DECtape
DECtape was a
magnetic tape storage medium used with early
Digital Equipment Corporation computers, including the
PDP-6,
PDP-8,
LINC-8,
PDP-10,
PDP-11,
PDP-12, and the
PDP-15. 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. Block size was 128 words. From a programming point of view, DECtape behaved like a very slow disk drive.
DECtape was controversial for its amazing similarity to
LINCtape, 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 appeared to many to have been "designed" by the simple process of systematically reversing every arbitrary design feature of LINCtape. Thus, LINCtape and DECtape had opposite placements of feed and takeup reels, wrote signals to tape with opposite magnetic polarities, and wrote the bits of a byte in different orders. These changes had no obvious engineering purpose other than to make LINCtape and DECtape different from each other. Mechanical dimensions, speeds, and signal characteristics were identical, and Digital's hybrid machines, such as the PDP-12, featured units were capable of operating in either a LINCtape or DECtape mode. Digital patented the DECtape; the patents were tested in court and ruled invalid.
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.
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.
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LINC - additional material on LINCtape lineage and operation
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TU56 DECtape Drive Information
