Editing 600 series connector (section)

===611 socket===

Similar to 610, but with the contacts of pairs one and two allowed to connect when no plug is inserted, contact 1 connecting to 2 and 5 to 6.

This is particularly designed for [[mode 3 (telephone)|mode 3]] connection. The incoming line to the mode 3 device is connected using pair one, and pair two is used as the outgoing line to other devices. If the mode 3 device is unplugged, the switch contacts maintain line connection to the other devices.

This function is not generally supported by modular connectors. Modular connectors providing suitable switch contacts are available but not common; Instead, mode 3 devices use two modular connectors, one for the incoming line and the other for the outgoing line. The convenience of using a single connector in this application is one reason for the continued use of 600 series connectors, but mixing this wiring scheme with modular connectors does not work well. For example, if a modern modem with its supplied 6P2C adaptor is plugged into a conventionally wired 611 mode 3 socket, the result is to disconnect all downstream devices.

In some sockets conductors 3 and 4 also connect when no plug is present, shorting pair three. Sockets are also available with cams to allow the switching functions to be selectively enabled as required. In some older sockets this could be achieved by simply bending the contacts; The cams merely implement this useful accidental feature in an intentional fashion.

Many recent 611 sockets have a stepped recess for the plug spigot, to accept a plug with a larger spigot but still provide a good fit for a standard 605 plug. Equipment requiring mode 3 connection can then be equipped with these larger spigot plugs, to prevent this equipment being connected to a 610 socket which does not provide mode 3 support. These plugs are common on older fax machines and modems, and used on some security and similar equipment. These plugs were also used on [[Telex]] machines within Australia.