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In an idle circuit, the central office supplies –48V (nominally) on the ring conductor with respect to the tip side. A ground-start PBX initiates an outgoing trunk seizure on an idle circuit by connecting of the ring lead to ground (maximum local resistance of 550 ohms). The central office senses
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from the local phone company and correctly install the telephone system at the PBX end – so that they match. Line equipment in most 20th-century central-office switches had to be specially rewired to create a ground-start line.
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on the line. To avoid glare, before the PBX originates an outgoing call, it must first verify that the CO has not already applied ground to tip. The PBX has 100ms to sense this condition.
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used coin-first ground start lines, with the starting ground connection provided by the coin itself, bridging a set of contacts as it passed through the coin chute.
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by going on-hook, or the central office initiates disconnect by opening tip. When the other end detects the loss of loop current, it also goes on-hook and the call
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Telephone companies typically provide two types of dial-tone switched circuits – ground start and
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In ground-start signaling, the central office initiates a call by grounding tip and putting the
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by a slide switch on the line card, all according to what the customer ordered.
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this condition and grounds the tip lead. When the PBX senses this, it goes
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since neither gets the expected response and no call can be initiated.
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At the end of either an incoming or outgoing call, the PBX initiates
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may inadvertently seize the line simultaneously, a condition called
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typically work on loop-start lines. On loop-start lines the PBX and
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A PBX user must be careful to order the correct type of
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193:and the rest of the call proceeds normally.
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