Editing DECtape (section)

== Technical details ==
[[File:Linc-8.jpg|thumb|A partially restored LINC-8,<br> including LINCtape drives]]
DECtape was designed to be reliable and durable enough to be used as the main storage medium for a computer's [[operating system]] (OS). It is possible, although slow, to use a DECtape drive to run a small OS such as [[OS/8]] or OS/12. The system would be configured to put temporary [[swap file]]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]]. Early [[hard disk]] and [[drum memory|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]], where early computer [[hacker (term)|hacker]] 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 {{convert|0.75|in|mm|abbr=on}} 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 code|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.<ref>[https://archive.org/stream/bitsavers_decdectape_3543487/DEC-00-HZTA-D_TU55_May69#page/n9/mode/1up ''TU55 DECtape 55 Instruction Manual''], DEC-00-HZTA-D, Digital Equipment Corporation, Maynard Mass., Sept. 1968; sections 1.4 and 1.5.</ref> This meant a "drop-out" on one channel could be tolerated; even a hole punched through the tape with a {{convert|0.25|in|mm|abbr=on}} hole punch will not cause the read to fail.<ref>{{cite book|url=http://www.bitsavers.org/pdf/dec/dectape/tu55/H-TU55_MaintMan_Nov66.pdf|title=Instruction Manual - DECtape Transport TU55|publisher=Digital Equipment Corporation|location=Maynard, MA|date=1966}}</ref>

Another reason for DECtape's unusually high reliability is the use of laminated tape: the magnetic oxide is sandwiched between two layers of [[mylar]], 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.<ref>{{Cite web|url=https://classiccmp.org/pipermail/cctalk/2015-November/015310.html|title=DECtape reliability?|first=Paul|last=Koning|date=November 12, 2015}}</ref>

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.