The Electronic Design April 1970, Vol. 18, No. 9. cover article, The minicomputer: machine with an endless future, highlighted minicomputers. They had been around for more than a decade and would continue to be important for many years after until the IBM PC arrived. This was also a time where mainframes were still crunching numbers, but minicomputers let you put computing on the desk or at least next to it.
Minicomputers were common in office, commercial, and industrial spaces, bringing computer access to the masses. It also provided local control of computing hardware. Timesharing systems with multiple terminals that had required a mainframe could be handled by minicomputers.
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What are Minicomputers?
Minicomputers were mid-range, general-purpose computers in the age of mainframes, the latter taking up large rooms and required special cooling. Minicomputers were small enough to sit on a desktop or in the corner of an office.
They were typically tied to printer terminals like the venerable Teletype or display terminals, often referred to as CRTs due to their cathode ray tube (CRT) main display like the Hewlett-Packard (HP) 2116 (Fig. 1). Storage devices included tape drives and hard-disk drives.
Early minicomputers tended to have lots of lights and switches to allow direct access to memory. First-stage bootloaders were often a few switch flips away.
Integrated-circuit technology of the time was small scale integrated (SSI), medium scale integrated (MSI), and large scale integrated (LSI) circuits built on transistor-transistor logic (TTL). Other transistor technologies were used, though, such as emitter coupled logic (ECL). A system usually consisted of multiple circuit boards.
Data formats varied depending on the vendor. For example, the 12-bit PDP-8 supports 36-bit floating-point numbers. Memory was also limited by the word size, which is why many systems were based on 16-bit registers. Memory bank switching was often employed to provide access to more physical memory. Virtual memory support was rare on low-end systems.
Minicomputers came in a range of sizes and architectures from many companies. Digital Equipment Corporation (DEC) was one of those companies, and the 12-bit PDP-8 (Fig. 2) was one of its more popular models. DEC also offered large-scale systems, including the 32-bit VAX series. DEC was eventually acquired by Compaq and then Hewlett-Packard.
The Data General Nova line of 16-bit minicomputers found a home in scientific and industrial settings (Fig. 3). The base system fit into a 3U rack and the register contents could be displayed on the front panel with switches to enter data typically for initial booting. Punched paper tape or magnetic tape often contained the second-stage loader that in turn booted applications directly or in an operating system.
As with many minicomputers, a single-chip implementation called the microNOVA was eventually built. Other vendors followed suit. The DEC LSI-11 supported 16-bit PDP-11 code. These one-chip minicomputers were often bypassed by users that went with the new microprocessor architectures.
There were dozens of minicomputer vendors, but the number decreased over time. Eventually, most disappeared or were absorbed by other companies. Some vendors like IBM and Unisys still exist, although their minicomputer lines are now history. Unisys was a combination of Sperry and Burroughs. Like IBM, these companies had product lines that included mainframes as well as minicomputers and ultimately personal computers (PCs).
Some other notable minicomputer vendors included Control Data, Wang Laboratories, Prime Computer, Pyramid Technology, Bendix, Harris Computer System, Honeywell, Interdata, Raytheon, and Tandem Computers.
Magnetic core memory was used in systems before the 1970s. However, most platforms transitioned to semiconductor memory as it became available and affordable.
What Were Minicomputers Used For?
Minicomputers were tasked with doing many of the functions originally performed by mainframes, from accounting to timesharing. Minicomputers were also integrated into scientific and industrial systems. While most systems had one or more computer terminals attached, the embedded solutions were sometimes headless, making the lights and switches useful when things went wrong.
Multiprocessor systems were typically relegated to mainframes as minicomputer designs were generally intended to reduce costs. Many minicomputer product lines grew, so having more than one processor wasn't uncommon. For example, the HP 2000 timesharing system that provided support for BASIC used a pair of HP 2100 minicomputers — one for communication and one to run user applications.
Most minicomputer systems included software as part of the package or it was exclusively available from the vendor. The same is true for compilers. BASIC, FORTRAN and COBOL stood out, although other languages like PL/I and vendor-specific languages were available, too.
The same is true for many of the peripherals like printers and terminals. The hardware and software were often integrated, such as Wang Laboratories' word-processing systems.
What Replaced the Minicomputer?
Technology advanced, with very large scale integration (VLSI) and beyond providing the functionality of a minicomputer in a single chip. Our latest ICs exceed even the largest minicomputer in terms of computational capability, storage, and so on.
What initially did in the minicomputer was the microcomputer and the single (usually) chip microprocessor, with the Intel 8008 being one of the opening shots. The Intel 8080 was the next and most popular step, showing up in platforms like the IMSAI 8080 (Fig. 4).
This class of hobbyist microcomputers and eventually commercial products was based on defacto standards like the S-100 bus (eventually defined as IEEE 696-1983) and operating systems such as Digital Research's CP/M, which eventially led to MS-DOS for the IBM PC.
While microcomputers like the IMSAI 8080 brought minicomputer functionality to a lower price point, the IBM Personal Computer (PC) turned these platforms from an expensive shared resource to a personal-computer system (Fig. 5). The 16-bit Intel 8088 and 8086 were behind these platforms, along with the PC Disk Operating System (PCDOS or simply DOS) and its Microsoft counterpart, MS-DOS.
Graphics on the PC was useful but primitive compared to today's platforms. Improved graphics and faster and better hardware allow Microsoft Windows to dominate the desktop and office space.
The cloning of the IBM PC BIOS (Basic Input/Output System) — originally developed by Gary Kildall for CP/M systems — and the ability to use de facto Industry Standard Architecture (ISA) expansion boards made all the difference. IBM PC clones expanded the market, and hardware and software vendors had a platform to target.
Not surprisingly, the embedded commercial and industrial uses of minicomputers was replaced by more compact microcontroller- and microprocessor-based solutions. Today, we have platforms like the Raspberry Pi and systems such as Sfera Labs' Iona Pi Max based on a Raspberry Pi Compute Module — and that's only one of the processors within the systems (Fig. 6).
The Raspberry Pi family highlights the massive advancement in computing power and reduction in size and power requirements compared to minicomputers. The Iona Pi Max has its own internal expansion and can run standard operating systems like Linux. Developers have access to an array of software tools including multiple programming languages and middleware frameworks, as well as amazing communication options using standards like wired Ethernet and wireless communication.
We've been focusing on the digital side of things as minicomputers rarely had analog data acquisition as part of the mix, except in scientific and industrial applications. However, these days, analog and mixed signal is very common, especially for industrial applications and IoT and edge-computing devices.
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Nowadays, people don't think twice about folding smartphones with terabytes of non-volatile storage, multiple cameras, built-in GPS, NFC support, and AI acceleration in the palm of their hands. Devices like Amazon's Echo Show (Fig. 7) provide voice interaction courtesy of Alexa. It represents just one of many systems having such capability.
Many have been born after the minicomputer revolution, although I started out with vacuum tubes and mainframes. It's been a whirlwind of technology over the decades, but it's surprising how much remains the same in terms of purpose and functionality. The main changes are in terms of price, performance and power requirements.
Of course, communication is one thing that's changed radically, and the neural-network AI revolution is just beginning. Though most will agree that the technology changes have been helpful to a majority of people, don't overlook the other issues that have changed over time; issues we don't cover here.
The internet has made exchange of data easier. However, it's also made it possible to beg, borrow, and steal things that weren't possible with minicomputers. Trying to get a letter out using a minicomputer word-processing system pales in comparison to dealing with a distributed denial-of-service (DDoS) attack on your phone or having to address someone stealing your computer credentials.
As noted in The minicomputer: machine with an endless future, "In the future, there is the possibility of a minicomputer in every kitchen and in every automobile." The minicomputer didn't make it into these applications, but tiny computers now reside in these locations — often with many cores and chips, not just one large minicomputer.
