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Input Signal Response Time

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    Last Modified: September 6, 2017

    Input signal response time describes the total time it takes to charge or discharge line load capacitance, propagate the signal to the User Program, and pass the signal through the debounce filter. Input signal response time applies to all input configurations, including single inputs, dual inputs, and inputs with test pulses.

    Use the following equations to calculate input signal response times.

    (1 to 0) Input signal response time:

    Input signal response time = Input discharge time + Debounce filter time + Debounce constant + 15 µs

    (0 to 1) Single Input:

    Input signal response time = Debounce filter time + Debounce constant + 18 µs

    (0 to 1) Single Input with test pulse:

    Input signal response time = (2 × Debounce filter time) + (3 × Debounce constant) + Test pulse width + 39 µs

    (0 to 1) Dual Input:

    Input signal response time = Debounce filter time + Debounce constant + Discrepancy time + Discrepancy constant + 18 µs

    (0 to 1) Dual Input with test pulse:

    Input signal response time = (2 × Debounce filter time) + (3 × Debounce constant) + Discrepancy time + Discrepancy constant + Test pulse width + 39 µs

    Debounce filter time, discrepancy time, and test pulse widths are user-configurable parameters in the Functional Safety Editor.

    Use the following table to determine the debounce constant based on debounce filter time.

    Table 1. Debounce Constant
    Debounce Filter Time Debounce Constant
    0 µs ≤ debounce filter time ≤ 16,383 µs 2 µs
    16,500 µs ≤ debounce filter time ≤ 2,047.875 ms 126 µs
    2,125 ms < debounce filter time ≤ 5 s 126 ms

    Calculate the input discharge time using the following equation or use recommended input discharge times based on configuration, external load, cable length, and cable capacitance.

    t = R × ( C + 600 p F ) × ln ( R × 0.8 m A + 5.7 V R × 0.8 m A + 30 V )

    Table 2. Recommended Input Discharge Times
    Configuration External Load[1] Cable Length/Capacitance Input Discharge Time
    • Single output
    • Dual output
    • Single output with internal test pulse
    • Dual output with internal test pulse
    High Impedance ≤10 m and ≤1.8 nF 1,000 µs
    High Impedance ≤50 m and ≤9 nF 10,000 µs
    High Impedance >50 m 100 ms
    ≤3 kΩ ≤10 m and ≤1.8 nF 40 µs
    ≤3 kΩ ≤50 m and ≤9 nF 1,000 µs
    ≤3 kΩ >50 m 10,000 µs
    • Single output with external test pulse
    • Dual output with external test pulse
    • Single output with external readback
    • Single input with test pulse
    • Dual input with test pulse
    >3 kΩ ≤50 m and ≤9 nF 1,000 µs
    >3 kΩ >50 m 10,000 µs
    ≤3 kΩ ≤50 m and ≤9 nF 40 µs
    ≤3 kΩ >50 m 10,000 µs
    • 1 When the output load on the DO channel is a DI channel on the same module, load is >3 kΩ.

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