AC input system for computer-based control system

In a computer system operative to provide control signals in response to a plurality of input signals selectively interrogated, each interrogated input signal normally representing an AC power voltage of fundamental frequency in the form of a first train of pulses of equal width having successive high and low digital levels at the frequency and phase of said AC power voltage;

• the combination of:
• means responsive to said AC power voltage for generating a reference signal in the form of a reference train of pulses having high and low digital levels of the same width as normally for said first train, at the same said frequency, and shifted therefrom by a predetermined delay;
• means responsive to said first train of pulses for sampling the digital level thereof;
• first upon the occurrence of one of a falling and rising edge in said reference train, secondly a half-cycle thereafter;
• thereby to derive consecutive first and a second samples thereupon;
• and
• means for comparing said first and second samples for detecting in relation to a predetermined lack of parity between consecutive first and second samples an indication of invalidity of said interrogated input signal.