F25 PDP1 IO

This "PDP-1 Input-Output Systems Manual (Preliminary Manual)" details the electrical, physical, and programming aspects of connecting external devices to the PDP-1 computer.

The document introduces two types of signal flow: information transfers (simultaneous 18-bit data movement) and control pulses (single-line signals for sampling or triggering actions). The primary method for I/O is the In-Out Transfer (IOT) command (operation code 72XXXX), where specific bits in the command address devices and control synchronization. IOT operations typically involve a two-step process: clearing the target register and then transferring information.

The manual describes how information is transmitted from the PDP-1's In-Out (IO) register to external devices (e.g., a buffer register for digital-to-analog conversion) and received into the IO register from devices (e.g., an analog-to-digital converter). It highlights the importance of synchronization between the faster computer and slower I/O devices, explaining how programs can wait for device completion or continue computations and resynchronize later.

Detailed operations are provided for standard I/O equipment:

• Point Plotting Oscilloscope: Uses the display command to plot points by transferring data from the Accumulator and IO register to X/Y coordinate buffers.
• Paper Tape Punch: Commands like ppa (alphanumeric) and ppb (binary) perforate tape based on data in the IO register.
• Typewriter: tyo sends characters for output, while tyi reads input characters into the IO register.
• Photoelectric Tape Reader: rpa (alphanumeric) and rpb (binary) read tape data into a buffer, which is then accessible to the IO register.

For more sophisticated I/O handling, the manual outlines:

• The Sequence Break System (SBS): An interrupt-driven mechanism for handling asynchronous device requests. It saves the program's state (Accumulator, Program Counter, IO register) and redirects execution to a specific location when an external device needs attention. A more advanced 16-channel SBS offers prioritized interrupts for multiple devices.
• High Speed Channels: An optional feature for direct memory access (DMA) block transfers between memory and I/O devices (like magnetic tape), designed to reduce program overhead and allow concurrent computations.

Appendices list available IOT commands with their definitions and provide a schedule of interconnections for various PDP-1 I/O lines.