Hi I would like to know what extcomm port is for control second modbus rs485 port

I use extcomm/0/0 for the first port but there is extcomm/1 and extcomm/2 what is the correct port?

To be specific I'll explain how to calculate the value to write in the analog output register.
For determinator i use: (((3.3 * deltaRef) / 4095) * (1 + (VDev / 10000)) * 3);
DeltaRef is (vref / vrefInt), if denominator is 0 i change it in 4095.

For nominator i use: ((Vdesired - (VOffset / 10000)) < 0) ? 0 : (Vdesired - (VOffset / 10000));

Then i calculate nominator / denominator.

I receive 0, so is for Voltage Output

I use Raspbian Stretch
I communicate with node.js using a npm library.
If i wrote 0 i receive 0 as value

7 approximately

I calculate value using formula and when i select 6V that is my desired volt, that formula produce 2294. I take the result and I write it to Analog Output 1 Modbus register

When I select 0V, the formula produce 0, but when i measure the output with multimeter i measure 1.066 V

If i set 6 Volt the result is 2294.784382284382

"Verify the functionality of the LED dimmer
connect the LED to the dimmer
connect the power supply to the dimmer
leave the 0-10V input unconnected
the LED should be off."

I leave the 0-10V input unconnected from unipi and led are On like 100% Luminosity.

"Verify the output voltage
Disconnect everything from the AO and AGND
connect a voltmeter (or multimeter set to DC voltage measurement) between AO and AGND (positive / red test lead to AO, negative / black test lead to AGND)
set the desired voltage via Modbus registers and verify the actual output voltage
set the voltage to 0V via Modbus registers and verify that the output voltage is 0V"

When I set 0V via Modbus multimeter reveals like 1.066 Volt and not 0 volt.

They are connected. I can't misure now the V in output, there is a way to be able to turn led completely off?

@martin-kudláček "With register "Analog Output 1" = 0 the output voltage should be 0V". I do that but still the light is on.

"You can simply verify the functionality with multimeter by measuring the output voltage between AO and AGND. " What I accomplish with this operation?