Real-Time Clock Options

Summary

This document provides a technical overview of the DK8-EA and DK8-EC real-time clock options for the PDP-8/E computer. These options are single-quad modules that plug into the OMNIBUS to trigger program interrupts at fixed intervals for tasks such as event counting or process sampling.

Key technical details include:

• Operational Differences: The DK8-EA generates clock frequencies (100 Hz or 120 Hz) based on primary power supply AC voltage, whereas the DK8-EC uses a 20-MHz crystal-controlled oscillator to derive frequencies of 1 Hz, 50 Hz, 500 Hz, or 5 kHz.

• Logic Components: The hardware utilizes three main logical sections:

  • Select Logic: Decodes IOT instructions (6131, 6132, 6133).
  • Interrupt Flip-Flop Logic: Manages the enabling and disabling of interrupt requests.
  • Clock Flag Logic: Handles the SKIP signal and interrupt servicing.

• Instruction Set: The system uses specific instructions to manage operation: 6131 (set interrupt), 6132 (clear interrupt), and 6133 (skip on clock flag).

• System Constraints: Because the clock is free-running, the software must be capable of handling simultaneous interrupts, and there is an inherent one-count uncertainty due to the unknown clock phase at the moment of enabling.