DDS238-2 Reactive Power wrong value

[sebitnt]

PROBLEM DESCRIPTION

Hello,
I am using 9.4.0(sensors) with an DDS238-2 attached via Modbus. Everything is working as expected except for the Reactive Power which is displayed wrong (much too high). It seems to me that it is an issue with an 16bit value being rolled over. Thank you for your help in advance!

REQUESTED INFORMATION

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• Device used (e.g., Sonoff Basic): ESP07
• Tasmota binary firmware version number used: 9.4.0
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• Flashing tools used: OTA
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TO REPRODUCE

See description.

EXPECTED BEHAVIOUR

Calculate and show the correct value.

SCREENSHOTS

ADDITIONAL CONTEXT

Nothing

(Please, remember to close the issue when the problem has been addressed)

[schrej-zz]

Hi,
I wonder, that noone already replied.
For me, it looks like we have a negative value overflow here.
Due to some effect like rounding, the value is negative, but the variable only expects positive values.
Haven't looked into the code yet, but I suggest to set the value to zero, when it becomes negative.
A negative value doesn't make sense indeed.
Joerg

[chaveiro]

Check if this is the reason : #12114

[sebitnt]

I have done a few tests. The problem is only present when the power flow is backwards through the meter. Connecting it the other way, tasmota is reporting correct values. This seems to me to be a overflow of a unsigned variable. Reactive power might be negative but tasmota does not expect this.

[arendst]

Thx for the investigation.

Tasmota does negative reactive power as can be seen here:

The issue is with the DDS2382 driver (xnrg_09_dds2382.ino - line 70) which decodes reactive power like:

      Energy.reactive_power[0] = (float)((buffer[33] << 8) + buffer[34]);

You might want to try the below change dline:

      Energy.reactive_power[0] = (float)(int16_t)((buffer[33] << 8) + buffer[34]);