Introduced late 1970's the SL1 was a SPC Digital switch with a capacity for more than 1000 extensions.

SL1 was one of the first digital SPC PABXs available in the UK.  It was originally developed in Canada by Northern Telecom and then manufactured and developed in the UK, under licence, by GEC.

The first SL1 systems were installed in the UK during the late ‘70s, and there are still a number in use now.

Whilst there are some early development models, there were three main variants of SL1.  The SL1 LE, which could support around 800 extensions; the VLE that could support around 2,000 extensions and the XL, which would support about 4,000 extensions dependent on trunking and usage.

SL1 was, in many ways, before its time.

  • It introduced SPC and TDM techniques into the PABX world.
  • It offered a wide range of time and cost saving features and facilities, which had never been available with analogue systems.
  • The SL1 was easier and cheaper to install and maintain then the earlier analogue systems.
  • It used internal 2.048M buses – a feature that would later prove useful with the developments of private and public ISDN networks.
  • The ‘footprint’ and power consumption of the SL1 proved to be much smaller than the earlier analogue systems.
  • As well as MF and dial-pulse telephones, the SL1 supported Featurephones for applications such as Manager/Secretary working and key-and-lamp emulation.
  • The SL1 used an ‘any-card-in-any-slot’ principle so that the system could be equipped to suit almost any application.

Later developments on SL1 introduced Automatic Call Distribution (ACD), giving integral Call Centres; Remote Peripheral Equipment (RPE) – allowing remote sites to be served from one central SL1 and data switching up to 19.2K.

Ongoing development of the SL1 by GEC brought about the launch of the SLX in the early ‘80s, effectively replacing SL1 as GEC’s PABX offering.

The SL1 is contained in a number of large blue cabinets, each measuring 130cm wide, 183 cm high and 51 cm deep.  Each can weigh up to 363kg, depending on equipment.  All SL1s are dual processor/memory systems, with the spare processor being in hot standby and the processors/memories being swapped every 24 hours as part of the system’s daily self-diagnostic routines.

The building blocks of an SL1 are groups, networks (or loops), shelves and cards.  An SL1 LE is a single group machine, with each group being able to accommodate 16 networks (4 used for ‘services’), each network supporting up to 4 PE shelves with each shelf accommodating up to 10 PE cards.  SL1 VLE and XL are multi-group systems, each having being able to support, in theory, 5 groups, although limitations in processor and memory capacity often limit the VLE to 3 groups.

SL1 systems operate from 240V, 50Hz main supply.  They require a good earth, being a 16mm for LE and 35mm for VLE and XL systems.  The system has integral rectifiers and power converters that produce the various voltages required around the SL1 – such rectifiers and converters being provided on a N+1 basis with common bussing so that the failure of any one unit does not affect system operation.  Most SL1s have battery backup, which will allow the system to continue to operate for a period, in the event of mains failure.

SL1 systems support MF/DP ER/TBR standard extensions, SL1/300 Featurephones, most analogue trunk signalling systems and DPNSS and DASS2 digital trunks.

Common System Cards 

BPC 50            Network Card                            go in network shelf

BPC 64            PE Buffer

BPC 60            Standard Extension Card            4 Ccts

BPC 14            SL1 (Featurephone) Card           4 Ccts

BPC 70            Exchange Line Card                    2 Ccts

BPC 72            DDI Exchange Line Card            2 Ccts

BPC 79            MF Receiver                               1 Cct

BPC 969/1021   Digital Trunk Signalling Link       go in network shelf

BPC 955            Digital Trunk Channel Switch     go in network shelf

PE cabinets can each accommodate up to 6 PE shelves.  Each PE shelf has its own PE buffer and up to 10 PE cards.  Each PE shelf is connected to the Test Jack Frame (TJF) by four cables.  The termination at the TJF may use a number of different systems.  If terminated on Krone strips, the interface circuits on each card appear on one Krone strip (i.e. 10 strips per PE shelf). 

At each SL1 installation there should be a comprehensive set of system manuals, which provide detailed system data.

System Programming

A number of programs (or overlays) are stored on the system tape.  There are three types of programs – service change, diagnostic and print.  Frequently used programs are:

Service Change

10                    Standard Extensions

11                    Featurephone Extensions

12                    Operator Consoles

13                    MF Receivers

14                    Trunk Data

16                    Route Data

17                    Configuration Record

18                    Short Code Lists

19                    Toll Denied and Translation Data

23 + 24            Customer Data

27                    Automatic Route Selection

28                    Departmental Hunt Groups

29                    Dial Tone Detect Data

81                    Authorisation Codes

82                    Automatic Call Distribution

83                    Common Class of Service

88                    Digital Trunk Data


2                      Traffic Measurement

3                      Call Trace

32                    Network and Peripheral Equipment

35                    Common Equipment (processors etc)

36                    Trunk Diagnostics

37                    Input/Output Devices

40                    Digital Trunks

43                    Data Dump


20, 21, 22, 80 and 92.

Specific hardware locations are referred to in programs using TNs.  A TN is made up of four numbers, separated by a space.  The four numbers represent the network, shelf, card and circuit number.  A typical TN may look like this  - 18 2 9 2.  Parts of a TN may be used to address system components (e.g. a network card or a PE shelf).


Superseded by the ISLX/BTEX


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Last revised: November 03, 2021