PXML No. 85 and BABT/PXML/PBX. 0055
APPROVAL No. S/1000/GF/1981/PR


Monarch 120 is a small PABX catering for up to 120 extensions and 40 exchange lines/peripherals. The system uses a single stage PCM time switch under the control of a central microprocessor. It is designed to be housed in normal office accommodation, and is mounted in a single cabinet approximately 1600mm high by 600mm square and powered directly from the 240Vac mains supply. The operator's console is a low profile desk mounted unit which is used both for call handling and as a simple teletype for limited communication with the machine.

The system will operate with both dial telephones and SSMF4 keyphones. Calls can be set up either automatically or via the operator's console, but all calls incoming from the public network are routed via the console. Outgoing calls may be dialled direct, but can be selectively barred on examination of the extension class of service and up to 5 dialled digits. Standard PABX facilities are provided from all extensions and a range of supplementary facilities, e.g. 3 party service, call diversion and ring when free are available to keyphone extensions.

The Monarch cabinet contains 7 shelves. The control shelf, mounted at the top of the cabinet carries the processor and switching equipment, together with clock, conference and tone generation cards. The power unit is mounted on the bottom shelf. The intermediate 5 shelves each contain extension line units, exchange line cards, auxiliary units (e.g. SSMF4 receivers) and a shelf multiplex. Each shelf conforms to the IEC specification, the control and line shelves being 5 units high (222mm) and the power supply shelf 3 units high (133mm). The shelves are completely pre-wired using an automatically wire wrapped backplane. They are interconnected by means of ribbon cable looping down from top to bottom of the cabinet, whilst connexions to the distribution frame are made via plug-in access.

The main system features are shown in Figure 1. Monarch employs digital switching using A-law (CEPT compatible) 2048 kbit/s PCM and single channel CODECs. Speech is digitally encoded at the extension line unit, switched via a single time switching stage to its appropriate destination, and then decoded to recover the original signal. Each line unit incorporates its own PCM CODEC and produces a 9 bit digital signal at 72 kbit/s, comprising 8 speech bits and one signalling bit. The signalling bit is part of an 8 bit signalling code, giving each line an 8 kbit/s signalling capability.

Line shelves accommodate a maximum of 32 ports comprising six 4 port cards and four 2 port cards. Extension line cards are 4 port cards and exchange/inter PBX line cards are 2 port cards. The shelf also carries the shelf multiplex card which takes the 72 kbit/s send data streams from the 32 input ports and converts them to two composite send data streams, one for speech (2048 kbit/s) and one for signalling (256 kbit/s). These data streams together with those from up to four similar shelves are passed to the digital switch and signalling interface cards on the control shelf. At the digital switch, speech signals from the five shelf multiplex cards are converted from serial to parallel form and multiplexed together. The speech samples from each of the 256 ports are loaded at a parallel rate of 2048 kbit/s on to an 8 wire highway and from this written cyclically into a 256 x 8 bit read-write memory. This memory therefore holds a speech sample for every port in the system. By reading out the required sample from the speech store during the time slot associated with the destination port a non-blocking connexion is established through the system. The address corresponding to this timeslot is held for the duration of the call in another 256 x 8 bit read-write memory called the connexion store.

The timeswitch has access for a maximum of 256 ports, 160 of which serve exchange/extension lines and peripherals; the remaining 96 providing for such items as exchange tones and conference working. The switch is completely non-blocking and the traffic carrying capacity of the system is limited solely by the capabilities of the processor software structure.

The signalling interface cards provide buffering and access capability between the 256 kbit/s signalling highways and the Central Processor Unit (CPU) data and address highways.

The CPU uses the Intel 8085 microprocessor and has provision for addressing up to 256 kbytes of memory. The software is highly structured in modular form and is written in CORAL high level language with insets of assembly code where run-time is critical. The concept of top-down design has been used to derive the software structure, resulting in complete independence between different areas which simplifies additions, modifications and testing.

The controlling microprocessor accesses the timeswitch, signalling interface cards and memory via the tri-state CPU bus (highway). The main system program is stored in Programmable Read Only Memory (PROM), and occupies typically 48 kbytes. Volatile data, such as call records, is stored in Random Access Memory (RAM, otherwise known as Read-Write Memory). The exchange configuration and class of service information (database) is stored in battery backed RAM to protect this data against power failure or card withdrawal.

Tones for the system are generated digitally, the appropriate PCM encoded samples of the tone waveforms being stored permanently in a read-only memory. Cadencing of tones is also provided on the tone generation card and each tone appears at a fixed timeslot on the PCM highway into the switch. The CPU therefore has only to provide the appropriate information to the connexion store to connect a given port to a particular tone.

Monarch 120 systems are now in service and bulk production is building up towards a level of over 2000 units per annum. Further development is in progress to enlarge the system and provide an increased range of facilities.

Potter A R, Monarch 120 - A New Digital PABX - POEEJ, Volume 73, Page 14, April 1980.

Sales Brochure - PH2904

Sales Brochure - MER227

Taken from
The Post Office Electrical Engineer' Journal
Volume 74, Part 1 - Dated April 1981

Post Office Press Notice

British Telecom has placed orders worth 45M with GEC and Plessey for Monarch 120 private automatic branch exchanges (PABXs).

Monarch, the only entirely British digital switching stored program control (SPC) PABX, was designed and developed by British Telecon1 and engineered by GEC and Plessey; this new order demonstrates British Telecom's continuing support for established British suppliers. The first production deliveries of these Monarch 120s, which are being supplied initially to custo1ners in central London, were made in October I980.

In addition, after extensive examination of small proprietary PABXs produced by manufacturers in the UK and elsewhere, British Telecom intends to negotiate a contract with Mitel Telecon1munications Ltd. of Slough for small-to-medium PABXs. When field trials begun last autumn are successfully completed, it is anticipated that an initial order for 5M will be placed; the equipment will be produced at Mitel's Slough factory.

Announcing this further round of PABX orders, Mr. David Cartwright. British Telecom's Customer Switching Product Group Manager, emphasized British Teleco1n's determination to ensure continuity of deliveries of modern systems by giving its suppliers better advance notice of requirements. He stated that the order with Mitel demonstrated British Telecom's determination to widen its product portfolio in order to offer customers, as quickly as possible, a range of first-rate products in ready supply.



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