LINC – Everything You Need To Know

Many of today’s most essential electronic devices started as nothing more than just a small idea in a laboratory at a university. This is especially true of the LINC — also known as the Laboratory Instrument Computer.

Conceptualized by a couple of grad students in the early 1960s, the LINC is widely regarded as the very first minicomputer and one of the most prominent predecessors to the personal computers we know and use today. But how did the LINC come to be?

What about the LINC made it so special at the time? And what kind of inventions did the LINC eventually lead to? Read on to learn the answers to these questions and more.

Three Facts about LINC

• Unlike almost every other groundbreaking invention at the time, the LINC’s design was public domain. This meant that creators Wesley Clark and Charles Molnar did not wish to own the rights to their invention, allowing other inventors to take and expand on their LINC concept freely without fear of infringement.
• The LINC proved to be a real game-changer in biomedical laboratories, where they were put to use for a variety of different functions involving the generation, conversion, or extraction of data.
• Adjusting for inflation, a LINC computer would have cost nearly $375,000 in today’s money. Remarkably, this price was considered affordable at the time.

LINC Specs

The Laboratory Instrument Computer’s specifications make it clear how ahead of its time the LINC really was. Not only is it largely considered to be the first interactive personal computer in the world, but its never-before-seen small size also made it a record-breaker. Here are some of its most important specs: 

• 12-bit word length
• A 1s complement for arithmetic
• A 2,048-word memory with a speed of 8 microseconds
• High-speed calculations
• 16 analog input channels
• A transfer rate of 125,000 words per second
• Two output channels 
• Six sets of relay contacts (DPDT)
• 16 digital pulse lines
• A power requirement of 1,000 watts or 115 volts

Additionally, a standard LINC core system was comprised of:

• A console module for various controls and indicators
• A terminal module with front-panel connections for I-O
• A display module with one mounted oscilloscope and various other controls
• A LINC tape module to hold the dual transport
• An electronics cabinet for the central processor and circuit boards
• A keyboard

The History of LINC: What To Know

In 1961, Wesley Clark, an electrical engineer at MIT’s Lincoln Laboratory in Massachusetts, realized that the time-sharing used by the time’s most advanced machines was not the only solution to the problem of direct access. Because Clark had contributed substantially to the development of the large TX-0 and TX-2 computers ordered by the US Navy, he proposed building a relatively inexpensive, general-purpose computer that could be controlled easily by biomedical researchers instead.

Initially ignored by Lincoln Lab’s upper management, Clark continued to work on his idea for a small computer in spite of the opposition. At the end of 1961, he disappeared from the Lab for a period of about three weeks. When he returned, he brought with him a complete design for a small computer with characteristics that marketing representatives would later call user-friendly. This design became the LINC.

Clark designed this early home computer to satisfy four basic criteria:

• Easy to program
• Easy to communicate with while in operation
• Easy to maintain
• Easy to process biotechnical signals directly

No computer in the early 1960s could even come close to fulfilling those objectives except for Clark’s LINC. Later, Clark and Charles Molnar added two shrewd criteria: It could not be too high to see over, and it must cost $25,000 at most. (This was the maximum amount a lab director could spend without higher-level approval.)

LINC originally had just 1 Kb of core memory (or 1,024 words), which was later expanded to 2 Kbs. It was designed for interactive use via a graphical user interface, complete with a 256 x 256 cathode-ray tube (CRT) display and four knobs to enter variable parameters that functioned similarly to that of a home computer mouse. The Soroban keyboard, used for alpha-numeric entry, had keys that locked down when pressed and popped back up once the early desktop computer had read them.

This effectively solved the problem of type-ahead. The LINC also allowed for removable media, such as two LINC tape drives (the predecessor of DEC tape, with each spool holding 512 blocks of 256 12-bit words or 512 bytes). Additionally, the LINC’s characters — which consisted of upper-case letters, Greek letters, and math symbols) took up six bits each.

A typical configuration of the machine included an enclosed 6’X20″ rack, four boxes holding tape drives, a small display, a control panel, and a keyboard. Analog inputs and outputs were also part of the basic design. In these, a tall cabinet sitting behind a white-Formica-covered table held two somewhat smaller metal boxes holding the same instrumentation, a Tektronix display oscilloscope over the “front panel” on the user’s left, a bay for interfaces over two LINC-Tape drives on the user’s right, and a chunky keyboard between them.

LINC Versions

Starting in 1964, LINC was manufactured commercially by DEC and Spear Inc. of Waltham, MA. During this time, a total of 50 were built, most at Lincoln Labs. They went for $43,600 each, which was actually considered quite affordable for this kind of technology at this time. 

As we see with desktop computers today, the LINC went through a series of different versions and saw various different additions throughout this period in the 1960s. For instance, the standard program development software at this time consisted of an assembler and screen editor.

LINC’s take on this software, known as LINC Assembly Program (LAP), was designed by Mary Allen Wilkes and integrated with the Assembler and File System. Interestingly enough, it was written for users, not computer professionals. LAP made it fairly easy to program LINC for biomedical experiments and, in its last version, was sufficiently flexible to allow for word processing.

When a scientist sat down at their LINC keyboard, they had a complete and comprehensible computer system at their disposal. Users were free to create a program and execute it in one sitting, making the LINC one of the most adaptable and unique pieces of technology then or now. Additionally, this brought on even more variations to the standard LINC setup.

One key example came when Wesley Clark was planning for the ARPANET, an early predecessor to the Internet. He suggested the use of LINCs (later named Interface Message Processors) to Larry Roberts as a way of standardizing the network interface and reducing the load on their local computers.

Other subsequent versions of the LINC include the PDP-4 and the PDP-5, the PDP-8, the LINC-8, and the PDP-12.

The Public Response

While the original version of the LINC only saw 50 units produced, the product’s public domain status made it very popular among other engineers and researchers throughout the 1960s.

However, future versions of the LINC and the adjacent PDPs saw increasing degrees of success among the general public as minicomputers (and, more broadly, computing technology in general) became more familiar and affordable.

Of all these subsequent updates to the LINC and the PSP, the most popular by far was the PDP-12. This was due to its vast amount of improvements and features over previous versions.

The image featured at the top of this post is ©A.C.Diller, CC BY 4.0, via Wikimedia Commons.