H-550 550dectapeCtlr

This document describes the DECtape Control Type 550, a system designed to offer the flexibility, speed, and storage of magnetic tape with the convenience of paper tape, specifically for use with PDP-1, PDP-4, and PDP-7 computers.

Key Characteristics and Operation:

1. Program Break Control: The 550 Control operates as a "program break control," transferring information word-by-word rather than in blocks. This allows for individual word access but requires more direct computer intervention.
2. Components: It consists of a control unit (electronic circuitry for logical and timing functions) and a transport unit (tape handling, drive mechanism, head switching). It can control up to four tape transport units.
3. Recording & Format: DECtape uses a Manchester-type polarity sensing recording technique across ten tracks (five duplexed channels), including a timing track, a mark track, and three data tracks. An 18-bit word is stored across six lines of three bits each.
4. Prerecorded Tracks: The timing and mark tracks are "prerecorded" (written by a specific program) and are crucial for operation. They provide program synchronization, block end detection, error checking, protection of control information, and enable block and word addressability.
5. Operational Modes: The system supports Search, Read, Write, and Move modes. These modes are managed by reading the mark track using an 8-bit "moving window" register, which decodes patterns and raises "flags" to signal the computer for data transfers or other actions.
6. Bidirectional Ability: A unique feature is its robust bidirectional capability, aided by "complement obverse" mark track patterns and end marks, ensuring consistent data interpretation regardless of tape direction.
7. Data Transfer: Information flows between the computer's accumulator/I/O, the control unit's in-out register, and a shift register, then to/from the tape heads. Timing pulses (TP1, TP0, TP2) derived from the timing track synchronize these transfers.
8. Error Detection: The system detects various error conditions, including "unable" (e.g., WRITE LOCK switch not on), "misindication" (flags not cleared), "off end" (tape reaches physical end), and "mark track error" (inconsistencies in mark track data).

Programming and Maintenance:

1. Programming Interface: Users interact with the DECtape via Input-Output Transfer (IOT) instructions to select units, control motion, set modes (e.g., Move, Search, Read, Write, Write Timing & Mark Track), and retrieve status information.
2. Preprogrammed Subroutines: Digital Equipment Corporation provides subroutines for basic search, read, and write operations. Specialized routines like "Microtog" are used for diagnostics and maintenance, while "Microtrieve" enables quick saving and retrieval of programs/data.
3. Maintenance: Maintenance involves both preventive and corrective measures. Preventive tasks include cleaning, visual inspection, and "marginal checks" (testing circuits at varied voltage levels to pre-empt failures). Corrective maintenance addresses issues like strobe timing errors and general circuit troubleshooting, often aided by specialized test programs (e.g., Rocker, Exerciser, Microtog).