DECtape

PDP-11 with TU56 dual DECtape unit

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. VAX/VMS support for it was implemented but did not become an official part of the product. DECtapes were 3/4 inch wide and formatted into blocks of data that could be read or written individually. One tape stored 184K 12-bit PDP-8 words or 144K 18-bit words. Block size was 129 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.

Details

DECtape was designed to be reliable and durable enough to be used as the main storage medium for an operating system. It was possible, if 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 files on a second DECtape drive, so as to not slow down access to the main drive with the system programs.

The design of DECtape and its controllers was quite different from any other type of tape drive or controller. Physically DECtape used dual-redundancy to keep the error rate low. Each bit was written twice across the width of the tape, using Manchester encoding (PE). During read, the two read heads for each bit were wired in series, so the resultant output was the sum of the two bit amplitudes. This meant a "drop-out" on one channel could be tolerated. You could even punch a hole in the tape with a 1/4 inch hole punch and that area would read correctly. Another reason for its unusually high reliability was the use of laminated tape: the magnetic oxide is sandwiched between two layers of mylar, rather than being on the surface as is common in other magnetic tape types.

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

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 in the public domain, and LINCtape drives were manufactured by several companies, including Digital. DECtape used the same transport mechanism as LINCtape, but the tape was run in the opposite direction, thus the supply and takeup reels were reversed. Mechanical dimensions, speeds, and signal characteristics were identical, and at least one system, the PDP-12 (with the TC12-F option), was capable of using either LINCtape or DECtape on the same transport.

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 is the subject of U.S. Patent 3,387,293 .

LINCtape's origin can be found in the magnetic tape system for the Lincoln Laboratory TX-2 computer. Best and Stockebrand's 1958 paper "A Computer-Integrated Rapid-Access Magnetic Tape System with Fixed Address" is the direct ancestor of LINCtape, including the use of two redundant sets of five tracks and a direct drive tape transport.

DECtape and DECtape II

DECtape II

DECtape II was introduced around 1979 and had a similar block structure, but on a special, pre-formatted DC150 miniature cartridge tape. 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. As a result, the TU58 was fitted to several different systems (including the PDP-11/24 and /44 and the VAX-11/730 and /750) as a commonly-available device for loading diagnostic programs. 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.

Like its predecessor, and like the RX01 floppies used on the VAX-11/780, a DECtape II cartridge had a capacity of about 256 kilobytes.

The TU58 was also used with other computers, such as the Automatix Autovision machine vision system and AI32 robot controller. TU58 drivers are available for modern PCs.

See also

• Leonard Hantman "MICROTAPE: Its Features and Applications" DECUS Proceedings Nov, 1963
• LINC - additional material on LINCtape lineage and operation

External links

• TU56 DECtape Drive Information
• DECtape Documentation at bitsavers.org