ION9000 Modbus Issue

[sbogavilli]

Hello everyone,

Checking if some experts had faced a similar issue?

Modbus Map used is version_004.005.000 (attached)

1) We are commissioning one of our sites with an ION 9000 meters and want to retrieve values via the Modbus interface.

We set the frequency to 25 Hz by simulating the input to the ION but the frequency in our GeoSCADA via Modbus is showing as 25.06 Hz (32-bit Floating Point)

We investigated the 2x 16 bit register values which are being sent across Modbus to represent 25 Hz, they are shown above as 16840 & 32768, these in Hex are 41C8 (=16840 Decimal) & 8000 (=32768 Decimal)

When we put the 32 bits combined 41C88000 into a calculator it does compute to 25.0625 Hz

We have checked this with two separate calculators

Floating Point to Hex Converter

IEEE-754 Floating Point Converter

We believe the value sent across to represent 25 Hz should be 41C80000

We only noticed this discrepancy on Frequency as it was stable at a value of 25, the difference albeit small is significant when measuring frequency.

The error is present on both addresses returning frequency

We also saw slight differences in current, voltage etc when we simulated those too so they potentially have a similar error.

2) There also appears to be a two register offset compared to what is quoted in the Modbus map, to address register 3110 I had to request register 3108.

[Mehran_Mehrnia · Accepted solution]

Hi @sbogavilli 

1- As mentioned in the ION9000 user guide, Electrical Characteristics, the frequency accuracy class according to IEC 61557-12 is Class 0.02, which means ±10 millihertz. In a 50 Hz system, this corresponds to an acceptable range of 49.99 Hz to 50.01 Hz.

However, as stated in the voltage table, the specified frequency accuracy will be maintained when the injected voltage frequency is within 50/60 Hz ±15%. This means that for a 50 Hz system, the claimed accuracy is valid when the frequency is between 42.5 Hz and 57.5 Hz.

The meter can still measure parameters when the frequency is outside the specified range, but the claimed accuracy is not guaranteed.

2- This is dependent on how the Modbus client interprets register addressing.

Zero-based addressing (starts at 0)
One-based addressing (starts at 1)

So:

A client using one-based addressing will read 3110 & 3111
A client using zero-based addressing will read 3109 & 3110

Based on the ION9000 Modbus map, register 3108 is not associated with any valid data, unless there is a custom configuration in the meter's Modbus map.

Best Regards,

-Mehran

L3 Expert Advanced metering