PABX - RELAY No. 1 & No. 2 

An extract from the IPOEE Paper No. 142 (1932)
PABX Installations by R. Dennison

Siemens Relay No. 1 and Relay No. 2 Systems are doubtless well-known, these being originally the R.A.T. Co's 24V and 32V systems respectively.  These two systems have in the past formed the basis of P.A.B.X. practice in this country and fulfil the requirements of the Post Office.

The whole of the operations are dealt with by relay groups, and the by-path principle of operation is employed. Such a principle is essential, or the number of relays involved would be excessive.

The relays are made up into groups which are necessary to perform the functions of storing, routing and connecting the calls to their various destinations.

Operation. The system operates on a "line finder" basis, the action of removing a receiver causing the calling extension to be connected to a free trunk termed an "out-trunk."  A schematic diagram is shown below. The "out-trunk" is associated with an A.B. Feed which forms part of the out-trunk, and contains the transmission elements, dialling tone, busy and ringing connections. The A.B. Feed so seized searches for a free Recorder, which, when found, is connected to the A.B. Feed via a Recorder Connector.  At this stage the caller hears dial tone and commences to dial The dialled impulses are repeated into the Recorder, which stores them until all have been received.  The Recorder then "marks out" and, with the aid of a common relay group termed a "Marker," sets up a connection between the called extension line and the A.B. Feed, upon which the caller is waiting.  The "Marker" can only be taken into use by one Recorder at a time. The connection is set up over an "in-trunk" via a "Trunk Connecting" Group.  When a connection is established between the two extensions, the Recorder and Marker, which comprise the by-path circuits, are released and become available to other callers.  If the called extension is engaged, then no connection is set up between the A.B. Feed and the called extension and the A.B. Feed will return busy tone to the caller.

Ten extension line equipments, in two units of five lines each, form one group, and are contained in a group case.  Four A.B. Feeds and a Trunk Connecting Group of corresponding size, are mounted in another group case which is wired for a maximum capacity of six A.B. Feeds.  Each Recorder is assembled into separate and smaller group cases.  Similar equipment is employed in the 32V system.

The whole equipment is therefore in unit form, and so many of each type of relay group may be assembled on suitable racks to form a P.A.B.X. of any desired size. The group cases are cabled to terminal assemblies located above each bay, and. may be cross-connected to equipment mounted in the same bay by straps, and to equipment in other bays by tie cables.

The system is very quiet in its operation, and can, therefore be installed in the room occupied by the manual board.  In the author's opinion it is an ideal P.A.B.X. system, being capable of extension to 240 lines in the case of the 24V system, and from 300 lines indefinitely in the case of the 32V system.  Standard P.G and other alarms are provided and direct access, tie line and bothway junction working on an automatic basis may be provided to Post Office specification by the addition of suitable relay groups and with the minimum of alteration to existing apparatus.

The 24 volt system will operate satisfactorily over a voltage range of 22-26 volts.
The 32 volt system operates over a voltage range of from 28-34 volts.



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Last revised: January 09, 2023