DECtape

PDP-11 with TU56 dual DECtape unit. Note the 6-armed "starfish" hubs holding circular white tape reels in place. Read/write heads are housed in the white vertical rectangles above the tape reels.

DECtape, originally called "Microtape", was a magnetic tape data storage medium used with many Digital Equipment Corporation computers, including the PDP-6, PDP-8, LINC-8, PDP-10, PDP-11, PDP-12, and the PDP-15. On DEC's 32-bit systems, VAX/VMS support for it was implemented but did not become an official part of the product lineup. DECtapes were 3/4 inch wide, and formatted into blocks of data that could each be read or written individually. Each tape stored 184K 12-bit PDP-8 words or 144K 18-bit words. Block size was 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, DECtape behaved like a very slow disk drive.

Origins

DECtape had its origin in the LINCtape tape system, which was originally designed by Wesley Clark at the MIT Lincoln Laboratory as an integral part of the LINC computer. 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 used a physically incompatible tape format (1/2 inch Tape on 10 inch reels).^[1]

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. At the time of introduction, plans were already in place for modifications to the data format to allow support of 12 and 36-bit word sizes.^[2] The LINC used the same transport, but with its own incompatible controller. The LINCtape and DECtape formats were incompatible, with the tape run in the opposite direction (reversing the positions of the supply and takeup reels), although the mechanical dimensions, speeds, and signal characteristics were identical, and eventually, the TC12-F tape controller on the PDP-12 supported both LINCtape and DECtape on the same transport.

On the PDP-12, the DECtape drives were tightly integrated into the LINC CPU instruction set. There were simple LINC instructions for reading and writing tape blocks using a single machine instruction.

While LINCtape was designed to support high-speed bidirectional block search, it only supported actual data read and write operations in the forward direction. DECtape used a significantly different mark track format to provide for the possibility of read and write operations in either direction. Some but not all DECtape controllers supported reverse read. This bidirectional data transfer capability was covered by U.S. Patent 3,387,293.

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 (OS). It was 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 files 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 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 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 was quite different from any other type of tape drive or controller at the time. The tape was 0.75 in (19 mm) 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 was paired with a non-adjacent track for redundancy by wiring the tape heads in parallel; as a result the electronics only dealt with 5 tracks: a clock track, a mark track and 3 data tracks. Manchester encoding (PE) was used. The clock and mark tracks were written only once, when the tape was formatted; after that, they were read-only.^[3] This meant a "drop-out" on one channel could be tolerated; even a hole punched through the tape with a 0.25 in (6.4 mm) hole punch would not cause the read to fail.^{[citation needed]} Another reason for DECtape's unusually high reliability was the use of laminated tape: the magnetic oxide was sandwiched between two layers of mylar, rather than being on the surface as was common in other magnetic tape types. This allowed 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.^{[citation needed]}

Eventually, a heavily-used or abused DECtape would start to become unreliable. The operating system was usually programmed to keep retrying a failed read operation, which often would succeed after multiple attempts. Experienced DECtape users learned to notice the characteristic "shoe-shining" motion of a failing DECtape as it was passed repeatedly back and forth over the tape heads, and would retire the tape from further use.

DECtape II

DECtape (top and lower left) and DECtape II (lower right) removable magnetic media

DECtape II was introduced around 1978 and had a similar block structure, but used a much smaller 0.150 in (3.8 mm) tape^[4] (the same width as an audio compact cassette). The tape was packaged in a special, pre-formatted DC150 miniature cartridge consisting of a clear plastic cover mounted on a textured aluminum plate. Cartridge dimensions were 2 ³⁄₈ by 3 ³⁄₁₆ by ¹⁄₂ inches (60 × 81 × 13 mm). The TU58 DECtape II drive had an RS232 serial interface, allowing it to be used with the ordinary serial ports that were very common on Digital's contemporary processors.

Because of its low cost, the TU58 was fitted to several different systems (including the VT103, 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 programs and microcode. The first version of the TU58 imposed very severe timing constraints on the unbuffered UARTs then being used by Digital, but a later firmware revision eased the flow-control problems. The RT11 single-user operating system could be bootstrapped from a TU58, but the relatively slow access time of the tape drive made 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, a DECtape II cartridge had a capacity of about 256 kilobytes. Unlike the original DECtape media, DECtape II cartridges could not be formatted on the tape drive transports sold to end-users, and had to be purchased in a factory pre-formatted state.

The TU58 was also used with other computers, such as the Automatix Autovision machine vision system and AI32 robot controller. TU58 driver software is available for modern PCs.^{[citation needed]}

Early production TU58s suffered from some reliability and data interchangeability problems, which were eventually resolved. However, rapid advances in low-cost floppy disk technology, which had an inherent speed advantage, soon outflanked the DECtape II and rendered it obsolete.

See also

• LINC — 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.