Editing DECtape (section)

==Origins==
DECtape has its origin in the '''LINCtape''' tape system,<ref name=DECbook/>{{rp||page=215|quote=The DECtape was derived from M.I.T.'s Lincoln Laboratory LINCtape unit}} which was originally designed by [[Wesley A. Clark|Wesley Clark]] at the MIT [[Lincoln Laboratory]] as an integral part of the [[LINC]] computer. There are simple LINC instructions for reading and writing tape blocks using a single machine instruction.<ref>Mary Allen Wilkes and Wesley A. Clark, 18: Magnetic Tape Instructions, [https://archive.org/stream/bitsavers_washingtoneferenceManualsLINCVol16Section2Programm_6160276/LINC_Vol_16_Section_2_Programming_the_LINC_Jun65#page/n87/mode/1up Programming the LINC], LINC Volume 16, Section 2, June, 1965; pages 80-104.</ref> 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]] computer, designed by Richard L. Best and T. C. Stockebrand. The [[TX-2 Tape System]] 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).<ref name=BestStockerbrand>R. L. Best and T. C. Stockerbrand, A Computer-Integrated Rapid Access Magnetic Tape System with Fixed Address, [http://dl.acm.org/citation.cfm?id=1457769.1457783 Proceedings of the Western Joint Computer Conference: Contrasts in Computers], May 6–8, 1958; pages 42-46.</ref><ref>Herbert R. Johnson, [http://www.retrotechnology.com/restore/linc_tapes.html#fit Tape reels and hubs - "fit"] section of [http://www.retrotechnology.com/restore/linc_tapes.html LINC, LINCtape, DECtape], November 26, 2013.</ref>

Digital initially introduced the Type 550 Microtape Control and Type 555 Dual Microtape Transport as peripherals for the [[PDP-1]] and [[PDP-4]] 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]] and 36-bit [[PDP-6]], even though this involved a change in recording format.<ref>Levin H. Campbell, [http://law.justia.com/cases/federal/appellate-courts/F2/653/701/313062/ Court ruling], Digital Equipment Corporation, Plaintiff, Appellant, v. Sidney A. Diamond, Etc., et al., 653 F.2d 701 (1st Cir. 1981), June 12, 1981; see paragraph 5 for the chronology of introduction.</ref><ref>Leonard M. Hantman, Microtape: Its Features and Applications, [https://archive.org/stream/bitsavers_decdectape_1101439/microtapeDecusDec63#page/n2/mode/1up Second Annual Meeting of the Digital Equipment Corporation User's Society (DECUS)], Lawrence Radiation Laboratories, Livermore, Nov. 18-19, 1963; see the ''Future Trends'' section, page 15.</ref>  The initial specifications for the Type 550 controller discuss a significant advance beyond the LINCtape, the ability to read and write in either direction.<ref>[https://archive.org/stream/bitsavers_decdectape_2552119/F-13_555brochure_May63#page/n0/mode/1up 555/550 Micro-Tape Dual Transport & Tape Control], Digital Equipment Corporation, May, 1963.</ref>  By late 1964, the Type 555 transport was being marketed as a DECtape transport.<ref>[https://archive.org/stream/bitsavers_decdectapeDec64_1008579/H-555_555dectapeDrive_Dec64#page/n5/mode/1up 555 DECtape Dual Transport], Digital Equipment Corporation, H-555, Dec. 1964; The start of Chapter 1 contains the term 'DECtape'.</ref>

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.<ref name=DECtapePatent>Thomas C. Stockebrand, Bidirectional Retrieval of Magnetically Recorded Data, {{US Patent|3,387,293}}, issued June 4, 1968.</ref>  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.<ref name=BestStockerbrand />

Eventually, the TC12-F tape controller on the [[PDP-12]] supported both LINCtape and DECtape on the same transport. As with the earlier [[LINC-8]], the PDP-12 is a PDP-8 augmented with hardware support for the [[LINC]] instruction set and associated laboratory peripherals.