My first job in 1981 was programing the Northern Telecom/ BNR SL-1 PBX. It had been licensed in the UK by GEC (the UK company, not the US one) from Northern Telecom, and I added some UK specific features. It was programed in a language called SL-1 that I believe was short for Switching Language 1. SL-1 was a high-level language that was then compiled to a byte code that was then interpreted by the CPU on the PBX. Is there a reference to the SL-1 programing language anywhere? I can find information on the SL-1 hardware and the Man-Machine interface used to configure the system, but not to the actual language that ran the PBX.

  • Nothing in the Internet Archive?
    – RonJohn
    Commented Feb 14 at 23:18
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    – dave
    Commented Feb 14 at 23:33
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    Having "1" as a name-suffix was popular for a while. How many such ever progressed to "2"? ;-)
    – dave
    Commented Feb 15 at 12:45
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    @dave - OS/2 did... Not that I ever saw OS/1 mind you!
    – Jon Custer
    Commented Feb 15 at 13:20
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    Per Wikipedia, the SL-1/Meridian switch was programmed in a language called 'Protel'. Does that look familiar?
    – dave
    Commented Feb 17 at 18:00

1 Answer 1


Bitsavers has a large collection of old computer manuals in PDF format. They only have a few manuals for Northern Telecom. For the Meridian they only have the Design Guidebook.

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