10.7
BINARY TRANSMISSION AND THE CONCEPT OF TIME
259
Figure 10.4
Neutral and polar waveforms.
this is called "changing the sense." Either way, a neutral loop is a dc loop circuit where
one binary condition is represented by the presence of voltage and the flow of current,
and the other condition is represented by the absence of voltage and current. Figure 10.3a
illustrates a neutral loop.
Polar transmission approaches the problem differently. Two battery sources are pro-
vided, one "negative" and the other "positive." Following the convention in Table 10.1,
during a condition of spacing (binary 0), a positive battery (i.e., a positive voltage) is
applied to the loop, and a negative battery is applied during the marking (binary 1) con-
dition. In a polar loop current is always flowing. For a mark or binary "1" it flows in one
direction, and for a space or binary "0" it flows in the opposite direction. Figure 10.3b
shows a simplified polar loop. Notice that the switch used to selected the voltage is called
a keying device. Figure 10.4 illustrates the two electrical waveforms.
10.7
BINARY TRANSMISSION AND THE CONCEPT OF TIME
10.7.1
Introduction
As emphasized in Chapter 6, time and timing are most important factors in digital trans-
mission. For this discussion consider a binary end instrument (e.g., a PC) sending out in
series a continuous run of marks and spaces. Those readers who have some familiarity
with the Morse code will recall that the spaces between dots and dashes told the operator
where letters ended and where words ended. The sending device or transmitter delivers
a continuous series of characters to the line, each consisting of five, six, seven, eight,
or nine elements (bits) per character. A receiving device starts its print cycle when the
transmitter starts sending and, if perfectly in step with the transmitter, can be expected to
provide good printed copy and few, if any, errors at the receiving end.
It is obvious that when signals are generated by one machine and received by another,
the speed of the receiving machine must be the same or very close to that of the transmit-
ting machine. When the receiver is a motor-driven device, timing stability and accuracy
are dependent on the accuracy and stability of the speed of rotation of the motors used.
Most simple data-telegraph receivers sample at the presumed center of the signal element.
It follows, therefore, that whenever a receiving device accumulates timing error of more
than 50% of the period of one bit, it will print in error.