![DECTape unit for the DEC PDP-11](https://upload.wikimedia.org/wikipedia/commons/3/32/DECTape_unit_for_the_DEC_PDP-11.jpg)
DECtape, originally called Microtape, is a
magnetic tape data storage
Magnetic-tape data storage is a system for storing digital information on magnetic tape using digital recording.
Tape was an important medium for primary data storage in early computers, typically using large open reels of IBM 7 track, 7-track, ...
medium used with many
Digital Equipment Corporation
Digital Equipment Corporation (DEC ), using the trademark Digital, was a major American company in the computer industry from the 1960s to the 1990s. The company was co-founded by Ken Olsen and Harlan Anderson in 1957. Olsen was president unt ...
computer
A computer is a machine that can be programmed to Execution (computing), carry out sequences of arithmetic or logical operations (computation) automatically. Modern digital electronic computers can perform generic sets of operations known as C ...
s, including the
PDP-6
The PDP-6, short for Programmed Data Processor model 6, is a computer developed by Digital Equipment Corporation (DEC) during 1963 and first delivered in the summer of 1964.
It was an expansion of DEC's existing 18-bit systems to use a 36-bit da ...
,
PDP-8
The PDP-8 is a 12-bit computing, 12-bit minicomputer that was produced by Digital Equipment Corporation, Digital Equipment Corporation (DEC). It was the first commercially successful minicomputer, with over 50,000 units being sold over the model's ...
,
LINC-8
LINC-8 was the name of a minicomputer manufactured by Digital Equipment Corporation between 1966 and 1969. It combined a LINC computer with a PDP-8 in one cabinet, thus being able to run programs written for either of the two architectures.
Archit ...
,
PDP-9
The PDP-9, the fourth of the five 18-bit minicomputers produced by Digital Equipment Corporation, was introduced in 1966. A total of 445 PDP-9 systems were produced, of which 40 were the compact, low-cost PDP-9/L units..
History
The 18-bit PDP ...
,
PDP-10
Digital Equipment Corporation (DEC)'s PDP-10, later marketed as the DECsystem-10, is a mainframe computer family manufactured beginning in 1966 and discontinued in 1983. 1970s models and beyond were marketed under the DECsystem-10 name, especi ...
,
PDP-11
The PDP-11 is a series of 16-bit minicomputers sold by Digital Equipment Corporation (DEC) from 1970 into the 1990s, one of a set of products in the Programmed Data Processor (PDP) series. In total, around 600,000 PDP-11s of all models were sold, ...
,
PDP-12
The PDP-12 (Programmed Data Processor) was created by Digital Equipment Corporation (DEC) in 1969 and was marketed specifically for science and engineering.see Mini-Computer section and press see more, then press see more again It was the third i ...
, and the
PDP-15
The PDP-15 was the fifth and last of the 18-bit minicomputers produced by Digital Equipment Corporation. The PDP-1 was first delivered in December 1959 and the first PDP-15 was delivered in February 1970. More than 400 of these successors to th ...
. On DEC's 32-bit systems,
VAX/VMS
OpenVMS, often referred to as just VMS, is a multi-user, multiprocessing and virtual memory-based operating system. It is designed to support time-sharing, batch processing, transaction processing and workstation applications. Customers using Ope ...
support for it was implemented but did not become an official part of the product lineup.
DECtapes
are 3/4 inch (19 mm) wide, and formatted into blocks of data that can each be read or written individually. Each tape stores 184K
12-bit PDP-8 words or 144K
18-bit words.
Block size is 128 12-bit words (for the 12-bit machines), or 256 18-bit words for the other machines (16, 18, 32, or 36 bit systems).
From a programming point of view,
[ because the system is block-oriented and allows random seeking, DECtape behaves like a very slow disk drive.
]
Origins
DECtape has its origin in the LINCtape tape system,[ which was originally designed by ]Wesley Clark
Wesley Kanne Clark (born December 23, 1944) is a retired United States Army officer. He graduated as valedictorian of the class of 1966 at West Point and was awarded a Rhodes Scholarship to the University of Oxford, where he obtained a degree ...
at the MIT Lincoln Laboratory
The MIT Lincoln Laboratory, located in Lexington, Massachusetts, is a United States Department of Defense federally funded research and development center chartered to apply advanced technology to problems of national security. Research and dev ...
as an integral part of the LINC
The LINC (Laboratory INstrument Computer) is a 12-bit, 2048-word transistorized computer. The LINC is considered by some the first minicomputer and a forerunner to the personal computer. Originally named the "Linc", suggesting the project's o ...
computer. There are simple LINC instructions for reading and writing tape blocks using a single machine instruction. The design of the LINC, including LINCtape, was placed in the public domain because its development had been funded by the government. LINCtape drives were manufactured by several companies, including Digital.
In turn, LINCtape's origin can be found in the magnetic tape system for the historic Lincoln Laboratory TX-2
The MIT Lincoln Laboratory TX-2 computer was the successor to the Lincoln TX-0 and was known for its role in advancing both artificial intelligence and human–computer interaction. Wesley A. Clark was the chief architect of the TX-2.
Specific ...
computer, designed by Richard L. Best and T. C. Stockebrand. The TX-2 Tape System
The TX-2 Tape System was a magnetic tape data storage technology from the late 1950s.R. L. Best and T. C. Stockerbrand, A Computer-Integrated Rapid Access Magnetic Tape System with Fixed AddressProceedings of the Western Joint Computer Conference ...
is the direct ancestor of LINCtape, including the use of two redundant sets of five tracks and a direct drive tape transport, but it uses a physically incompatible tape format (½-inch tape on 10-inch reels, where LINC tape and DECtape used ¾-inch tape on 4-inch reels).[R. L. Best and T. C. Stockerbrand, A Computer-Integrated Rapid Access Magnetic Tape System with Fixed Address]
Proceedings of the Western Joint Computer Conference: Contrasts in Computers
May 6–8, 1958; pages 42-46.
Digital initially introduced the Type 550 Microtape Control and Type 555 Dual Microtape Transport as peripherals for the PDP-1
The PDP-1 (''Programmed Data Processor-1'') is the first computer in Digital Equipment Corporation's PDP series and was first produced in 1959. It is famous for being the computer most important in the creation of hacker culture at Massachusetts ...
and PDP-4
The PDP-4 was the successor to the Digital Equipment Corporation's PDP-1.
History
This 18-bit machine, first shipped in 1962, was a compromise: "with slower memory and different packaging" than the PDP-1, but priced at $65,000 - less than half th ...
computers, both 18-bit machines. DEC advertised the availability of these peripherals in March and May, 1963, and by November, planning was already underway to offer the product for the 12-bit PDP-5
The PDP-5 was Digital Equipment Corporation's first 12-bit computer, introduced in 1963.
History
An earlier 12-bit computer, named LINC has been described as the first minicomputer and also "the first modern personal computer." It had 2,048 12 ...
and 36-bit PDP-6
The PDP-6, short for Programmed Data Processor model 6, is a computer developed by Digital Equipment Corporation (DEC) during 1963 and first delivered in the summer of 1964.
It was an expansion of DEC's existing 18-bit systems to use a 36-bit da ...
, even though this involved a change in recording format. The initial specifications for the Type 550 controller discuss a significant advance beyond the LINCtape, the ability to read and write in either direction. By late 1964, the Type 555 transport was being marketed as a DECtape transport.
The tape transport used on the LINC is essentially the same as the Type 555 transport, with the same interface signals and the same physical tape medium. The LINC and DEC controllers, however, are incompatible, and the positions of the supply and take-up reels were reversed between the LINC and DEC tape formats. While LINCtape supports high-speed bidirectional block search, it only supports actual data read and write operations in the forward direction. DECtape uses a significantly different mark track format to provide for the possibility of read and write operations in either direction, although not all DECtape controllers support reverse read.
DEC applied for a patent on the enhanced features incorporated into DECtape in late 1964.[Thomas C. Stockebrand, Bidirectional Retrieval of Magnetically Recorded Data, , issued June 4, 1968.] It is notable that the inventor listed on this patent, Thomas Stockebrand, is also an author of the paper on the TX-2 tape system from which the LINC tape was derived.
Eventually, the TC12-F tape controller on the PDP-12
The PDP-12 (Programmed Data Processor) was created by Digital Equipment Corporation (DEC) in 1969 and was marketed specifically for science and engineering.see Mini-Computer section and press see more, then press see more again It was the third i ...
supported both LINCtape and DECtape on the same transport. As with the earlier LINC-8
LINC-8 was the name of a minicomputer manufactured by Digital Equipment Corporation between 1966 and 1969. It combined a LINC computer with a PDP-8 in one cabinet, thus being able to run programs written for either of the two architectures.
Archit ...
, the PDP-12 is a PDP-8 augmented with hardware support for the LINC
The LINC (Laboratory INstrument Computer) is a 12-bit, 2048-word transistorized computer. The LINC is considered by some the first minicomputer and a forerunner to the personal computer. Originally named the "Linc", suggesting the project's o ...
instruction set and associated laboratory peripherals.
Technical details
DECtape was designed to be reliable and durable enough to be used as the main storage medium for a computer's operating system
An operating system (OS) is system software that manages computer hardware, software resources, and provides common services for computer programs.
Time-sharing operating systems schedule tasks for efficient use of the system and may also in ...
(OS). It is possible, although slow, to use a DECtape drive to run a small OS such as OS/8
OS/8 is the primary operating system used on the Digital Equipment Corporation's PDP-8 minicomputer.
PDP-8 operating systems which precede OS/8 include:
* R-L Monitor, also referred to as MS/8.
* P?S/8, requiring only 4K of memory.
* PDP-8 4K ...
or OS/12. The system would be configured to put temporary swap file
Swap or SWAP may refer to:
Finance
* Swap (finance), a derivative in which two parties agree to exchange one stream of cash flows against another
* Barter
Science and technology
* Swap (computer programming), exchanging two variables in the ...
s on a second DECtape drive, so as to not slow down access to the main drive holding the system programs.
Upon its introduction, DECtape was considered a major improvement over hand-loaded paper tapes, which could not be used to support swap files essential for practical timesharing
In computing, time-sharing is the sharing of a computing resource among many users at the same time by means of multiprogramming and multi-tasking.DEC Timesharing (1965), by Peter Clark, The DEC Professional, Volume 1, Number 1
Its emergence a ...
. Early hard disk
A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical data storage device that stores and retrieves digital data using magnetic storage with one or more rigid rapidly rotating platters coated with magnet ...
and drum drives were very expensive, limited in capacity, and notoriously unreliable, so the DECtape was a breakthrough in supporting the first timesharing systems on DEC computers. The legendary PDP-1 at MIT
The Massachusetts Institute of Technology (MIT) is a private land-grant research university in Cambridge, Massachusetts. Established in 1861, MIT has played a key role in the development of modern technology and science, and is one of the m ...
, where early computer hacker
A hacker is a person skilled in information technology who uses their technical knowledge to achieve a goal or overcome an obstacle, within a computerized system by non-standard means. Though the term ''hacker'' has become associated in popu ...
culture developed, adopted multiple DECtape drives to support a primitive software sharing community. The hard disk system (when it was working) was considered a "temporary" file storage device used for speed, not to be trusted to hold files for long-term storage. Computer users would keep their own personal work files on DECtapes, as well as software to be shared with others.
The design of DECtape and its controllers is quite different from any other type of tape drive or controller at the time. The tape is wide, accommodating 6 data tracks, 2 mark tracks, and 2 clock tracks, with data recorded at roughly 350 bits per inch (138 bits per cm). Each track is paired with a non-adjacent track for redundancy by wiring the tape heads in parallel; as a result the electronics only deal with 5 tracks: a clock track, a mark track and 3 data tracks. Manchester encoding (PE) was used. The clock and mark tracks are written only once, when the tape was formatted; after that, they are read-only. This meant a "drop-out" on one channel could be tolerated; even a hole punched through the tape with a hole punch will not cause the read to fail.
Another reason for DECtape's unusually high reliability is the use of laminated tape: the magnetic oxide is sandwiched between two layers of mylar
BoPET (biaxially-oriented polyethylene terephthalate) is a polyester film made from stretched polyethylene terephthalate (PET) and is used for its high tensile strength, chemical and dimensional stability, transparency, reflectivity, gas and aro ...
, rather than being on the surface as was common in other magnetic tape types. This allows the tape to survive many thousands of passes over the tape heads without wearing away the oxide layer, which would otherwise have occurred in heavy swap file use on timesharing systems.
The fundamental durability and reliability of DECtape was underscored when the design of the tape reel mounting hubs was changed in the early 1970s. The original machined metal hub with a retaining spring was replaced by a lower cost single-piece plastic hub with 6 flexible arms in a "starfish" or "flower" shape. When a defective batch of these new design hubs was shipped on new DECtape drives, these hubs would loosen over time. As a result, DECtape reels would fall off the drives, usually when being spun at full speed, as in an end-to-end seek. The reel of tape would fall onto the floor and roll in a straight line or circle, often unspooling and tangling the tape as it went. In spite of this horrifying spectacle, desperate users would carefully untangle that tape and wind it laboriously back onto the tape reel, then re-install it onto the hub, with a paper shim to hold the reel more tightly. The data on the mangled DECtape could often be recovered completely and copied to another tape, provided that the original tape had only been creased multiple times, and not stretched or broken. DEC quickly issued an Engineering Change Order (ECO) to replace the defective hubs, to resolve the problem.
Eventually, a heavily used or abused DECtape begins to become unreliable. The operating system is usually programmed to keep retrying a failed read operation, which often succeeds after multiple attempts. Experienced DECtape users learned to notice the characteristic "shoe-shining" motion of a failing DECtape as it is passed repeatedly back and forth over the tape heads, and would retire the tape from further use.
On non-DEC computers
Computer Operations Inc (COI) of Beltsville, Maryland
Beltsville is a census-designated place (CDP) in northern Prince George's County, Maryland, United States. The community was named for Truman Belt, a local landowner. The 2020 census counted 20,133 residents. Beltsville includes the unincorporated ...
offered a DECtape clone in the 1970s.
Initially, COI offered LINC-tape drives for computers made by
Data General
Data General Corporation was one of the first minicomputer firms of the late 1960s. Three of the four founders were former employees of Digital Equipment Corporation (DEC).
Their first product, 1969's Data General Nova, was a 16-bit minicomputer ...
, Hewlett-Packard
The Hewlett-Packard Company, commonly shortened to Hewlett-Packard ( ) or HP, was an American multinational information technology company headquartered in Palo Alto, California. HP developed and provided a wide variety of hardware components ...
and Varian, with only passing reference to its similarity to DECtape. While DECtape and LINC tape are physically interchangeable, the data format COI initially used for 16-bit minicomputers was distinct from both the format used by the LINC
The LINC (Laboratory INstrument Computer) is a 12-bit, 2048-word transistorized computer. The LINC is considered by some the first minicomputer and a forerunner to the personal computer. Originally named the "Linc", suggesting the project's o ...
and the format used on DECtape. When COI offered the ''LINC Tape II'' with support for the DEC PDP-8
The PDP-8 is a 12-bit minicomputer that was produced by Digital Equipment Corporation (DEC). It was the first commercially successful minicomputer, with over 50,000 units being sold over the model's lifetime. Its basic design follows the pionee ...
, PDP-11
The PDP-11 is a series of 16-bit minicomputers sold by Digital Equipment Corporation (DEC) from 1970 into the 1990s, one of a set of products in the Programmed Data Processor (PDP) series. In total, around 600,000 PDP-11s of all models were sold, ...
, Data General Nova
The Data General Nova is a series of 16-bit minicomputers released by the American company Data General. The Nova family was very popular in the 1970s and ultimately sold tens of thousands of units.
The first model, known simply as "Nova", was ...
, Interdata 7/32
The Model 7/32 and Model 8/32 were 32-bit minicomputers introduced by Perkin-Elmer after they acquired Interdata, Inc., in 1973. Interdata computers are primarily remembered for being the first 32-bit minicomputers under $10,000. The 8/32 was a ...
, HP 2100
The HP 2100 is a series of 16-bit minicomputers that were produced by Hewlett-Packard (HP) from the mid-1960s to early 1990s. Tens of thousands of machines in the series were sold over its twenty-five year lifetime, making HP the fourth largest mi ...
, Honeywell 316
The Honeywell 316 was a popular 16-bit minicomputer built by Honeywell starting in 1969. It is part of the Series 16, which includes the Models 116 (1965, discrete), 316 (1969), 416 (1966), 516 (1966) and DDP-716 (1969). They were commonly used f ...
and several other computers in 1974, the drive was priced at $1995 and was explicitly advertised as being DECtape compatible.
In 1974, DEC charged COI with patent infringement. COI, in turn, filed a suit claiming that DEC's patent was invalid on several grounds, including the assertions that DEC had marketed DECtape-based equipment for over a year before filing for the patent, that they had failed to properly disclose the prior art, and that the key claims in the DEC patent were in the public domain. The US Patent and Trademark Office
The United States Patent and Trademark Office (USPTO) is an List of federal agencies in the United States, agency in the United States Department of Commerce, U.S. Department of Commerce that serves as the national patent office and trademark ...
ruled DEC's patent invalid in 1978. The court case continued into the 1980s.
DECtape II
DECtape II was introduced around 1978 and has a similar block structure, but uses a much smaller tape (the same width as an audio compact cassette
The Compact Cassette or Musicassette (MC), also commonly called the tape cassette, cassette tape, audio cassette, or simply tape or cassette, is an analog magnetic tape recording format for audio recording and playback. Invented by Lou Ottens ...
). The tape is packaged in a special, pre-formatted DC150 miniature cartridge consisting of a clear plastic cover mounted on a textured aluminum plate. Cartridge dimensions are . The TU58 DECtape II drive has an RS-232
In telecommunications, RS-232 or Recommended Standard 232 is a standard originally introduced in 1960 for serial communication transmission of data. It formally defines signals connecting between a ''DTE'' (''data terminal equipment'') such a ...
serial interface
In computing, a serial port is a serial communication interface through which information transfers in or out sequentially one bit at a time. This is in contrast to a parallel port, which communicates multiple bits simultaneously in parallel. T ...
, allowing it to be used with the ordinary serial ports that are very common on Digital's contemporary processors.
Because of its low cost, the TU58 was fitted to several different systems (including the VT103
The VT100 is a video terminal, introduced in August 1978 by Digital Equipment Corporation (DEC). It was one of the first terminals to support ANSI escape codes for cursor control and other tasks, and added a number of extended codes for special f ...
, PDP-11/24 and /44 and the VAX-11/730 and /750) as a DEC-standard device for software product distribution, and for loading diagnostic program
A diagnostic program (also known as a Test Mode) is an automatic computer program sequence that determines the operational status within the software, hardware, or any combination thereof in a component, a system, or a network of systems. Diagno ...
s and microcode
In processor design, microcode (μcode) is a technique that interposes a layer of computer organization between the central processing unit (CPU) hardware and the programmer-visible instruction set architecture of a computer. Microcode is a laye ...
. The first version of the TU58 imposed very severe timing constraints on the unbuffered UART
A universal asynchronous receiver-transmitter (UART ) is a computer hardware device for asynchronous serial communication in which the data format and transmission speeds are configurable. It sends data bits one by one, from the least significan ...
s then being used by Digital, but a later firmware revision eased the flow-control problems. The RT11 single-user operating system can be bootstrapped from a TU58, but the relatively slow access time of the tape drive makes use of the system challenging to an impatient user.
Like its predecessor DECtape, and like the faster RX01 floppies used on the VAX-11/780
The VAX-11 is a discontinued family of 32-bit superminicomputers, running the Virtual Address eXtension (VAX) instruction set architecture (ISA), developed and manufactured by Digital Equipment Corporation (DEC). Development began in 1976. In ad ...
, a DECtape II cartridge has a capacity of about 256 kilobytes. Unlike the original DECtape media, DECtape II cartridges cannot be formatted on the tape drive transports sold to end-users, and have to be purchased in a factory pre-formatted state.
The TU58 is also used with other computers, such as the Automatix
Automatix Inc., founded in January 1980, was the first company to market industrial robots with built-in machine vision. Its founders were Victor Scheinman, inventor of the Stanford arm; Phillippe Villers, Michael Cronin, and Arnold Reinhold o ...
Autovision machine vision
Machine vision (MV) is the technology and methods used to provide imaging-based automatic inspection and analysis for such applications as automatic inspection, process control, and robot guidance, usually in industry. Machine vision refers to m ...
system and AI32 robot controller. TU58 driver software is available for modern PCs running DOS
DOS is shorthand for the MS-DOS and IBM PC DOS family of operating systems.
DOS may also refer to:
Computing
* Data over signalling (DoS), multiplexing data onto a signalling channel
* Denial-of-service attack (DoS), an attack on a communicat ...
.TU58 Driver
/ref>
Early production TU58s suffered from some reliability and data interchangeability problems, which were eventually resolved. However, rapid advances in low-cost floppy disk
A floppy disk or floppy diskette (casually referred to as a floppy, or a diskette) is an obsolescent type of disk storage composed of a thin and flexible disk of a magnetic storage medium in a square or nearly square plastic enclosure lined w ...
technology, which had an inherent speed advantage, soon outflanked the DECtape II and rendered it obsolete.
See also
* LINC
The LINC (Laboratory INstrument Computer) is a 12-bit, 2048-word transistorized computer. The LINC is considered by some the first minicomputer and a forerunner to the personal computer. Originally named the "Linc", suggesting the project's o ...
additional material on LINCtape lineage and operation
References
External links
TU56 DECtape Drive Information
DECtape Documentation
at bitsavers.org
VT103 manual
at bitsavers.org. Appendix A describes the TU58 interface protocol.
{{Digital Equipment Corporation
DEC hardware
History of computing hardware
Computer-related introductions in 1963