IBM System/360
Featuring the IBM 9020
The IBM System/360 (S/360 for short) was a family of mainframe computer systems announced by IBM on April 7, 1964, and delivered between 1965 and 1978. S/360 was the first family of computers designed to cover both commercial and scientific uses and a complete range of applications from large to small. The design distinguished between architecture and implementation, which allowed IBM to release a group of compatible designs at different prices. Most models used microcode to implement the instruction set, and featured 8-bit byte addressing and fixed-point binary, fixed-point decimal, and hexadecimal floating-point calculations. The S/360 family introduced IBM's Solid Logic Technology (SLT), which packed more transistors onto a circuit card than previous technologies, allowing more powerful but smaller computers to be built.
What is the aim of this site?
The Background
I started my career as an Air Traffic Engineer with the UK’s National Air Traffic Services (NATS). NATS came into being as an Air Traffic Control Service provider in 1972 as part of the UK Civil Aviation Authority (CAA). The CAA is broadly equivalent to the Federal Aviation Administration (FAA) in the US.
I was initially trained to operate and maintain such aviation-related equipment as radar, navigation aids, and communication systems. The basic training also included courses on “computer techniques”. I was posted to the London Air Traffic Control Centre, located within RAF West Drayton in London, a few miles north of Heathrow Airport. This centre was comparable to an ARTCC in the US.
In the early 1970s NATS purchased, via the FAA, an IBM 9020D air traffic control computer complex which was installed at the West Drayton centre. This complex consisted of various components drawn from the S/360 family. In 1979, after gaining experience on several smaller computer systems, I was selected to attend a year-long 9020D engineers’ course at the FAA Academy in Oklahoma City. There we (five UK hardware engineers each year) were trained to IBM Customer Engineer level on all the constituent 9020D elements. As well as the mainframe computers and input/output channels the training covered peripherals such as disk and tape drives and high-speed printers.
On my return from Oklahoma, I joined one of the “9020 Watches” which looked after the system on a 24/365 basis as part of the centre’s watchkeeping roster. After several years I was promoted to Head of 9020 Hardware and moved to a management role in charge of all aspects of 9020D hardware support, alongside a colleague who was responsible for 9020D software support. While in this role I managed the decommissioning and disposal of the 9020D, and assisted with the installation of its replacement system (another IBM mainframe complex) from an operational point-of-view. I stress operational as I was just one cog in the wheel. There were tens of ‘project people’ far cleverer than me, but I contributed operational knowledge and experience.
Because all hardware engineering support of the 9020D was in-house we had far more on-site documentation than the usual IBM customer. For example, we had copies of hardware schematics and logic diagrams down to component level. On a day-to-day basis we were entirely independent of IBM, except for ordering replenishment spare parts. Speaking of which, the initial procurement included a recommended spares package of thousands of individual items. Most of these remained on the shelves until they were scrapped along with the main complex!
The Restoration
Well, the main complex wasn’t completely scrapped. The combined 9020D operational (triplex) and support (simplex) systems included a total of eight S/360 mainframe computers. Seven of these were scrapped (“there’s gold in them there mainframes!”) but one was saved from the hands of the metal reclaimers.
That mainframe was an S/360 Model 65. It differs from a standard Model 65 in that it has several 9020-specific features added. It has everything a base Model 65 has, plus some bells and whistles. All commercial S/360 mainframes had a range of additional features a customer could order. However, the 9020 features could only be ordered through government contract, as the 9020 was supplied by the IBM Federal Systems Division rather than the commercial IBM Data Processing Division.
After being safely cosseted in secure (and expensive!) storage for over three decades I am now restoring that Model 65 to working condition, a process that commenced in late 2024. It is to my advantage that, as well as being CE-trained on the system, I was the one who personally dismantled it in 1990, taking care and making copious notes. I also have all the necessary documentation and many spare parts, should they be necessary.
The Objectives
This site will not try to be “all you wanted to know about System/360”. IBM’s S/360 family is so iconic and revolutionary that there are many existing excellent sources of information. Instead, it will focus on technical detail directly related to the restoration of this particular Model 65. The objectives are fourfold:
1 – Out of historical interest, to document the challenges and solutions encountered during the restoration. As well as (hopefully!) being of interest in themselves, they should also prove useful when maintaining the restored system in years to come.
2 – To enable future maintenance procedures to be successfully undertaken by others. This involves making all the relevant technical information available online. But more than this, to show how the various S/360 maintenance procedures are actually put into practice. It is one thing to have a maintenance or theory of operation manual and understand the words. It is another to find one’s way around a hundred thousand (literally) ’scope points in practice and know what one is looking for at each point, for example. CE training wasn’t just ‘reading the books’. We were shown how to do things. For now, there are still some ex-CEs who have that knowledge. But not for long…
3 – To document the IBM 9020 system in general. Twenty-nine systems were delivered in total, including the one in the UK. (Incidentally, the UK system was identical to those in the US. A non-standard 60Hz power supply was even installed to avoid any hardware changes.) There is a growing body of 9020 technical literature publicly available online, more so since I have been uploading many of my 9020 documents to Bitsavers. On this site I hope to provide an overview to pull that information together in layman's language.
4 – To explain some of the finer points of S/360 technology and maintenance features not covered elsewhere on the internet. These will be applicable to S/360 generally, not just 9020.
As the second objective implies, I hope the restored mainframe will live on in working condition well into the future. Indeed, given the technology (IBM's legendary 'built like a battleship' quality of that era) and the component-level information available, I don’t see why ‘for ever’ isn’t a possibility.
The Apology
Of necessity, the information on this site will evolve in a piecemeal fashion. I will try to develop a sensible page/article structure as I go along. I have several other projects on the go, and I think it is more important to ‘do it’ and progress rather than ‘write it up’ – which is probably why I was in Operations and not Projects!
I expect updates will follow the same pattern as the practical restoration. I will try and post things in chunks as I go along, but please bear with me. If I waited until the site was finished before publishing it would never be seen!
The old site from 2012 If you came here expecting to find the "old site" you can find a copy here.