delay, ringing voltage is applied to the line (where required). The PABX immediately detects the grounded tip conductor and will not allow an outgoing call from the PABX to use this circuit, thus avoiding glare. In a similar fashion, if a call originates at the PABX and is outgoing to the local serving exchange, the PABX grounds the ring conductor to seize the line. The serving switch recognizes this condition and prevents other calls from attempting to terminate the circuit. The switch now grounds the tip conductor and returns dial tone after it connects a digit receiver. There can be a rare situation when double seizure occurs, causing glare. Usually one or the other end of the circuit is programmed to back down and allow the other call to proceed. A ground start interface is shown in Figure 7.8. Terminology in signaling often refers back to manual switchboards or, specifically, to the plug used with these boards and its corresponding jack as illustrated in Figure 7.9. Thus we have tip (T), ring (R), and sleeve (S). Often only the tip and ring are used, and the sleeve is grounded and has no real electrical function. Answer Relay Ground Start Contact Relay Contact 200 I Ringing Ground Detector Detector Network 48 Vdc Switch 400 n Nominal 200 n network Nominal Current Ground Detector Start Circuit Relay Termination Contact Bias Battery Ring Ringing Relay Contact Ringing Generator -Loop- Switch Figure 7.8 Ground-start interface block diagram. (From Figure 2-7 of Ref. 8, reprinted with permission.) 7.9 METALLIC TRUNK SIGNALING Switchboard Switchboard Figure 7.9 Switchboard plug with corresponding jack (R, S, and T are ring, sleeve, and tip, respectively). 7.9 METALLIC TRUNK SIGNALING 7.9.1 Basic Loop Signaling As mentioned earlier, many trunks serving the local area are metallic-pair trunks. They are actually loops much like the subscriber loop. Some still use dial pulses for address signaling along with some form of supervisory signaling. Loop signaling is commonly used for supervision. As we would expect, it prov