- For the programming language, see LINC 4GL.
The LINC (Laboratory Instrument Computer) was a 12-bit, 2048-word computer. The LINC
and the PDP-8 can be considered the first mini computers and
perhaps the first personal computers as well. Although its instruction set was small,
it was larger than the ingenious and tiny PDP-8 instruction set.
The LINC sold for about $50,000. It interfaced well with laboratory experiments. Analog inputs and outputs were part of the
basic design. It was designed in 1962 by Charles Molnar and
Wesley Clark at Lincoln Laboratory,
Massachusetts (Part of MIT), for NIH researchers. The LINC's design was literally in the public domain, perhaps making it
unique in the history of computers. 24 LINC computers were assembled in a summer workshop at MIT. Digital Equipment Corporation (starting in 1964) and Spear
Inc. of Waltham, MA manufactured them commercially. Software was designed by M.A. Wilkes, the last version named LAP6.
The Control Panel
The LINC control panel was capable of much more than single-stepping programs. It was, to all intents and purposes, the
debugger. It allowed execution to be stopped when the program counter matched a set of switches. More important, another function
allowed execution to be stopped when a particular address was accessed. The single-step and the resume functions could be
automatically repeated. The repetition rate could be varied over four orders of magnitude by means of an analog knob and a
four-position decade switch, from about one step per second to about half of the full speed. Running a program at one step per
second and gradually accelerating it to full speed provided an extremely dramatic way to experience and appreciate the speed of
the computer.
LINCTape
A very noteworthy feature of the LINC was the LINCtape, seen in the photographs below. It was a fundamental part of the
machine design, not an optional peripheral, and the machine's OS relied on it. In retrospect, the easiest way to understand the
LINCtape's role is to think of it as a linear diskette with a slow seek time. The magnetic tape drives on large machines of the
day stored large quantities of data, took many minutes to spool from end to end, but could not reliably update blocks of data in
place. In contrast, the LINCtape was a small, nimble device which stored about 400K, had a fixed formatting track allowing data
to be repeatedly read and re-written to the same locations, and took less than a minute to spool from one end to the other. The
tape was formatted in fixed-sized blocks, and was used to hold a directory and file system. A single hardware instruction could
seek and then read or write multiple tape blocks all in one operation.
Filenames were six characters long. The file system allowed for two files--a source file and an executable binary file to be
stored under the same name. In effect it was a 6.1 filename in which the extension was restricted to "S" or "B". Since the basic
LINC had only 1024 12-bit words of core memory (RAM)--and the big, expanded LINC had only 2048--normal operations depended
heavily on swapping to and from LINCtape. (Digital later patented and marketed an extremely similar design under the name
DECtape; Digital's patents on DECtape were eventually tested in court and found invalid).
LINCtape is also fondly remembered for its reliability, which was distinctly higher than that of the diskettes which
supplanted it. LINCtape incorporated a very simple form of redundancy--all data was duplicated laterally in two locations across
the tape. LINC users demonstrated this by punching holes in a tape with an ordinary office paper punch. Tape damaged in this way
was perfectly readable. The formatting track made operation almost independent of tape speed, which was, in fact, quite variable.
As can be seen in the pictures below, there was no capstan; the motion of the tape
during reading and writing was directly controlled by the reel motors. There was no fast forward or rewind—or, you might say,
reading and writing was performed at fast forward and rewind speeds. In some modes of operation, the data transfers were audible
over the built-in loudspeaker and produced a very characteristic series of harsh bird-like squawks with varying pitch.
The Keyboard
The LINC keyboard, manufactured by company named Soroban Engineering, had a set of keys with locking solenoids for each key.
When the user pressed a key, the LINC would lock all the keys in their current up or down positions, read the key into a hardware
register, then, when the running program read the register, the hardware would release all locking solenoids. This had the effect
of slowing down typing and preventing even 2-key rollover. This keyboard was abandoned in
the LINC-8 and PDP-12 follow-on computers (see below).
Text Display
The capital letter M as it was displayed in a 4 by 6 character cell on the LINC screen
The LINC hardware allowed a 12-bit word to be rapidly and automatically displayed on the screen as a 2-wide by 6-high matrix
of pixels, making it possible to display full screens of flicker-free text. The standard display routines generated 4 by 6
character cells, giving the LINC one of the coarsest character sets ever designed.
The display screen was a CRT about 5 inches square which was actually a standard Tektronix oscilloscope with special plug-in amplifiers. These special plug-ins could be replaced with
standard oscilloscope plug-ins for use in diagnostic maintenance of the computer. Many LINCs were supplied as kits to be
assembled by the end user, so the oscilloscope came in handy.
Teletype Output
Printed output on an ASR-33 Teletype was controlled by a single pole relay. A
subroutine would convert the LINC character codes into ASCII and use timing loops to toggle the relay on and off, generating the
correct 8-bit output to control the Teletype printer.
Laboratory Interface
The LINC included bays for two plug-in chassis allowing custom interfacing to experimental setups. Analog-to-digital and
digital-to-analog converters were built in to the computer and each could be accessed by a single machine instruction. Six relays
were also available.
The LINC-8 And PDP-12 Computers
PDP-12 Computer At The First Vintage Computer Festival East
Digital Equipment Corporation manufactured two next-generation
LINC-compatible computers, the LINC-8 and PDP-12. The first follow-on, the LINC-8, booted (slowly) to a PDP-8 program called PROGOFOP (PROGram OF
OPeration) which interfaced to the separate LINC hardware. The PDP-12 was the last and most popular follow-on to the LINC. It was
a capable and improved machine, and was more stable than the LINC-8, but architecturally was still a shotgun marriage of a LINC
and a PDP-8, full of many small technical glitches. (For example, the LINC had an overflow bit which was a small but important
part of the LINC's machine state; the PDP-12 had no provision for saving and restoring the state of this bit across PDP-8
interrupts.)
The MINC-11 Computer
Digital produced a version of the PDP-11/03 called the MINC-11, housed in a
portable cart, and equippable with Digital-designed laboratory I/O modules supporting capabilities such as analog input and
output. A programming language, MINC BASIC, included integrated support for the laboratory I/O modules. MINC stood for
"Multi-Instrument Computer." The name undoubtedly was intended to evoke memories of the LINC, but the 16-bit machine had no
architectural resemblance to, or compatibility with, the LINC.
The "LINC" in Beneath a Steel Sky
A fictional computer in the 1994 computer game Beneath
a Steel Sky has the name LINC; it has no resemblance to or known connection with the historical LINC.
See also
External links
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