|Introduced in the 1970's, the PDX was an SPC switch with a capacity for 800
Reputed to be the first Digital switch in the UK. A joint venture between Rolm (USA)
SYSTEM GENERAL DESCRIPTION
1.1 GENERAL INFORMATION
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
The information contained in this document is applicable to Plessey series 800 Private
Digital Exchange (PDX 800) systems.
1.2 PHYSICAL DESCRIPTION
1.2.1 General Information
The Plessey PDX system consists of the following major components:
(1) Equipment Cabinet(s)
(2) Operators 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 Operators
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
1.2.3 Equipment Cabinets
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
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 Operators Console
Plessey PDX may use one, or more, Operators 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
1.2.7 Service Equipment
A service teleprinter is used for instructing the systems 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 FUNCTIONAL DESCRIPTION
1.3.1 The Plessey PDX is a stored-programme, computer-controlled telephone switching
1.3.2 The Equipment Cabinets house the network switching circuits and their power
supplies. All connections to station equipment, trunks, and Operators Consoles are
made via the distribution frames. The service equipment and optional Call
Information Logging (CIL) recording device are connected to the Equipment Cabinets,
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
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
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 Operators 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 partys Coder and the called partys
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 partys 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 partys 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 Operators Console
The Operators 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
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
PABXs. Add on interface arrangements are available to suit particular tie-line
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