H-551P DECtape Control 551 Preliminary

This document is a preliminary technical manual (H-551P) for the Digital Equipment Corporation (DEC) DECtape Control Type 551, copyrighted 1965. It contains proprietary information intended for customers to use and maintain DEC equipment.

The manual describes the DECtape Control Type 551 as a core component of a DECtape system, which connects up to four Type 555 Dual DECtape Transports (eight drives) to the PDP-6 Arithmetic Processor via the Data Control Type 136.

Key aspects covered in the manual include:

• Purpose and Functional Description: Explains that the DECtape system stores information at fixed positions on magnetic tape, allowing random block replacement. It outlines the 551's role in handling read/write operations, block and error detection, and tape motion control, synchronizing with prerecorded timing and mark tracks. It details the 10-track redundant recording format and Manchester phase recording for data reliability.
• System Architecture: Illustrates the relationship between the DECtape Control, Data Control, and Tape Transports, including data flow (3-bit, 6-bit, 36-bit words).
• Recording Format: Provides in-depth information on the 5-channel tape format, the role of timing and mark tracks (for control codes, block marks, checksums, end-of-tape zones), and how the symmetrical format enables both forward and reverse reading/writing. It also describes key internal hardware registers.
• Theory of Operation: Details the operational logic, including initialization, command decoding (read, write, block mark, timing/mark track operations), data transmission control, checksum computations, and comprehensive automatic error checking (mark track, active, illegal operation, incomplete block detection). It also covers status checking and program interrupts.
• Interface: Lists detailed interface signals between the DECtape Control and the I/O Bus, Type 555 Tape Transports, and Data Control Type 136.
• Maintenance: Provides extensive procedures for both preventive maintenance (mechanical checks, power supply verification, specific module adjustments, marginal checks) and corrective maintenance (troubleshooting, repair guidelines, validation tests, and log entries). It also includes recommended spare parts lists (modules, semiconductors, pulse transformers, and miscellaneous components).
• Engineering Drawings: An appendix provides reduced copies of block schematics, circuit schematics, and other engineering drawings, along with an explanation of circuit symbols and module identification.

Notably, the "Programming" chapter (Chapter 5) is marked "To be supplied," indicating the preliminary nature of this specific manual.