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WattNode Modbus Frequently Asked Questions (FAQs)

This page lists customer questions about the WattNode® Modbus® meter and how they were answered or resolved.

The most common customer problems with the Modbus WattNode result from configuration problems with the Modbus master.

  • Registers are sometimes documented with leading “3”s or “4”s to indicate holding or input registers. This results in register numbers like “41221”. But the leading “4” is not actually part of the register number and isn’t included in the Modbus packet. To avoid confusion, we do not include a leading “4” in front of our register numbers, but some tools expect or will display a leading “4” anyway.
  • Some WattNode values are 32 bits long and must be transferred as two 16-bit registers. Most Modbus master devices support two register orderings (one sometimes called “reversed”) for long integers and floating point numbers. If this isn’t configured correctly, the values will be nonsense.
  • The Modbus specification calls for registers to be documented as one-based, but communicated as zero-based, so register 1221 appears as 1220 in the Modbus packet.

A good starting point is to read the WattNode meter’s integer frequency register (at the one-based address of 1221), since it uses the simplest number format (a single 16-bit integer register) and it is unaffected by installation problems like a CT installed backwards or CT connected to the wrong phase.

Frequently Asked Questions (FAQs)

  • What is the largest positive number that a 32-bit floating point register can represent?
Internally, the WattNode use a 64 bit energy accumulator and then outputs that as a 32 bit IEEE-754 floating point number, which supports only six or seven decimal digits of resolution. See the last entry in the table BACnet and Modbus Energy Resolution.

Modbus Programming Tips