Technical data

24 Theoretical and general applications www.westermo.com
Data communication...
...is extremely important in order to increase
productivity
Increases in automation also place demands on reliable data communications between
units and the systems that control and those producing and measuring. Data commu-
nication is the nervous system that forms the basis of increased efficiency and competi-
tiveness. Irrespective of whether it concerns manufacturing, installation, transport or
healthcare.
Interface
Agreement regarding the signal type, how they should be converted and transmitted is
not enough. Agreement is also required regarding the type of connector and the volt-
age levels they need to support, in other words, the physical and electrical interface.
There is also a logical interface, which defines the significance of the signal.
A protocol controls how the signals are built up, how communications are initiated,
how they are terminated, the order of transmitting and sending, how to acknowledge
a message, etc. There are many different protocols, for example, PROFIBUS, Comli,
Modbus, etc.
The physical interface defines how equipment is connected as well as the design
of the connector.
The electrical interface defines the electrical levels and what these denote
(ones or zeros).
Logical interface defines what the signals signify.
The most common interfaces
The most common interface for data communication via computer equipment’s
serial port is RS-232/V.24, which usually uses a 9-/25-pos. D-sub connector. According
to the recommendations for RS-232/V.24, the cable between connected units should
not exceed 15 metres (49 ft). Different modems can be used to achieve greater trans-
mission distances depending on the communications media available (e.g. fibre, copper,
telecommunication circuit). V.24 (European CCITT standard) or RS-232-C (American
ITU-T standard) are two standards that are in principle identical, see the table on page
25. V.24 describes the physical standard while V.28 is the electrical standard. That is
why you sometimes see the interface described as V.24/V.28.
The interface describes and defines the connector’s pins, the signals and voltage
levels supported.
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