PDP-X Technical Memorandum #11

This 1967 PDP-X Technical Memorandum #11, authored by S. Booth, outlines considerations for the DECtape tape format for the PDP-X computer, explicitly excluding I/O commands or status words.

The core proposal is to use an 8-bit byte as the basic unit of parallel transfer, compatible with other PDP-X I/O components. The suggested format for PDP-X uses three lines of tape to record these 8 bits (plus optional parity).

To achieve flexibility and compatibility, two read/write assembly/disassembly modes are proposed:

• Mode 1: Assembles three tape lines into one 8-bit byte. This is intended for normal PDP-X system usage, interchange, and general formatting operations.
• Mode 2: Assembles two tape lines into one 8-bit byte. This mode is specifically designed for tape formatting, diagnostics, and enabling communication/interchange with non-PDP-X machines (like PDP-8/9, which use a 2-line per byte format).

Regarding data block structure, the document suggests blocks of 128 or 256 words, influenced by existing PDP-8/9 usage.

Key considerations and justifications include:

• A deliberate trade-off where an 11% reduction in data density (1 unused bit out of 9 total tracks) is accepted to simplify hardware for assembly/disassembly, deemed reasonable given the low cost of DECtape storage.
• The benefits of the two modes include smoother programming, improved tape formatting capabilities, and crucial compatibility with other PDP machines.
• Transfer timings are estimated, with Mode 1 at approximately 70 µs per byte. The memo also briefly touches upon future possibilities like a ROS controller for read/write in opposite directions.