"The phone is a remarkably complex, simple device,
and very rarely ever needs repairs, once you fix them." - Dan/Panther

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SG-1 Introduction

Started by Keelan, September 21, 2016, 01:54:49 PM

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Well, I may as well start a thread here as well... The piece of equipment that got me into telephone collecting in the first place was a Nortel SG-1 PABX that I bought at an auction of a filthy motel that had gone out of business and was due for destruction. I thought I was only buying the phones out of the place, I didn't realize that the refrigerator sized cabinet in the basement was mine too.

The SG-1 has been discussed here before, but I'll give a brief rundown of it. It was heralded as the first fully electronic PBX. The control was fully digital, with an analogue speech pathway, but it was solid-state from end to end. The SG-1 used a pulse amplitude modulated speech highway, which connected two phones together for a brief moment thousands of times a second, with some filtering to fill in the blanks. In the case of the SG-1, the sample rate was around 12KHz. The speech highway had 24 separate time slots, but even providing dial tone consumed a time slot.

From a technical point of view, the neat thing about the SG-1 is that there are no ASICs. It's built out of off-the-shelf parts, mostly 7400 series TTL logic. While it lacks the brrrp-click-clunk-tick-tick-tick aesthetic of a fully electromechanical system, its operation is by no means a black box. The Practices don't go into the inner workings of the system in great detail, so the exactly how it works is still quite mysterious, but it's still possible to trace the operation of all the circuits.

I've started to take a closer look at this system, so I'm starting a video series to look in to it. My systems are currently not at my house, so I've started the examination with an overview, aided by some SG-1 cards I have in storage:


liked your video.... neat old technology. you should have shown the whole system, cabinet and all....


I would have if I could have. I don't have access to the whole system right now, it's in storage 8 hours to the east of me.


Over 25 years ago, I had the chance to get an SG-1, but after being reminded of how large and heavy it was, I opted for the second option, a Mitel SX-20. It had a better chance of fitting into my apartment. If I had had a place to store it, I would have taken both. I remember getting a former employer to replace their old (and malfunctioning) Siemens PBX with an SG-1, at CKOV in Kelowna, back about 1977 or 78.