PXML No. BABT/PXML/23 PBX.0016 (TSX50) & BABT/PXML/23 PBX.0004 (SX50)
APPROVAL No. NS/1412/23/F/600046 (SX50)

Click here to see information on BT's withdrawal of Subscribers Private Metering (SPM)

First produced by Mitel in 1985 this switch was originally called the SX500, with BT naming it the Dekara.  In 1986 the system was renamed as the Mitel SX50 and the BT TSX50.

The SX50 is a micro processor controlled switching system that uses a digital switching matrix for the connection of calls. TSX 50 was released in 1987 with Generic 3543.02 (UK1) software.  The introduction of Generic 3543.03 (UK2) software in May 1989 allows the use of a Visually Handicapped Operators Console (VHOC) attachment and, with a new power supply PSU2A increased system capacity from 80 to 160 ports.

System capacity will depend on the type of power supply unit (PSU 1A or PSU 2A) - see table below.  Note that the quoted capacities cannot all be achieved at the same time, due to a limitation of 10 interface card slots per system.

Generic 3546.01 (UK3), released in June 1990, introduced Automatic Route Selection (ARS), Remote Access (RMATS), and other facility enhancements detailed in the system documentation.  Increased capacity can only be offered when using Generic 3543.03 or 3546.01 software with the PSU 2A.

In 1994 DASS2 connectivity was announced and this used the Interconnect 200 box in conjunction with the SX50 DNC card (which supported DPNSS).   It is debatable whether BT sold any.

The system could be fitted with Superset 4, Superset 400 type telephones or one switchboard.

                     (Generic 3543.02/3546.01)     (Generic 3543.03/3564.01)

   EXTENSIONS              80 (5 CARDS)               160 (10 CARDS)

   EXCHANGE LINES          24 (3 CARDS)                32 (4 CARDS)

   PRIVATE CCTS                                    12 (6 MOTHER CARDS) or
   SSDC5                ANY MIX UP TO 8             24 (6 MOTHER CARDS) or
   AC15A                 (4 MOTHER CARDS)          + MAX OF 6,MOTHER CARDS

   TX14 EXTENSIONS        32 (4 CARDS)                32 (4 CARDS)

In exceptional circumstances, where six module mothercards are fully equipped with AC15A and/or DC5A modules and a remote maintenance module is also required, a seventh module mothercard can be provided. However, it must be equipped with the remote maintenance module only.
Standby Power may be provided using BT Power System 250G, which is an Uninterruptable Power Supply.

Made by Mitel Telecom Limited
Tel 01291 430000

For TSX 50 approved as described in BABT/PXML/23 PBX.0016.
For SX 50 approved as described in BABT/PXML/23 PBX.0004.

BT systems were later shipped from Mitel with the Generic 3546.01 (UK3) software module as standard.

The Lithium battery in the Generic Module must be replaced every ten years. The battery can be replaced WITHOUT powering down the system.   Click here for battery type.
Note: if the Lithium battery is replaced, when there is no power, then loss of customer data will occur.

Remote access facilities for maintenance will be provided as a standard feature on all Generic 3546.01 (UK3) systems.

The System warranty is 15 months from the manufacture date. Repaired cards carry outstanding system warranty or 3 months whichever is the greater. New cards carry 15 months warranty from date of manufacture. The TSX 50 will carry a date code identification (DD/MM/YY) in addition to a bar code, thus enabling the user to ascertain warranty status without contacting Mitel repair centre. The date code is applied to chassis consoles and the solder side of PCB's.

Power Unit 1A can only be used on systems with a maximum of 80 port capacity.
Power Unit 2A may be used on all systems to a maximum of 160 port capacity.

To maintain VHOC sites and 160 port capacity systems software level G3543.03 is required. As this software level is backward compatible with all previous TSX50 systems.

Taken from BT information.

General Information on the SX50 - UK3 variant (1989)



The SX50 is an Integrated Communications System using an innovative, compact modular design which allows easy system expansion, fault finding and card replacement. SX50 is a digital electronic system, using stored program control and advanced digital architecture.

SX50 has a very wide range of features and can accommodate (in the UK) up to 160 extensions, 32 Exchange Lines, 24 Private Circuits, and up to 32 SUPERSET 4 telephones, depending on system configuration. SX50 can be adapted by data changes to match a wide range of public exchanges. Although cost-effective as a basic telephony switch, the SX50 has been designed as an integrated voice and data switch. This means that it can be adapted to handle future communications applications including voice, data, text and image.

SX50 meets the requirements of BSI (BS6450) for Private Branch Exchanges. SX50 is approved by the British Approvals Board for Telecommunications (BABT), Approval Number NS/1412/23/F/600046.

SX50 provides the wide range of features normally provided with MITEL switching systems, as well as:-

  • Digital operation

  • Low power consumption

  • Small size

  • Small number of types of assembly, for simplified spares holding

  • Simultaneous use of MF4, dial pulse and SUPERSET telephones

  • Selection of fixed or flexible extension numbering scheme

  • Selection from a wide range of applications packages

  • Operation with or without SX50 Console

  • Operation with SUPERSET 4 Console

  • Visually Handicapped Operator's Interface Unit

  • Capability of Private Circuit interconnection with other PABXs

  • Call Information Logging

  • Emergency switching in the event of a power supply failure

Main Components
The main components of an SX50 system are as follows:-

  • A wall mounted SX50 equipment cabinet containing Printed Circuit Cards.

  • The operator's console.

  • The Test Jack Frame.

  • Cables connecting the power supply unit to the wall socket, between the equipment cabinet and the console, and connecting the Extension and Trunk Cards to the Test Jack Frame.

The Backplate for the SX50 equipment cabinet is fixed to a wall. The Card frame is attached to the Backplate. The Motherboard is inserted, followed by the power supply unit which is clipped to the side of the frame. Finally the Generic Module is clipped to the right hand side of the frame and connected to the Motherboard.  The printed circuit cards can now be  inserted. After this the cables are installed from the equipment cabinet to the mains socket and to the Exchange Lines and extensions, via the Test Jack Frame.

The cable to the console can be connected through the Test Jack Frame or directly to the Control Card.

SX50 can be used with loop disconnect dial, MF4 keypad and self contained (Decadic) keypad telephones in any combination. In addition, MITEL SUPERSET 4 electronic telephones which provide many additional facilities, can be used. Telephones to be connected directly to Exchange Lines in the event of a power failure MUST be compatible with the local Exchange. If Earth Calling Exchange Lines are used, an Earth Recall button must be provided for these telephones.

Customer Data Entry
After installation, the installation engineer powers up the system and carries out customer data entry from the console. This data defines the various features required by the customer and matches the system to the local Exchange Lines. The configuration of an SX50 system can be altered by redefining some of the data. The customer is allowed to change some of the data if required. A security code prevents unauthorised access to the system.

Traffic Considerations
The digital switching matrix in the SX50 is totally non-blocking for the maximum configuration of 32 Exchange Lines or 160 extensions.

Every five years the small lithium battery in the Generic Module must be replaced. Apart from that, no routine maintenance is required or recommended.  Click here for battery type.

Diagnostics and Fault Correction
The SX50 uses comprehensive automatic diagnostic procedures which run continuously and indicate, on the console, any faults that may be detected. In general, if a non urgent fault is detected, the system isolates and busies out the faulty circuit and continues operation though possibly with reduced performance. Detailed diagnosis of faults can be carried out at the console. Site work is limited to changing specific plug-in modules or boards and the modular design makes this quick and simple. Background diagnostics can also be run on demand.

Emergency Switching
If SX50 suffers a power failure, the system automatically goes over to Emergency Switching Mode, in which up to 10 Exchange Lines can be mechanically switched to selected extensions. These extensions are then powered from the local exchange, enabling some external communication from the system. When power is restored, normal operation resumes. Customer data is held during power failure by the Lithium battery in the Generic Module.

A printer or recorder can be used:-

  • for Call Information Logging,

  • for logging the use of Wake-Up Operator Controlled calls in a hotel,

  • for printing out the contents of the extension meters,

  • by the maintenance staff for Customer Data Dump and Load and logging Alarms.

Environmental Characteristics
Briefly, SX50 requires an office environment which remains within the limits:

  • Temperature 0 degrees C to 40 degrees C

  • Relative humidity 10% to 90% non condensing

SX50 is resistant to electro-magnetic interference expected in ordinary conditions and does not emit electro-magnetic radiation likely to affect most users. There is no fan and acoustic noise is well below accepted limits.

The equipment cabinet consists of three main components, a Backplate, a Card frame and a Cover. The SX50 Cabinet must be wall mounted.

An assembled, fully equipped equipment cabinet, weighs approximately 21kg.

Generic Module
The Generic (or Memory) Module is fastened to the right hand side plate of the Card frame. It is an enclosed circuit board containing the system memory and a Lithium battery which protects the memory from loss of data for up to 10,000 hours if the mains power supply fails.

Power Supply Units
Two Power Supply Units are available, the 140W unit and the 225W MK2 unit. The 225W unit provides extra capability by allowing a larger system configuration to be used. The Power Supply Unit is clipped to the left side of the Card frame and can be readily removed.

The 140W Power Supply Unit is used with configurations up to 80 extensions or 24 Exchange Lines. If more than 80 extensions or 24 Exchange Lines are to be connected to the system, the 225W Power Supply Unit MUST be installed.

The 225W MK2 Power Supply Unit requires 1.3 amps at 240 volts and provides a continuous output of 225 Watts. The 140W Power Supply Unit requires approximately 1 amp at 240 volts and provides a continuous output of 140 Watts. Both Power Supply Units are protected by fuses.

Control Card
The Control Card MK3 is situated at the back of the Card frame, parallel to the wall, and plugs directly into the Power Supply Unit. The Control Card carries:-

  • Connectors for up to ten Peripheral Cards.

  • The Central Processing Unit.

  • Digital cross point switches.

  • Call processing circuits.

  • Timing and synchronisation circuits.

  • MF4 circuits.

  • Music on Hold/Paging circuits.

  • Music on Hold/Paging Terminal Strip.

  • Console jack.

  • RS-232 port.

  • seven segment LED display.

  • Four Status switches.

  • One Reset switch.

  • Night Bell Terminal Strip.

  • Voltage Test Points.

The Peripheral Cards all plug into the Control Card, perpendicular to the wall, so that their front edges are exposed. It is possible to withdraw the Peripheral Cards sufficiently to allow the Control Card to be removed and replaced, without having to remove the front connectors on the Peripheral Cards.

Music on Hold/Pager and MF4 circuits. These audio and control circuits for Music on Hold and for the paging system are now carried on the Control Card MK3. Additional circuits allow SX50 to support MF4 signalling over Exchange Lines.

Peripheral Cards
Five types of Peripheral Card (Extension, SUPERSET, Trunk, Digital Network Card and Universal Card) are used in SX50. These are available in the following versions:-

  • Universal Card

  • 16 circuit ONS (Extension) Card

  • 8 circuit ONS (Extension) Card

  • 8 circuit COV (SUPERSET) Card

  • 8 circuit Trunk Card with 50 Hz Meter Pulse Detectors

  • 4 circuit Trunk Card with 50 Hz Meter Pulse Detectors

  • Digital Network Card

The Universal Card is designed for a variety of peripheral functions and is configured by attaching removable modules. Up to four single-sized modules or two double-sized modules can be plugged in to each Universal Card. The modules use two connectors per single module or three connectors per double module. A group of four red LEDs on the front of the Universal Card indicate the status of the modules and a single green LED indicates the status of the card. There is a connector at the front for the cable to the Test Jack Frame. Three types of plug-in module are available for different configurations of the Universal Card.

These are:-

  • SSAC15A Module. This is a single sized module. Each module provides the circuits for. one SSAC1S Private Circuit

  • SSDC5 Module. This is a double sized module carrying the circuits for one SSDC5 E&M Private Circuit.

  • RMATS Module. This single sized module allows data entry and fault location to be carried out from a remote Maintenance Centre.

The ONS (Extension) Card. Two versions of this card provide the circuits required for either eight or sixteen dial pulse or MF4 extensions. There is a connector at the front for the cable to the Test Jack Frame and there are also red LEDs to show the status of the extensions and one green LED to indicate the status of the card.

The COV (Control Over Voice) (SUPERSET) Card. This card can accommodate eight SUPERSET 4 telephones. Each COV Card has eight red LEDs to display the status of the SUPERSETs and one green LED to indicate the status of the card. There is a connector at the front for the cable to the Test Jack Frame.

Trunk Card. Each Trunk Card provides the circuits required for four or eight Exchange Lines. A connector is provided at the front for the cable to the Test Jack Frame and there is a row of red LEDs to give an indication of the status of the Exchange Lines together with one green LED which indicates the status of the card.

The Operator's Console is a moulded plastic unit, weighing 1.8 kg. A connecting cable at the back of the Console can be run to the TJF or directly to the Control Card. On the left side of the Console is a double socket for plugging in the operator's handset or headset On the right side there is a standard 12 key dial pad with the numerals 0 to 9 and * and #. On the right of the dial pad there are an additional four keys used for adjusting the display contrast and the volume of the bell. On the front panel of the console there is a Liquid Crystal Display (LCD) two characters high and forty characters wide which is used to display information to the operator or maintenance engineer. Directly below the LCD display is a row of five marked 'Display Assigned' keys. A further three rows each of seven keys with their functions marked above them are also fitted on the console.

Operation of the console is fully described in the Console Operating Instructions.

The functions of the keys are briefly described here.

Display Assigned Keys
The upper five keys are Display Assigned Keys. They have no permanent markings but always assume the functions shown in the display above them. These displays change according to the mode of operation at the time. When a key does not have a display, pressing it does nothing. During data entry, the functions are chosen from:


and during normal call handling, the five keys become:

GRP 1, GRP 2, GRP 3, DIAL 0 (or PRI 0), and RECALL

These represent the six categories of call that the operator can select for attention.

Function Keys
There are 21 function keys. Fourteen of these keys can have their assignments changed by data entry. There are 15 possible key assignments. These keys are used during call handling and are fully described, together with their operation in the Console Guide. The default functions of these keys are listed below for reference:

An ACCOUNT key for use with Account Codes can also be assigned by data entry.

Superset 6 with DSS attachment


The SUPERSET 4 is a microprocessor controlled electronic telephone. It has a row of multi-function keys, linked to a visual display, for PABX feature access which eliminates the need to remember access codes. It also has 15 line selection keys and a line status display which gives the SUPERSET 4 the facilities of a key and lamp system. Any unused line selection keys can be used as Abbreviated Dialling keys.

In addition, the SUPERSET 4 offers full Loudspeaking Telephone working, ringer pitch and volume controls, an On-hook Dialling facility and a dedicated Hold key.

Up to 32 SUPERSET 4 electronic telephones can be installed on an SX50 system.

One SUPERSET 4 can be used as a SUPERSET Console if required. 

SX50 can accommodate up to 10 Peripheral Cards. These cards can work in almost any combination, so that the required customer configuration can be achieved. There are several circuit card limits which MUST NOT be exceeded. Depending on the Power Supply Unit fitted, these maxima are:

  • With the 140W Power Supply Unit, 24 Exchange Lines, 8 Private Circuits, 80 extensions, (or 32 SUPERSET 4s and 16 extensions).

  • With the 225W Power Supply Unit, 32 Exchange Lines, 24 Private Circuits, 160 extensions, 32 SUPERSETS. The installer can configure the SX50 in any way he chooses but MUST NOT exceed the circuit card limits. The choice of circuit cards MUST be made carefully, so that there are no more than 10 cards and that the limits are not exceeded.

Circuit Cards
Up to 10 Peripheral Cards can be installed, but the following numbers of individual circuit cards must not be exceeded when using the 140W Power Supply Unit, NOT MORE THAN:-

  • five ONS (Extension) Cards,

  • four COV (SUPERSET) Cards,

  • three (8  circuit) Trunk Cards,

  • six (4circuit) Trunk Cards,

  • four Universal Cards,

  • one Digital Network Card, (NOTE: This card uses two card slots)

  • eight SSDC5 and SSAC1SA modules,

  • one Music on Hold/Pager/MF4 module.

  • one RMATS module can be installed.

With the 225W Power Supply Unit installed, the limits are:-

  • ten ONS (Extension) Cards,

  • four COV (SUPERSET) Cards,

  • four (8-circuit) Trunk Cards,

  • eight (4-circuit) Trunk Cards,

  • six Universal Cards,

  • one Digital Network Card, (NOTE: This card uses two card slots)

  • twelve SSDC5 modules,

  • twenty-four SSAC15A modules,

  • one RMATS module can be installed.

These rules allow many possible configurations.

Trunk Cards The SX50 has a maximum capacity of 24 Exchange Lines, (140W Power Supply Unit) or 32 Exchange Lines with the 225W Unit. These lines can be accommodated by either 8 circuit Trunk Cards, or combinations of 4 and 8 circuit Trunk Cards so that the limits are not exceeded.

Extension Cards
The SX50 has a maximum capacity of 80 extensions (140W Power Supply Unit) or 160 extensions with the 225W Unit. These extensions can be accommodated by either 16 extension ONS Cards or combinations of 8 and 16 extension ONS Cards so that the limits are not exceeded.

SX50 has a maximum capacity of 32 SUPERSET 4 telephones using either Power Supply Unit. These SUPERSETs can be accommodated by up to 4 x 8 circuit COV Cards.

Note: Each SUPERSET is equivalent to TWO normal extensions in capacity calculations.

Universal Cards
Universal cards provide connections for various modules. Up to four modules can be connected to each Universal Card. SX-50 has a maximum capacity of 8 Private Circuits (140W Power Supply Unit) or 24 Private Circuits using the 225W unit These can be any combinations of SSDC5 circuits (which use a double module) or SSAC15A circuits (which use a single module), as long as the totals are not exceeded.

In addition a combined Music on Hold/Pager/MF4 module can be installed, using one module position. (Note that these circuits are now available on the MK3 Control Card).

The Universal Card can also carry a single RMATS module which uses one module position. Refer to Volume Two, Applications Manual.

Digital Network Card
A Digital Network Card using two card slots, can be installed to provide 30 channels of voice and data between two or more SX50 with Digital Network Cards installed. Refer to Volume Two, Applications Manual for further details of the Digital Network Card.

System Expansion
Because SX50 is a modular system with plug-in printed circuit cards, it can be expanded by installing additional plug-in cards in the equipment cabinet. The smallest SX50 installation would have 4 Exchange Lines and 8 extensions. This could be increased by installing extra circuit cards to achieve the desired configuration.

Apart from the additional cards or modules, expansion requires additional customer data entry and cabling. If more than 80 extensions or 24 Exchange Lines are required, the 225W Power Supply Unit MUST be installed.

An Interface Unit, designed for use by visually-handicapped Operators is available with the SX50.

Visually-handicapped Operators Console


SX50 System Features
160 Port Working
SX50 can support full 160 port working, with up to a maximum of 32 Exchange Lines or 160 extensions.

The 225W Power Supply Unit MUST be installed on systems with more than 80 extensions.

Abbreviated Dialling
This feature allows SX50 to store a maximum of 90 or 900 telephone numbers. If the option to store 90 numbers is selected, then each number can contain up to 26 digits. If the option to store 900 numbers is selected, then it is not possible for all the numbers to contain 26 digits. Numbers can still contain up to 26 digits, but a system digit limit will be reached. (For example, it is normally possible to store more than 800 numbers containing 10-digits each). The operator and permitted extension users can dial these numbers by dialling a three or four digit access code.

Click here for Maintenance information


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Last revised: February 21, 2023