Have you ever wondered what the first Nortel digital PBX telephones looked like? They were heavy duty wedge sets called QSU60 and QSU61 and were introduced in 1975. They also had add on KEM modules. These telephones were the first electronic multi line phones to use electronic ring tones, call buzz tones and Red LED lights to show line and feature status. In 1975, the telephone world was far away from the ATT Merlin, Rolm CBX and NEC Dterm sets. Handset across top 1a2 desk sets and wall sets dominated the market, so these newer wedge sets with one tiny wire running into the back of the phone must have looked really futuristic in 1975.
Also these QSU telephones were the first to use a single 3 pair modular line cord running to the phone, as opposed to a 1a2 phones which has a large 25 pair wire coming into the back of the phone. The phones were so heavy that if you wanted to wall mount them they needed a special bracket plate to hold them to the wall.
According to Chris Edwards, a lead Nortel developer on the SL1 system the phones worked this way:
The sl1 sets were digital. Each had a propriety integrated circuit made by BNR (Bell Northern Research) called an ssd. Stands for scan-and-signal-distributor. The sl1 sets used an extra pair of wires to send control messages ( like indicating key presses and lighting key led lights).
In the 1980s and early 1990s my Dads federal office used these as their office phones. They were replaced in 1993 with NEC Dterm sets. I remember as a teenager arguing with the head IT guy telling him that all he needed to do was buy new cards and he could upgrade the SL1 system to the latest M2000 sets and Meridian Mail and he wouldn’t believe me. What a waste.
I personally used these phones while volunteering at a hospital in NW Indiana while in high school. The phones made a loud clunky noise when hung up over the Paging system. The RLS button was invented with these sets and provided a clean silent release over the paging. If you pressed the switchook after a page you’d get a pop, as opposed to the silent RLS button.
These phones were seen in many 1980s movies such as on the desk of Clark Griswald in Christmas Vacation and in Tom Hanks office on “Big” the movie.
The following screen grab shows the different styles of the first generation “Meridian” digital sets.
This second photo appears to show an updated version of the SL1 sets where the Red LEDs indicators were updated to an LCD panel. These phones appear to be the P centrex phones that worked directly off the DMS100. These were eventually updated to the M5000 sets first the boxy “Delta” looking sets such as the M5209 and then the M5316 and M5008 which look identical to the Meridian 1 telephones.