Historical Notes on the Carrier Switches on Avaya Red PBX systems

Originally part of the Tribute to System 75, relocated to a blog post, with an update since it’s original post in 2015.

The System 75 was a fully new system that could handle infinite technologies such as ISDN, PRI, T-1, and later IP and packet switching. At the same time, AT&T also marketed the not so bleeding edge PBX called the System 85, which was a Band-Aid version of the Dimension. But this version would use shared hardware compatibilities with the System 75. In fact the System 85 had some interesting features such as AUDIX “Unified Messaging”, ports could max out to over 30,000 extensions and support up to 40 attendant consoles. Well, not the 302 console I posted earlier, but a boxy one that was used for the Dimension, so boxy it earned the top 6 position as the Ugliest Operator Consoles in 2016 from this site.

Another system called the System 25 was based almost entirely on the Merlin system with the code, and features. The System 25 is much comparable to a Merlin Magix or Legend system of today, which eventually replaced this odd setup.

What was common with all the cards and the type of carriers is the boards were cross compatible and hardware for phone lines and trunks could be interchanged during a cutover to a larger system. There was a reason why there was color coded labels on the boards, back in those days System x5 systems required tone clocks, processors and auxiliary connections  to be together, line cards and those alike could be free floating if the customer chose so. Later versions of the Definity system would not have color coded labels and the cards could go in whatever fashion.

system75 8


This cabinet can weigh as much as a stainless steel refrigerator, about an 800 pound voice gorilla and can support up to 700 lines.

It’s kinda strange how the power unit is located right next to the processor and tape unit…wouldn’t that cause problems?

To the left and right of each shelf  has a power supply, for each row, it uses two sources of power. What’s interesting in this first version they all have power on and off (or kill switches.)  Newer versions had power in only.

The bottom area was changed to support battery backups, the power supply and it’s tone generator in later years of the Definity/MultiVantage/Communication Manager systems of this type of carrier.



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