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I have a problem trying to read the power from a power meter reading pulses. I already did a search on the forum and found 2 or 3 topics but would still like to ask if someone can give me any suggestions to increase accuracy.

At the moment the power meter is set to output 1imp/Wh. The output is connected to a V120, not to the high speed input because they are already all used, but to a normal input and at 15kW the error is of 2.5kW which is to much.

At the moment in the 2.5ms subordinate I'm reading the value of SDW3 (2.5ms counter) at every pulse to get the time between pulses, and then in a normal subordinate calculate the kW from that. I tried to use immediate read but VisiLogic says it not compatible with the hardware (is it only for high speed inputs or it's not supported by the V120?).

If it helps I can also set the power meter to output 1 imp/10Wh if the 1imp/Wh is to fast for the normal input. The power meter works in the range between 5kW and 20kW so from 1.38 to 5.55 imp/sec.

Is there any "trick" to improve accuracy?  Or what am I doing wrong?


Attached is a sample of how I'm doing the reading and calculations.


Thank You


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another question, the scan time in SI0 is between 11-14ms. The plc has 19 inputs used out of 20. As I understand SI0 gives the scan time of 1 cicle eg. in that time it reads 1 channel. Assuming that the total scan time should be around 200-250ms?

Without the immediate read, using only the 2.5ms interrupt the plc will check the input every 2.5ms or every 200-250ms?


Thank You.

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  • MVP 2023

All the inputs are checked each scan (it's just a bitmap).  Your scan of 11-14 ms is typical, and that's how often the inputs are checked.


An Immediate Read for each input in an interrupt routine will read them once every time the interrupt is run but it is not available in the V120 instruction set.  Again - V130.


Joe T.

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