Supplied by Pye/TMC the EBX8000 was a SPC reed switched telephone system with an ultimate capacity of 8000 extensions.

Introduced in the late 1970's.

EBX8000 Stored-Program Control PABX
by V. A. E. FOUNTAIN, M.1.T.E., M.B.C.S.t
An Extract from the P.O.E.E.J Vol 74, page 134 (July 1981)

Since the early 1970s the British Post Office has permitted the private supply and installation of approved large (over 100 lines) private automatic branch exchanges (PABX's). A considerable number of PABX's are now approved and include an increasing proportion of stored-program-controlled electronic systems. One of the largest of these is the EBX8000, catering for up to 8000 extensions and about, 2000 exchange or tie lines.

The EBX8000 is an electronic space-division private automatic branch exchange (PABX) using a reed-relay switching matrix under stored-program control (SPC), and provides the sophisticated facilities that are available to users of modern PABX's. The EBX8000 was evaluated by the British Post Office (BPO) during 1980 and given type-approval; it was one of a number of SPC PABX's, from different manufacturers, that were offered to the BPO between 1970 and 1980. Some 30 EBX8000 installations are now in service.

The EBX8000 caters for up to 8000 extensions with up to 2000 exchange or tie-line connexions. Large-scale integration (LSI) allows a degree of distributed processing of operations such as timing and scanning of peripheral circuits; for example, exchange lines and tie lines. The characteristic features of SPC exchanges are incorporated in this design; that is, ease of maintenance and introduction of facilities, such as call forwarding, automatic ring back, etc. The system can be expanded in steps of 128 extensions and 16 exchange or tie lines. The exchange is enclosed in a number of cabinets which can vary from a minimum of 5 to a maximum of about 60.

The particular features of this exchange are:-

  1. its large capacity,
  2. reed switching enabling high-reliability2, high-quality transmission and low power consumption to be achieved,
  3. extension telephones that can be MF4 signalling keyphones or 10 pulses/s telephones, no modifications are required when either is fitted,
  4. low power consumption, so that natural air convection is sufficient for cooling purposes, and no special environment is required, and
  5. quiet operation and powered by BPO standard 50V float-charged batteries, low voltages (for example, 5V and 12V) being derived from duplicated high-reliability quiet DC-to-DC converters.

With a view to economy throughout the size range, the basic design requirements of the EBX8000 system were that:-

(a) the switching central control would be problem oriented, that is, purpose built, and
(b) the expansion of the exchange would be in small increments.

The processing load of the central computer control is reduced by distributed processing, decentralizing operations such as timing, scanning and taking simple decisions. This decentralization is realized by the application of LSI techniques.
Gradual expansion is possible as fundamental units, which can be added according to traffic demands, have been developed. For the peripheral equipment, expansion is further facilitated by the construction system which uses detachable shelves and pluggable cabling.
Since the exchange is principally operating from a common control, the central control unit (CCU) is duplicated to guard against interruptions. This duplication extends to the subsystems and the bus lines to the peripheral equipment in such a way as to minimize the chances of an overall breakdown.

The trunking diagram shows an outline of the main system elements, which are discussed below. The elements shown within the broken lines are assembled into single or twin shelf units which are described later in the Physical Design section of this article.

Each extension has an associated line circuit (LCT); 64 LCTs form a group. Each group is continually scanned by a line-state scanner (LSS) which operates in conjunction with the line state memory (LSM). The complete scanning cycle takes 160 ms over a maximum of 128 LSSs. In this way, a change of state of any LCT is detected and the information forwarded via a bus to the CCU.

The data processing unit (DPU), which controls all parts of the system, processes the line-circuit status information and generates a marker command. The marker (MKR) translates this command in conjunction with the access matrices into switching signals that cause the relevant reed-relay crosspoints in the switching network to be energized and thus establish a path.

Each extension is connected to the line link network which provides concentration to a first mixing network (IMN), totalling 4 stages of switching. In the larger exchanges a second mixing network (IIMN) is always supplied, to provide a total of 6 stages of switching.

The calling party is first connected to a digit receiver which forwards dial tone to signify that the number can be dialled. The dialled digits are forwarded to the CCU and analysed by the DPU which then initiates the main actions. The first action is to select a peripheral circuit (PCT) and the second is to set up an appropriate path through the switching network via a MKR as before.

Different types of calls are handled by different PCTs; connexions from an operator to an external line, or an extension, involve the use of an operator circuit, whereas external calls involve either an exchange-line circuit (ELC) or a tie-line circuit. An internal call requires a connecting circuit

Up to 16 PCTs are connected to one peripheral processor unit (PPU), which scans each PCT in a fixed sequence every 10ms. Information from each PPU is sequentially transferred via a bus system to the CCU under the guidance of a peripheral message multiplexer (PMM), which can handle up to a maximum of 32 PPUs. The exchange can be equipped with up to 4 PMMs.

There are 2 identical CCU systems which operate synchronously in a dual mode; that is, main and hot stand-by. Both systems are continually monitored by the alarm and disconnecting unit (ADU) which, under the direction of the DPU, is capable of transferring control from one CCU to the other. In the event of a fault, an alarm is raised, the fault-finding programs are executed by the DPU and the ADU disconnects the malfunctioning units.

The man machine unit (MMU) is used in conjunction with the MMU control panel and various input/output devices; for example, teletypewriters, etc.

The strength of SPC systems, as compared with other systems, lies in the centrally available data. Since all the essential information is available simultaneously, the programs can take complex decisions of widely differing natures. The advantage of semi-permanent data placed in the data store (DST) is that it can be changed by simple instructions. Some examples of data tables in the DST are:-

(a) switching network mapping,
(b) dialled-digit analysis for routeing and trunk barring,
(c) service classes,
(d) translation from directory number into line number,
(e) extensions for group hunting,
(f) fault reports, and
(g) metering data.

The EBX8000ís central control has been specifically designed for this application, rather than being an adaptation of a general purpose computer. Routine housekeeping tasks (for example, scanning peripherals) are carried out by an outer ring of distributed autonomous microprocessors; this ring obviates the need for a priority interrupt hierarchy and all communications with central control have the same priority level.

Communication with the special purpose DPU (the mini-computer part of the CCU) is at the level of elementary telephone operations. The number of different operations the machine can perform (the instruction package) must be such that elementary telephony operations are carried out in an efficient manner.

The most elementary operations the machine can perform are micro-instructions; that is, the smallest constituent program step, stored in read-only memory (ROM). With the micro-instructions, control is ultimately effected. A microinstruction may be read memory, set accumulator, set program counter etc. Several micro-instructions are put together so that they form an instruction; for example, load accumulator, store from accumulator. A number of instructions can be assembled to form a master instruction; that is, a basic telephony function, which is realized in dynamic random access memory (RAM). A number of master instructions are coupled together (concatenated) by link tables. A link table represents a telephony function and by concatenating a number of link tables a telephone facility is formed. A number of facilities are gathered into a package, and facilities package telephony can be achieved; that is, each customer can have a set of facilities specific to his requirements.

Thus the program is built up in several levels:-

Micro-Instructions - ROM
Instructions - RAM
Master Instructions - RAM
Link Table - RAM
Facilities Package - RAM

The micro-instructions, instructions and master instructions are located in a program store (PST) physically separate from the DST holding the link tables and facilities package allotting tables.

The PST contains 4 relatively independent program categories:-

(a) control program,
(b) call-processing program,
(c) man-machine-communications program, and
(d) system-assurance program.

The utilization of different component types within the EBX8000 system is shown below:-

  1. Reed relays used in the switching matrix, relay-sets and disconnexion of faulty system elements.
  2. LSI devices used in the data store, master instruction store, micro-program store, PPU and LSM.
  3. Medium-scale and small-scale integration devices used throughout the system in low-power transistor transistor logic devices.
  4. Transistor circuits used for loop detection in relay sets and LSSs, driving and guarding circuits in the MKR, drivers and receivers for the bus line system from the CCU to the LSSs and PPUs, and in the tone generators and receivers.

All integrated circuits and transistors are hermetically sealed.

All components are fitted on printed-wiring boards which slide into shelves. The card dimensions are
40 x 28 mm and can accommodate 64 reed crosspoints or 2 PCTs.

The circuit cards are assembled into shelves in such a way that a shelf, or a combination of 2 shelves, constitutes a functional unit with relatively few connexions to other equipment. The shelf wiring is wrapped; the shelf units are detachable, and their connexions to other units are exclusively of the plug-in type.

Several different types of shelf are used, and the equipments accommodated on these different shelf types are:-

(a) a PPU and 16 CCTs,
(b) a PPU and 16 ELCs,
(c) DC-to-DC converters,
(d) ringing and tone generators,
(e) IIMN and access relays, and
(f) IMN and LCTs.

A cabinet can house 6 single or 3 double shelves. The dimensions of a cabinet including cable runway are: height 2.2 m, width 0.9 m and depth 0.45 m. The cabinets, which are self-supporting, can be arranged in rows back-to-back against a wall, or in free standing single rows. The standard length of a row is 6 cabinets. The power distribution, however, is laid out for a maximum length of 12 cabinets. Any shorter length of row may be used at will.

The installation of the EBX8000 is a further step in the rapid evolution of private telephone exchanges, serving the ever expanding requirements of the business customer. Its electronic technology base allows an increasing level of sophisticated features to be provided economically.
SPC PABX's will, no doubt, progress to form the nucleus of the integrated office information system of the future.

The author wishes to thank his BPO colleagues and the Communications and Control Division of Philips Business Systems for their assistance in preparing this article.


MICHAELSON, J., and POULSON, S. Use of Facilities in an EBX8000 PABX. Philips Telecoms. Review, Vol. 37, No. 2, May 1979.

VAN DER SCHAFF, R. T. Experience with Stored Program Controlled PABX EBX8000. International Conference on Private Electronic Switching Systems, Apr. 1978.

VAN DER SCHAFF, R. T. EBX8000, a Stored Program Control PABX. Philips Telecoms. Review, Vol. 33, No. 3, Sept. 1975.

JUNG, M. M. Random Overflow of Calls between Sub-Groups in a Full-Availability Group of Lines. Philips Telecoms. Review, Vol. 37, No. 2, May 1979.

DE KONING, G. C. M. How the Software for Philips SPC Telephone Systems is Assembled. Philips Telecoms. Review, Vol. 35, No. 2, June 1977.


BACK Home page BT/GPO Telephones Search the Site Glossary of Telecom Terminology Quick Find All Telephone Systems

Last revised: December 18, 2010