PRIVATE MANUAL BRANCH EXCHANGES
MULTIPLE AND LAMP SIGNALLING
PMBX. 1A with multiple and lamp signalling is designed to cater for very large establishments and comprises a main distributing frame, a suite of two or more switch sections, one or more apparatus racks and a power plant.
The main distributing frame is of conventional design, that shown in the illustration below, being of the wall type, and is extensible.
The switch sections, constructed of wood, are of modern design, being free from panelling and cornices, and have iron supports at the rear for mounting, cord, pilot, cord test and operator's circuit apparatus only, thereby ensuring accessibility to the interior equipment.
The dimensions of each section are:-
4 ft. 6 in. high, 2 ft. 2 in. wide, and 2 ft. 6 in. deep over the keyboard which is 2 ft. 6 in. high, so that special chairs for the operators are unnecessary.
The jack field is divided into two panels, the multiple being complete over four
panels, i.e., two sections. Strips of 10 jacks and lamps are used for the exchange and
inter-switchboard multiple and strips of 20 for the extension multiple, the lamp strips
being dispersed throughout the respective multiples according to the loading of each
The maximum capacity of a fully equipped PMBX. 1A is:-
The keyboard is initially equipped with 12 cord circuits, the maximum capacity being
16, each circuit comprising two supervisory lamps, speaking and ringing key, and dialling
and ring-back key. Miscellaneous circuit apparatus, such as hand generator keys, coupling
keys and night service jacks are suitably mounted at the top of the jack field.
The apparatus racks are of two types, PMBX. No. 1 and No. 1/35, each constructed of an angle iron framework 7 ft. x 2 ft. 9 in. x 1 ft. 5 in. Apparatus rack No. 1 accommodates terminal strips for terminating the apparatus and cables from the switchboard and M.D.F.; a fuse panel for battery feed to all circuits; sleeve resistor coils; extension line relays when required and relay sets for exchange and inter-switchboard lines.
MDF and two apparatus racks No. 1 are shown right.
Apparatus rack No. 1/35 is provided when a large number of inter-switchboard lines is
M.D.F. and Two Apparatus Racks No. 1 required and accommodates up to 35 lines.
Power supply is normally a 24 V. battery floated from a.c. mains by a rectifier, but where a.c. supply is not available double batteries with suitable charging equipment would be used.
Line relays for extensions up to 200 ohms line resistance are omitted; over 200 ohms, provision is made on racks No. 1 for the inclusion of P.O. 600 type relays; all other relays are P.O. 3000 type.
Two No. 1 and One No. 1/35 Apparatus Racks are shown left
Taken from the Ericsson catalogue, edition No. 49
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