Topic: DMD0375
BRX Input Filters
The on-board discrete inputs on all of the BRX hardware platforms and the inputs on the BRX HSIO modules (BX-HSIO1, BX-HSIO2 ) can be configured to use digital filtering. Filters like this are typically used on inputs that are operating in electrically noisy environments to remove both "false positives" and "false negatives". They do this by requiring that the input signal for a discrete input must remain above or below the input's hardware threshold level longer than the filter time before the CPU will see the state change. Once the state change is recognized, it must remain at the opposite level longer than the filter time before the CPU will see the opposite state.
For those interested in how the filter is implemented internally, refer to the following diagram. The internal counter increments or decrements by 1 count every 13.33ns until it reaches the filter value. At that point it will initiate a state change if applicable. As shown, spurious counts in the wrong direction delay transitions in progress.
Choose Preferred Filter Scale sets the format for the values entered in the form for all of the fields. The Filter Scale can be specified in the following forms:
Note: be sure to select the Filter Scale before entering values in fields. If you change the Preferred Filter Scale after entering values then any values that are not valid in that scale will be set to 0. The Filter Scale can be specified in the following forms:
A frequency in the range of 0 - 250000 Hz.
A time value in nanoseconds in the range of 0 - 111848093.
The number of 13.33 nanosecond clocks in the range of 0 - 8388607.
An input filter value of 0 will use the default filter value of 1 microsecond. Selecting one form to specify the filter value will automatically show the filter value in the other two forms.
BRX models that have 120 / 240 VAC inputs will automatically have an internal filter value of 60 Hz (~4 ms) for the filter value that will not be displayed, any value entered will be in addition to the internal 60 Hz.
For BRX models that have 24 VDC / VAC inputs, if the input voltage is 24 VAC you need to manually set the filter value to a minimum of 60 Hz (~4 ms) to prevent the input from flickering OFF as the AC input signal cycles below the minimum voltage threshold.
Note: the default Input Filter value of 10ms (25Hz / 750000 clocks) is an appropriate value when the High-Speed I/O inputs are used with normal-speed discrete input devices such as proximity switches or push buttons. But the default value is NOT appropriate when the inputs are used for high-speed input devices like encoders. You will want to change this value to a much faster response time (lower filter value) to allow those high speed device to work as expected. Do-more Designer will display the following message box any time one of the High-Speed I/O inputs is selected for use in one of the high speed functions without changing the default filter time.
See Also:
BX-HSIO1 / BX-HSIO2 High-Speed I/O Modules
BRX Timer / Counter / Pulse Catch Functions
BRX Pulse Width Modulated Outputs