KEY SYSTEM 4+12

Key System 4+12
A proprietary SBS

Edinburgh Telephone Area is about to market trial a proprietary small business system (SBS)-known as Key System 4+12. Twenty are to be installed. The Key System 4+12 has many similarities to the PO developed 'Herald' call connect system to be introduced in London during 1980.

System outline
The versatile, electronic Key System 4+12 offers both intercom and direct public or private exchange access. Designed to be installed in offices, it is small and silent in operation and includes advanced features normally associated with more elaborate systems, such as hold, transfer and conference facilities. It provides for a maximum of 4 exchange lines and 12 extensions. A 4-wire cable connects the extension instruments to a central switching cabinet, two wires being used for speech and two for signalling. The photograph shows the central switching cabinet or Processor Unit and a station instrument. The units are fully factory wired. Standard plug-in printed wiring boards (PWB) are inserted to meet the specific requirement exchange lines and extensions-of each installation. There are four different types of PWB, a fully equipped unit having a total of thirteen.

Digital control
The switching element for the speech circuits uses a single stage of reed relays. The folded matrix arrangement only needs a single crosspoint to be operated per call, the number of cross points provided giving a non-blocking system so that all extension stations can be used simultaneously. Using a time-sharing digital technique, the signalling system caters for simultaneous bothway signalling. The control equipment looks at each extension in a cyclic scan and during each interrogation period the extension instrument is sent eight clock pulses. In periods between the first and fifth clock pulses, the station instrument has access to the central equipment during which 1t sends five data pulses. These five data pulses provide a possible 32 codes which satisfies the maximum signalling requirement of the system. Clock pulses six to eight are used for signalling from the central equipment to the extension instrument.

Maintenance
A handbook has been prepared to help maintenance staff in localising faults. Maintenance of the Processor Unit is based on defective card identification and replacement procedures, that is, associating the loss of certain facilities on the system with the functions of specific cards. Fault location down to a specific card is achieved using logical deduction with the aid of faulting/flow charts and tables contained in the handbook. A faulty card, once identified, is changed with a serviceable replacement. The defective card is then returned to the maintenance AEE who arranges for its repair.

Two complete sets of spare cards and power supplies are held in two purpose built suitcases designed to protect the cards during transportation between customers' premises and the designated holding/storage area. When a faultsman is called to repair the Processor Unit he collects a suitcase. returning it to the holding point once the repair has been completed. This method ensures that a faultsman has all the necessary equipment to deal with most faults and any spares deficiency can be easily monitored.

Training
Familiarisation training has been given to 21 faultsmen at STB/HO's Engineering Training Centre in Edinburgh. This one-day course gave them a working knowledge of the facilities available and the operating principles of the system. Special emphasis was placed on faulting techniques using the flow charts. tables and diagrams contained 1n the maintenance handbook.