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.

Continue reading



As part of the last post, reminding people to comment here, as opposed to my other site; Gary Sager, from the System 75 development team responded (I’m going to be honest unfairly) about my one line opinion wishing the System 75’s core operating system should’ve been UNIX. I believe he took my words too literally. Because I can’t easily move comments from one WP site to another, here is the original text.

In your web page on Oryx/Pecos for the System 75, you ask “it makes you wonder why was this not UNIX?” I think I am qualified to answer the question, being the architect and principal programmer for the operating system (you will find my name on the BSTJ article). At the time the system was being developed we were using the Intel 286 processor to keep costs down. That chip does not have virtual memory hardware, so I got to spec the hardware to implement that. At the time, UNIX was certainly not up to the real-time requirements of telephony on such hardware. BTW, before I really got into the implementation, I had to pass muster with Ken Thompson. The issue of using UNIX never came up. Perhaps today some form of UNIX might be viable on the more powerful chips available, but UNIX is still not a real-time system. Running UNIX as an application on a real-time system might work.

While I understand the abstract differences, I was talking mostly of it’s self healing nature of Oryx/Pecos and the more friendlier end user interface of the “Definity” line of PBX systems. I know people would quickly come to my defense and know my frustrations and strong opinions of UNIX in general, from a professional user, not a technical engineer. You’re welcome to opine to my defense in the comments – to the people I won’t name! 🙂

Anyways, I’m hoping he enjoyed this site with future loyalty and I always appreciate direct comments from the people behind the subjects in this case the vintage Avaya PBX offerings. It’s becoming rare because if Bell or Grey was still alive, you know they’d probably respond to me too! 😉

As a friendly reminder, there are pages on this site where you can contact me. All non pages enable you to comment with a real time response. The former “Dial Zero” page act’s as it’s Facebook-like wall. As much as it’s clutter to me when I visit others sites, I felt well its like the Wall, hell with it!

I do thank him for reminding me to put the final touches in getting the System 75 papers to be digitized. As some of you know, I had a lousy couple of years which has stalled this project. The finished project is to have the entire article as opposed to some other stuff I posted in the same tribute branch of pages. Please give me some time to complete!


AT&T Technical Journal July/August 1985 Cover – 5ESS Switch


This a work in progress. 

I found this copy at NH State Library and looked at this in tandem of getting copies of the System 75 tell all, as some came out blurry. I’m working on the latter book as a virtual exhibit as some will be published on this site very soon.

The 5ESS tribute will be a year long subject, AT&T posted video a while back on the first ESS back in the 1960s. The 25th anniversary of the long distance blackout occurred in January impacting 4ESS systems throughout the country on Martin Luther King’s holiday in 1990.

I like the cover though.

AT&T Technical Journal Jan 1985/System 75 Tell All, Attendant Console



Some things don’t change. Sometimes familiarity you shouldn’t mess around with.

In this case, the operator switchboard or Attendant Console as remained mostly the same since 1984, first released for the System 75 PBX. This model # was 302 series, but rarely is branded as such, only in the administrative terminal sessions to add stations and alike. There were 4 different series of 302s, suffixed with letters

A was the model shown below, very Merlin looking, and used button caps similar to Nortels, but more smaller.

B had a different display, with a white instead of metallic.

C replaced the button caps and supported 2 wiring environments. Prior to 302C, you needed the full 8 wires of copper to connect the console. C also introduced modular connections for the hand or headset. Prior to C, the console automatically logged in operators if they plugged in that dual 1/4″ jack into the console. Because of the change of jacks, the operator would had to press a few buttons to log them in.

302D was introduced around the turn of the century and required 2 wire environments only. 302C could work in ether/or environment


At some point, as of this writing in 2015, the 302 Series is officially dead and is no longer sold as new by Avaya as many operators have evolved to administrative assistants doing multiple things and split the calls between those groups, or in some operator environments have been replaced by s0ftware based sets. Operator sets don’t come cheap, and including its Direct Extension Selection or DXS can cost about $1,100 new!

The DXS has evolved, but the  general idea hasn’t changed. In fact the Merlin looking industrial designed remained in tact right to the EOS. The first generation had 8 buttons on the very bottom acting as “Hundreds groups select”. Simply put, if you have dial plan from 4000 to 6000, the bottom 8 buttons would be labeled “40”, “41”, “42” and “53”, “54”. etc. So if you want to call extension 5138, you would hit the bottom “51” key from hundreds group, then  press “38” in the 100 array keys. You can monitor a group of hundreds at a time and in later models as the System 75 evolved from supporting a few hundred extension into the behemoth of the Definity PBX supporting tens of thousands of lines, the Hundreds group keys maxed to 20, so you could monitor a couple thousands of lines if you had that model.




This picture is interesting because its very rare for any AT&T/Avaya Red publication to show exploded views of any of their equipment. This was standard operating procedure for Nortel if someone wanted to replace a part in a console or a set, however it was the job of AT&T to Lucent to Avaya to let this type of work be done by them or vendors.

AT&T Technical Journal Jan 1985/System 75 Tell All, Wiring Panel





This is one of the figure pictures in the AT&T Technical Journal, and the tell all of the System 75 PBX. What’s interesting about the wiring is modular, I’m not sure if the RJ45 jacks come from Amphenol to the cross connect or the voice drops.  I thought most System 75 setups in those days (and probably in the mid 90s) were 66 blocks where you’d take an Amphenol from the PBX and ether splice the wires to a 66 block or plug in it if the block had a female Amp adaptor.

system75 3

New Virtual Exhibit – A System 75 PBX Article

I’ve been lucky to obtain access and finally got to the Holy Grail of modern office telephony – that Avaya’s marketing department would never want you to know! We like to focus on other systems, and platforms like the ESS for an example, but what me interested in phone systems in general was the AT&T era of the 1980s.

Thanks to the N.H. State Library, and Rebecca, one of the reference librarians, I got access to the book taking a recent trip to Concord and got nearly 100 pages worth! AT&T at the time which sold the System 75 (later named the Definity ECS, MultiVantage, Communication Manager to Aura, marketed by Lucent and Avaya over the years) did an entire tell-all of how they developed the system, how they developed it, the hardware background, the software background and how they ate their own dog food, as some AT&T sites were the first test groups. Developing the System 75 from concept to market took about 3 years and was on the market by the time of the publication in January 1985.

Despite the 30 year old publication, the kernel and hardware architecture basically remained the same and such architecture helped its way through newer technologies such as ISDN, packet and IP switching and later Voice over IP. It was this concept and system that would have descending companies tout 90% of the Fortune 500 wither company wide or a few locations using this type of communications using the System 75/Definity/CM/Aura platform. It wasn’t really until the last decade did such entity (Avaya) tout such customer base, which probably has eroded significantly under companies like Microsoft and Cisco with their “unified communications offerings”

I’ll be posting this little by little over the course of the next couple of months, with my own take. It’s surprising it wasn’t a form of a white paper or publicized elsewhere, it’s a great read and it’s a rare find, my job is to make it easy for the people who would be so interested in reading this.

Stay Tuned!