. The configuration of these devices is often similar to that of modems, but they
have additional and different characteristics that vary according to the model used.
E52340
PABX
caller
person called
1001101011100
A/D D/A
CPU
Guide to Integration
Modbus / Jbus