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VII
I
INTRODUCTION
No matter what programs exist within it, a computer by
itself is useless; there must be a way of entering data into it
and getting data out of it. For most types of applications,
this means connecting some terminals to the computer.
That is where the field of "data communications" enters
the picture.
There are several ways to connect a terminal to a computer.
To arrive at a particular way, you must compare a number
offactors and make a series of decisions. Mter selecting the
necessary equipment and cables, you must then physically
connect the terminal to the computer (or to the modem, if
that is what you have chosen) and configure the terminal
for use with the particular type of data communications
link.
This section is divided into four parts:
1. The .first is a general discussion that should help you
decide what type of equipment and cabling you need for
each desired data link.
2. The second tells you how to physically install the ter-
minal.
3. The third tells you how to configure the terminal to
operate properly with the selected type of data link.
4. The final part provides programming reference ma-
terial for someone who is writing a data comm driver or
controller program to communicate with an HP 2624A
in a point-to-point environment.
SELECTING EQUIPMENT AND CABLES
To select the most suitable and least expensive combina-
tion of equipment, cables, and/or common carrier
(telephone company) facilities, you must approach the
overall situation in a systematic fashion. For each
available data communications port on each available
computer, you make a series of decisions starting at the
most general level and working downward from there.
Each decision determines the next set of capabilities to be
considered.
Before proceeding with the decision making process, let us
briefly define the most important terminology as it per-
tains to HP 2624A data communications.
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 telephone network, or a
leased telephone line) and it may also include
a pairof modems (one ateach end of the line).
Point-to-Point:
A data communications configuration in
which a single terminal is connected to a host
computer over a data link. The
HP
2624A is
designed for use with a point-to-point data
link.
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 characters is random. The
HP
2624A is designed for use with an
asynchronous data link.
Half Duplex:
A data link in which data can be transmitted in
only one direction at a time. Each time the
direction of the data flow is reversed the
modems on each end of the line must switch
from "transmit" state to "receive" state (or
vice versa). This state transition is called a
"line turnaround".
Full Duplex:
A data link in which data can be transmitted in
both directions simultaneously.
Character Mode: When the terminal is operating in character
mode, it sends data characters to the com-
puter one at a time as they are typed through
the keyboard.
Block Mode:
When the terminal is operating in block mode,
data characters typed through the keyboard
are merely stored in display memory. When a
block transfer is subsequently triggered (by
the host computer or by pressing the
II1II
key
or another appropriately defined key), a
group of data characters are sent from the
terminal to the computer as a block.
A point-to-point configuration is the standard form of data
communications within the industry (it is sometimes
referred to as a "Teletype-compatible" data link). Point-
to-point is supported by most computers. At any given
time, it accomodates only one terminal per data link; it
may, however, operate in either character mode or block
mode.
You must now define the major characteristics of the data
link by making the series of decisions illustrated in figure
7-1. You will notice that in that figure the overall set of
decisions is organized as a tree structure and that when
you make each choice you then follow the associated
branch to the next set of alternatives.
For each desired point-to-point data link, you must decide
whether you want a hardwired or modem connection.
A hardwired connection, where feasible, is the cheaper
alternative because it eliminates the use of modems and
common carrier (telephone company) lines.
7-1
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