Function Blocks
FI Frequency Input
160 HC900 Hybrid Control Designer Function Block Reference Guide Revision 11
2/07
Example
Say you configure the block as follows.
Pulse Width (Range) = 500µsec (10Hz-500Hz)
Frequency Span Low = 50Hz
Frequency Span High = 450Hz
Output range in EU = 0 - 100
Bias = 8
Failsafe = Upscale (upper output range)
Assume the module is receiving a 255Hz signal, including noise. Assume 2% of pulses are noise, that is,
are less than the minimum pulse width 500 µsec. The block rejects this 2% and does not count them in the
frequency. The remaining 98% valid pulses are counted, which is an effective signal of 250Hz. From the
output equation above,
OUT = [(250–50) / (450-50)] * (100 – 0) + 0 + 8
= [200/400] * 100 + 8
= 0.5 * 100 + 8
= 50 + 8
= 58 EU
If the frequency of the effective signal (>500 µsec pulse width) changes to outside the specified range of
10-500Hz, a failure-to-convert error occurs and the output goes to failsafe, in this case upscale (100).
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