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Data Communications
INTRODUCTION
7
Data communications (datacomm) refers to the transfer of data between the terminal and a
host computer.
The datacomm system can be of either the point-to-point or multipoint type. Considerations
involved in selecting the type are discussed early in this section. In addition, for each type of
system, this section discusses the following items:
• Considerations involved in selecting either a modem or hardwired connection.
• Installation.
• Configuration.
• Considerations involved in selecting either an asynchronous or synchronous system
(multipoint systems only).
• Considerations involved in selecting either a daisy chain or DSN data link type connection
(multipoint systems only).
• Information required for writing programs which interface with the datacomm system.
Following is a list of datacomm terms with their definitions, as used in this discussion.
Data Link
The means by which a terminal is connected to a host computer. This always
includes some type of communications line (a coaxial cable, the public tele-
phone network, or a leased telephone line), and it may also include a pair of
modems (one at each end of the line).
Point-to-Point
A data communications configuration in which a single terminal is con-
nected to a host computer over a data link.
Multipoint
A data communications configuration in which two or more terminals are
((chained" together so as to share a data link to a host computer.
Asynchronous
A mode of transmission in which each data character is framed by a ((start
bit" and one or more ((stop bits". The interval between successive data charac-
ters is random.
7-1
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