Problem

Address offset vs. reference number

Users and tools can mix zero-based protocol offsets with one-based or 3xxxx/4xxxx reference notation.

Why it matters

The Modbus application protocol defines PDU starting addresses from zero. A Pymodbus issue also shows a user explicitly mapping 30001 to address 0.

What to do

Sources

• Modbus Application Protocol modbus.org
• Pymodbus #2565 github.com