555/550 Micro-Tape Dual Transport & Tape Control

This document provides a preliminary technical overview and operational specifications for the Digital Equipment Corporation Micro-Tape Type 555 Dual Tape Transport and the Type 550 Tape Control.

Key Features and Technical Specifications:

• Transport Capabilities: The Type 555 consists of two independent tape drives handling 250-foot reels of 3/4-inch Mylar tape. It operates at 80 inches per second with a transfer rate of 180,000 bits per second.
• Data Organization: Information is stored in blocks of 256 18-bit words using a 10-track recording head. The system utilizes redundant recording for reliability and a serially coded "Mark Track" to allow for exact, drum-like tape addressing.
• Control System: The Type 550 Control manages up to four 555 Transports (8 drives) and handles the assembly/disassembly of data between the tape and the computer (PDP-1 or PDP-4). It manages various modes including Search, Read, Write, and Move.
• Operating Logic: Data is validated using check sums at the end of each block. The control uses flags (Data, Block, and Error) to signal the processor during tape operations.
• Programming: The document includes an instruction list for the PDP-4, detailing commands such as Read (mmrd), Write (mmwr), Select (mmse), and various status/flag checks to facilitate integration with the computer.