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DNP and Modbus Data Scaling in ION Meters

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DNP and Modbus Data Scaling in ION Meters

ION Meters support both DNP and Modbus protocols for SCADA communications, but determining the raw values isn't always straight forward. When scaling is 1 and the input values are within the bounds of the register the raw values will equal the source values of the DNP Slave Export or Modbus Slave modules.

 

How do you determine the values when the scaling is not a factor of 10, and how to validate those values. Each export module in the meter can be scaled differently and looking at the setup registers for the module will help verify the scale. In general the value/bit will be the input range/output range. The input range for DNP modules will be IONFull minus IONZero, and output range will be DNPFull minus DNPZero. The input range for Modbus modules is InFull minus InZero, and output range is OutFull minus OutZero. There may also be an offset that needs to be accounted for if the equivalent DNP value of 0 is not equal to a source value of 0.

 

An example for DNP would be a meter that is setup for frequency that is setup for IONZero equal to 58, IONFull equal to 62, DNPZero equal to -32768, and DNPFull equal to 32767. In this example frequency also has an offset of 60. The input range would be 62 - 58 which equals 4. The output range would be 32767 - -32768 which equals 65535. The Hz/bits would be 4/65535 which equals 0.000061036 Hz/bit or 16383.75 bits/Hz. If the source value is 59.98 as an example, the raw value would be (59.98-60) Hz * 16383.75 bits/Hz = -328 bits rounded. Which is the raw value we would expect in the DNP Master. This same process can be used when calculating the expected Modbus value from the meter. Register format and bounds should also be accounted for when calculating raw values. For example, if a meter has a scaling of one for primary volts phase - phase and is setup for unsigned 16bit registers. A 765kV meter would have 765,000 volts on the source input which would be above the maximum value for a 16bit unsigned register. The DNP master would only see 65,335. 

 

Calculating raw values by hand, or using a spreadsheet can be time consuming and may be susceptible to error. Using a protocol analyzer to validate data can be more reliable and efficient. Once configured can quickly validate scales and raw values to compare with SCADA during meter commissioning and installation. We can provide a similar solution as a protocol analyzer with our Easergy T300 RTU using the HU250 module. If you would like more information please reach out to your regional account executive or subject matter expert.

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