microtapeDecusDec63

This document describes the Microtape system, introduced by Digital Equipment Corporation (DEC) in 1963, as a novel magnetic tape storage solution designed to combine the flexibility, speed, and storage capacity of traditional magnetic tape with the physical convenience of paper tape.

The system comprises the Type 555 Dual Microtape Transport (featuring two drives, small 3.5-inch reels of 3/4-inch Mylar tape, operating at 80 inches/second with a data transfer rate of 90,000 bits/second) and the Type 550 Microtape Control.

Key features and technological innovations include:

• Manchester type polarity-sensed recording with a dedicated, pre-recorded timing track, ensuring high reliability and insensitivity to tape speed variations.
• Redundant recording across five information tracks, with data read by analog summation of two heads for enhanced error detection and prevention.
• Bi-directional read/write capabilities, facilitated by "complement obverse" mark patterns that appear consistent regardless of tape direction.
• Individually addressable blocks and words, enabled by a crucial "mark track" that provides program synchronization, error checking, block/word identification, and protection for control information.
• A non-start-stop operation mode suitable for continuous, large data transfers, minimizing delays.
• Flexible block formatting, allowing blocks of varying lengths.
• Ease of physical use, with simple reel loading and small, portable reels.

The Microtape system is supported by programmed subroutines for basic operations (Search, Read, Write) and specialized utilities like MICROTOG (for diagnostics and tape preparation) and MICROTRIEVE (for program/data storage and retrieval). It notably supports multi-programming during search operations, optimizing performance in multi-user environments.

Highlighted applications include:

• Efficient storage and retrieval of programs and data, offering portability and in-place modification.
• Support for on-line, multi-user systems by enabling fast random access and minimizing waiting times.
• Continuous data logging for sampled information, avoiding interruptions inherent in start-stop systems.
• Potential for efficient sorting and merging algorithms due to its continuous motion and bi-directional capabilities.

Future developments aim to enhance compatibility with newer computer architectures (e.g., PDP-5 and PDP-6) by supporting variable word lengths and automated check sum handling.