My PLC requires a “six-digit” register address but the WattNode manual only documents “five-digit” register addresses.

The official Modbus.org standard specifies that five-digit register addresses be documented. However, some PLCs (like the Modicon 984) use a six-digit addressing convention. To convert a five-digit register address to the equivalent six-digit one, add a leading digit of “4” for a holding register or “3” for an input register. For example, to read the WattNode’s integer frequency register at address 1221, you would enter 401221 to read it as a holding register or 301221 to read it as an input register. The last part of this message thread discusses the issue: http://www.control.com/thread/1026238596