The Northen Telecom SG-1/SG-1A "Pulse" EPABX (Electronic Private Automatic Branch Exchange) is commonly regarded as the first fully electronic PBX. The system combined digital controls with an analog speech highway. Unlike most other analog systems at the time, the SG-1 did not have a hard wired speech path. Instead, it used time division multiplexing to multiplex all voice channels onto a single pair of wires, the "speech highway".
From US patent 3,444,326
Northern Telecom's description of the technology inside the SG-1 is a mouthful: Pulse amplitude modulated time division multiplex with resonant energy transfer. The SG-1 speech highway is divided into 24 individual timeslots, with a per-timeslot sample rate of 12KHz.
Read more about my SG-1 story here.
I have ended up with two SG-1As. This is my first, equipped for 40 lines:
This is a view of the trunk cards (brown), which control access to the central office from the local extensions. The design of the Pulse also put things like dial tone generators on this shelf. Below the brown cards are the yellow line cards. Each of the line cards serves two extensions. Some of the yellow cards are common equipment, leaving 20 actual line cards (40 lines) on each shelf. This system is equipped for 120 lines. I picked this unit up a little while after my first.
This is what the back of the shelves look like:
This is the operator's console. That big white thing coming out of it is a 150 pair (300 wire) cable that connects the console to the cabinet.
No CPU? No problem! A not-so-deep-deep-dive into the design of the SG-1.