Lesson 1/Learning Event 1
b. Signal Compatibility.
To pass from one communication system to
another, signals must be compatible electrically; they must also be
compatible in content.
(1) Electrical compatibility.
The signals to be transferred through
the interface must be electrically compatible with the end equipment items
as well as with the transmission medium.
(a) Signal Type.
Assume that a teletypewriter is the end item of a
communication system, and that neutral DC teletypewriter signals are being
delivered to the interface.
Assume further that the teletypewriter is
wired to receive and interpret polar signals. It is evident that one of
two procedures must be followed; either the neutral signal must be
converted to a polar signal by a neutral-polar repeater at the interface,
or the teletypewriter circuit must be rearranged to receive the neutral
signal direct without conversion at the interface.
(b) Polarity.
Direct current must be in the proper direction to
satisfy the requirements of the teletypewriter. If the current is in the
wrong direction (reversed), a "turnover" may occur, resulting in
misprinting or garbling of the received messages. If it becomes necessary
to reverse the direction of current through a teletypewriter, the
inversion is usually accomplished at the patching panel located at the
point of interface. DC telegraph loops are ungrounded to permit this type
of patching.
(c) Transmission speed.
The speed characteristics of the incoming
signals must match those of the receiving printer.
For example, if the
incoming signals are generated by a sending device operating at a speed of
100 wpm and the terminating printer is adjusted to receive signals at 60
wpm, it is obvious that misprinting must occur.
(d) Transmission medium. The transmission medium includes all the wire
and interconnecting facilities between two communicating systems.
The
characteristics of the transmission medium determine to a large extent
what types of signals can be sent over the system.
In the case of DC
signal pulses, it is necessary that the transmission medium of both
interfaced systems be capable of passing the signal with equal ease and
with a minimum of distortion.
(2) Content compatibility.
Content compatibility is related to the
information characteristics of the signal. The content capability of the
circuits joined at the interface must be similar. As an illustration, the
operator should not patch telephone circuits to teletypewriter DC loops.
If he attempted to make such
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