Digital Filtering

Digital filtering time is the settling time to within 1 LSB of the

 

analog-to-digital converter on the module. (Often digital filtering is specified

 

as a time constant in milliseconds. With a time constant specification, it will

 

take the input 4 to 5 time constants to reach 99% of the final value.)

 

The value entered is the actual settling time.

 

 

 

NOTE: In the SIMATIC 505±2557 module, the value used in digital filtering

 

is not a time constant but is the settling time for the system to reach the full

 

resolution of the analog-to-digital converter (ADC).

 

 

 

When filtering is enabled, the actual resolution of the module is a full 16

 

bits. The filtering function performs a dithering operation for the least

 

significant bits.

 

Default Filter Settling Time If the digital filtering bits are enabled via

 

the WY register and the Y32 output and no settling time values are written

 

to the module, then the default digital filter settling time of 250 msec is

 

automatically used.

 

Filtering and Alarms If filtering is enabled, then the filtered data will

 

be used for alarm comparisons; that is, the data will first pass through the

 

digital filter and its associated settling time and then be compared to any

 

low or high alarm setpoint. This prevents alarm conditions that are

 

attributable to noise.

 

Changing the Settling Time When new filter data is written to the

 

module, the microcomputer must recompute the filter time constants. This

 

operation takes 25 msec and no new data is written to the controller during

 

this time.

 

Numerical Range Values loaded into the module for digital filtering are

 

expressed as 16-bit unsigned integers 0 to 65535 in units of milliseconds.

 

 

 

NOTE: Signed integers will be interpreted as unsigned values.

 

 

SIMATIC 505±2557 Installation and Operation Guide

Advanced Function Programming 3-21

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Siemens 505-2557 installation and operation guide Digital Filtering