Ground start
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In telephony, a ground start or GST is a method of signaling from a terminal or subscriber local loop to a telephone exchange, in which method a cable pair is temporarily grounded to request dial tone. Most middle 20th century American payphones used "coin first" ground start lines, with the starting ground passing through the coin itself.
[edit] Ground start trunk
Local telephone companies typically provide two types of dial tone switched trunks -- ground start and loop start. PBXs work best on ground start trunks because those trunks can give them an on hook signal allowing for timely clearing (telecommunications). Many will work -- albeit intermittently -- on both types.
Normal single line phones and key systems typically work on loop start lines. A PBX user must be careful to order the correct type of trunk line 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 CO switches had to be specially rewired to create a ground start DDCO line. Crossbar switch did it by a paper sleeve on the Vertical Off Normal contact, 5ESS switch by translation, and DMS-100 by a slide switch on the line card, all according to what the customer ordered.
Technically, a ground start trunk initiates an outgoing trunk seizure on an idle circuit by applying a momentary connection to ground (maximum local resistance of 550 ohms). In an idle circuit, the CO is giving -48v (nominally) on the Ring and an open on the Tip. When the PBX merely shorts the pair, this does not operate the Line Relay in the CO, hence the line remains idle. In order to start the line, the PBX applies a ground to the shorted pair, which operates the Line Relay, causing the CO to send dial tone. Along with dial tone, the CO puts a ground on the Tip side. The PBX senses the resulting voltage difference across the pair, releases its Ground Start condition, maintains the connection as a simple current loop, and outpulses the telephone number. When conversation is finished and the line is to be cleared, the CO removes the short across the pair, and the PBX accepts this on-hook condition as indicating an idle line.
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