|Introduced late 1970's the SL1 was a SPC Digital switch with a capacity for more than
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.
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 GECs PABX offering.
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.
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:
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
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).
Last revised: November 03, 2021