e off-hook impedance of the ring end of the network to the other and in some cases dial tone may be detect circuit to the exchange. Incoming calls from the exchange to returned to the PABX while the PABX still has the line seized. The the PABX are ringing current superimposed on the d.c. of the system relies on both parties clearing. For this reason, and the call exchange battery. The capacitor in the ringing detector circuit collision mentioned earlier, high traffic PABX trunk circuits use blocks the d.c. exchange battery and the a.c. ringing signal is earth calling or ground start signalling. presented to the ring detector circuit, which generally consists of an opto coupler which isolates the line from the low level electronic 34.3.2 Earth calling circuits. The a.c. ringing activates the opto coupler and the signal is presented to the system. It is normally the system software which Earth calling signalling (also known as ground start) was introduced performs the persistency checking on the signal and timers are set when automatic direct outward dialling from the PABX extensions to monitor the on and off periods of the ringing signals. was introduced and the telephone operator was removed from As no common international standards exist for ringing signal monitoring the progress of the call and hence the disconnect signal. cadences, different public exchanges provide different ringing sig- The major reason for the introduction of this signalling system was nals depending on the individual operator's specification, SO the to prevent call collision during the silent period of the ringing signal. PABX circuit must be flexible enough to cater for different require- The principle of operation of the earth calling signalling is shown ments. in Figure 34.5. To request service the PABX applies local earth via The problem that can arise with loop calling lines is call collision a relay contact to one leg of the pair from the public exchange. The due to the line seizure being applied by the public exchange before exchang