ns for binary transmission, an open loop (no current flowing) and a closed loop (current flowing). Keep in mind that we could reverse this, namely, change the convention and assign marking to a condition of current flowing or closed loop, and spacing to a condition of no current or an open loop. 4 As mentioned, Receiver Keying device termination Keying device Receiver termination Figure 10.3 Simplified diagram illustrating a dc loop with (a) neutral keying and (b) polar keying. 4In fact, this was the older convention, prior to about 1960. 10.7 BINARY TRANSMISSION AND THE CONCEPT OF TIME + Voltage 0 Volts Neutral waveform + Voltage 0 Volts - Voltage Polar waveform 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 