Introduced in the 1970's, the PDX was an SPC switch with a capacity for 800 extensions.  

Reputed to be the first Digital switch in the UK.  A joint venture between Rolm (USA) and Plessey.



1.1.1 Contents

The System General Description is supplied as an introduction to the Plessey PDX. The document contains general information, brief physical and functional descriptions, physical and electrical characteristics, a summary of system features, and a description of optional equipment.

1.1.2 Scope
The information contained in this document is applicable to Plessey series 800 Private Digital Exchange (PDX 800) systems.


1.2.1 General Information

The Plessey PDX system consists of the following major components:
(1) Equipment Cabinet(s)
(2) Operator’s Console(s)

1.2.2 The Equipment Cabinets contain the computer common control, power supplies, and various electronic modules that comprise the network switching centre. The Operator’s Console is used to control distribution of incoming calls and to provide such features as intercept, recall, paging, and call assistance. Extension-line telephones are connected to the Equipment Cabinets via plant wiring, distribution frames, and interconnecting cables. Trunks are terminated at the distribution frame and inter-connected to the Equipment Cabinets.

1.2.3 Equipment Cabinets
The Plessey PDX may contain one, two, or three Equipment Cabinets depending on the number of extension and exchange lines serviced. Each cabinet is enclosed at the front and rear by key-locked doors which are easily removed for maintenance. Blowers, located at the bottom of the cabinet, provide cooling air for electronic assemblies. Power supplies, a Power Distribution Panel, and the Automatic Program Loader are also located near the bottom of the cabinet.

1.2.4 Each Equipment Cabinet is divided into two bays. Each bay contains a three shelf card cage into which various printed circuit assemblies (cards) are installed. The shelves and the card slots are numbered for rapid location of a particular card. Shelf and slot numbering is explained in System Configuration Information. Cards may be rapidly removed and replaced for maintenance or system expansion purposes.

1.2.5 The cards installed in the card cage slots are divided into two functional categories, (a) groups of cards and (b) discrete (single-function) cards. The card groups are dedicated to a particular function; such as computer memory and processing, extension-line interface, and trunk interface. The interface groups have a special printed circuit assembly with plugs for interconnecting the Equipment Cabinets and Main Distribution Frame. Discrete cards are used for such functions as registers, senders, and service equipment interfaces.

1.2.6 Operator’s Console
The Plessey PDX may use one, or more, Operator’s Consoles. The console is a desk-top, compact unit. The console basically consists of three assemblies and a handset. The top cover assembly is removable to gain access to the keyboard and displays. The frame assembly contains four printed circuit assemblies mounted in .a frame. The base assembly contains the speech circuit for the console. The modular construction facilitates rapid maintenance of the console, and allows indicating lamps and fuses to be replaced without major disassembly.

1.2.7 Service Equipment
A service teleprinter is used for instructing the system’s computer when moves and changes are performed. The service teleprinter is connected to the Equipment Cabinet when local operation is selected. Alternatively, the service teleprinter may be located remotely, and instructions transmitted via a telephone line. The service teleprinter is also used to list the self-test error table, system traffic table, perform specific tests, and establish a system password (security key). Self-test is part of the system programme which periodically tests critical system functions and records any errors in a table for reference by maintenance personnel.

1.2.8 Spare printed circuit assembly kits are provided for use in Plessey PDX maintenance. The spares kits also include power supplies and other critical assemblies. A special cable is provided with the Plessey PDX which allows most cards to be removed and exchanged while the system is powered.


1.3.1 The Plessey PDX is a stored-programme, computer-controlled telephone switching system. 

1.3.2 The Equipment Cabinets house the network switching circuits and their power supplies. All connections to station equipment, trunks, and Operator’s Consoles are made via the distribution frames. The service equipment and optional ‘Call Information Logging’ (CIL) recording device are connected to the Equipment Cabinets, as required.

1.3.3 Power Distribution
The Equipment Cabinets contain the power supplies which provide operating voltages for the various electronic assemblies used for network switching and control.

Speech battery voltage and ringing are also supplied. A Power Distribution Panel is used for termination of all power supply input and output voltages. The panel contains fuses for power supply circuits, and indicators which allow the circuit status to be visually monitored.

1.3.4 Computer Common Control Group
The Computer Common Control group consists of a Central Processing Unit (CPU), memory and memory control logic, and a real time clock. The system operating programme is loaded into memory, and controls the network switching operations. The CPU is used as follows: to monitor extension line status (on-hook/off-hook), control tone generation (dial tone, ringing tone, etc), establish connections between parties, assign registers and senders to parties, and perform other system control functions. The manner in which these functions are executed is controlled by the system operating programme, stored in memory.

1.3.5 The Real Time Clock card is used to inform the CPU of elapsed time. This information is required so that the CPU can activate the howler tone, recall parties to the operator, and perform other system timing operations. The CPU supplies data to the TDM Network Control group, and receives data from the Serial Device Interface card.

1.3.6 TDM Network Control Group
The TOM Network Control group serves as the interface between the Computer Common Control group and the switching cards. This group consists of two Control cards and individual shelf Expander cards. The Plessey POX is a TOM (Time Division Multiplexed) system. The switch cards are sampled and controlled based on an assigned time-slot or time-window principle.

1.3.7 As shown in Figure 1 .4, a voice channel signal (channel 2) is sampled at T1. The voice sample is digitised and output to the digital bus. At time T2, the channel 2 voice sample digital word is selected and converted back to an analogue (speech) sample. Sample signals at T1 and T2 are spaced apart to allow for transmit and receive delays. A second channel is sampled at T3, output to the digital bus, and converted back to an analogue sample at time T4. Notice that sample signals T3 and T4 are also spaced apart (same intervals as T1 and T2).

1.3.8 This example illustrates the operation of the TOM Network control group. Commands are loaded into the Control cards from the CPU. The ‘transmit’ commands (sample analogue signal, digitise, and output to the digital bus) and the ‘receive’ commands (select digital word and convert to analogue signal) are sent to various switch cards at regular intervals by the shelf Expanders. Commands are spaced properly to allow correct transmit-to-receive delay intervals and assigned to unique time slots, so that more than one voice channel connection may be made, using a common digital bus to transmit and receive sampled speech signals.

1.3.9 Interface Groups
There are three types of Interface groups used in the Plessey POX. Two groups are used for extension lines: standard and long line and the third for exchange and tie lines (trunks). The Operator’s Console is controlled by means of a trunk group. Figure 1.5 illustrates a typical connection between an Extension-Line group and a Trunk group.

1.3.10 All connections between Interface groups are made via a digital bus. Analogue signals (2-wire) from the single-line telephone are separated into 4-wire transmit and receive signals. The transmitted signal is applied to a Coder which provides a digital representation of the sampled analogue signal. The analogue signal is sampled and digitised at 12 kHz. The TDM Network Control group supplies the commands to activate the extension-line group for both the transmit and receive paths.

1.3.11 After an offhook condition is detected, the Coder is activated so that digital words are output to the bus at a 12 kHz rate. At the trunk group, another set of commands are used to seize the trunk and activate a Decoder. The complete transmit path is: from the handset, through the hybrid and Coder to the digital bus, from the digital bus through the Decoder, through the hybrid to the trunk. The TOM Network control group supplies commands which activate the calling party’s Coder and the called party’s Decoder. A similar set of commands is used for the receive path, so that a two-way connection is established.

1 .3.12 Each interface group will service 16 channels and contains a Coder and Decoder. The 12 kHz sampling frequency allows the full voice frequency bandwidth to be preserved. The information transferred between groups on the digital bus is Pulse Code Modulated (PCM) as a 12-bit word (one sample).

1.3.13 Register and Senders
The Plessey PDX can be configured to service either, or both, pushbutton (MF) or rotary (dial pulse) extensions and trunks. In any configuration, a Plessey POX contains a Tone Generator and a MF Register. The Tone Generator is used to supply dial tone, busy tones, ringing tone etc. The MF Register is used to store pushbutton tones, and then forward them to the CPU. Both the Tone Generator and MF Register are controlled by the TOM Network Control group. The Tone Generator synthesizes a digital representation of a single or dual-frequency tone which is output to the digital bus. The MF Register accepts digitised dual-frequency tones from a push button telephone via the digital bus, and converts the digital word to analogue signals (tone pair). If the tone pair passes certain tests, the equivalent decimal value of the tone pair is sent to the CPU.

1.3.14 When rotary equipment is installed, or when rotary trunks are accessed, one or more Rotary Registers and Rotary Senders may be installed. The Rotary Sender converts a digital value into dial pulses. The Rotary Register converts dial pulses into a digital value, representing a dialled digit.

1.3.15 Conference Bridge Group
The Conference Bridge cards allow up to eight, 6-party conferences. The Conference Bridge contains a totalizer which sums the samples from all conferencing parties and then outputs the stored sum, minus a particular party’s sample. The result of this operation is that each party in the conference hears the sum of all parties, less its own contribution.

1.3.16 The TDM Network Control group issues control commands to the Conference Bridge cards. During each sampling interval, the bridge loads a sample from the digital bus for each party in the conference. It then outputs the sum, minus the party’s own sample, to each party in the conference. The entire operation occurs during each sample interval: both the totalising of samples and the output to parties in the conference.

1.3.17 PDX Bypass-Group
The optional PDX Bypass group allows direct-trunk access in the event of power failure or Computer Common Control group malfunction. When a failure is detected, the Bypass cards connect extension -line telephones directly to an exchange line. The extensions may be dedicated bypass instruments, not used during normal operation, or they may be standard extensions, used during normal operation.

1.3.18 I/O Interface Group
Two serial device interfaces are provided for the service teleprinter and for an optional CIL listing device. The service teleprinter channel is accommodated on the Loader Interface card, which also serves as an interface between the Automatic Program Loader and the Common Control group. The CIL teleprinter channel is accommodated on the Alarm/CIL Interface card, which also provides alarm outputs. If a magnetic tape drive is used for CIL output, a Magnetic Tape Interface card is available.

1.3.19 Operator’s Console
The Operator’s Console is connected to the Equipment Cabinets by means of two conductor pairs. One pair is used for voice frequency signals; the second pair is used for data communication. Data is sent between the console and the cabinets in serial form, as opposed to the parallel word format used on the digital bus. This scheme allows control and status data to be transmitted over a single conductor pair.

1.3.20 The console displays: the status of the Plessey POX system, extension status, source, class of service, extension number, and time of day. When a console key is pressed, the switch code is sent to the CPU for appropriate action.

Extension features are available as a function of the extension Class of Service (COS). Features are assigned to a particular COS, which is then assigned to a particular extension. This assignment is accomplished at the time that the system software is generated.


Items described in the following paragraphs may be supplied with the basic Plessey PDX system.

CIL Recording Device
The Basic Plessey POX contains the software and interface hardware to support the call information logger (CIL). Either a listing device (printer) or magnetic tape transport can be used with the system.

1.7.2 Pox Bypass Group
The POX Bypass group consists of two 4-Channel PDX Bypass cards and the PDX Bypass Motherboard. This group is used for direct trunk connection.

1.7.3 Universal Tie Line Group
The Universal Tie Line group is provided for inter-connection of the Plessey PDX and other PABX’s. Add on interface arrangements are available to suit particular tie-line requirements.

1.7.4 This group uses two of the 4 Channel Universal Tie Line Interface cards to service eight two-way tie lines.

1.7.5 Redundancy Option
Critical Electronics back-up is provided by an option which may include an extra MF Register, Tone Generator, Rotary Register, Rotary Sender, and Conference Bridge group. A redundant Common Control Electronics option is also available for back-up of the Computer Common Control and TDM Network Control groups. When this option is supplied, the Critical Electronics option should also be specified.

1.7.6 MF-to-Rotary Conversion Option
When conversion from standard pushbutton (MF) telephone signals to rotary (dial pulse) signals is required, specifying this option ensures that the appropriate number of Rotary Sender cards are provided.

1.7.7 Advanced Features
Advanced features which use the # key on pushbutton telephones are optional. Advanced features include Abbreviated Dialling, Camp-On, Follow-me, Privacy, Do not Disturb, etc.

1.7.8 System Forwarding
The alternate answer feature which allows call forwarding on busy and/or no answer, is a system opt ion.

1.7.9 Route Optimisation
Route optimisation is a system option enabling the automatic selection of the least costly method of placing an external call.

1.7.10 Automatic Reload
The system software file is automatically reloaded from a backup unit in the event of extended loss of mains power causing depletion of the exchange battery, or after shutdown of the exchange.

Taken from the Plessey PDX system description paper 1976


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Last revised: November 03, 2021