ModSnmp Manual v3.09 - 6.3.2. Modbus settings
Modbus Size. If you have selected Holding Register or Input Register as the Modbus Type, you should select the register size in bits (16, 32 or 64). If you selected Float as the SNMP Type, you must select 32 or 64. Note that 32-bit and 64-bit register sizes are non-standard.
If you have selected Coils or Discrete Inputs as the Modbus Type, you should select or enter the number of Coils or Discrete Inputs that you want to transfer in each SNMP instance.
Modbus Address. Enter the Modbus address of the first variable in the group. Subsequent variables in the group will have consecutive addresses, unless you are using 32-bit or 64-bit Modbus registers and have selected Word Registers in the Device Type page.
Note that you may have to adjust the addresses published in device manuals - see Section 6.1, “Modbus Variables”.
Modbus Offset and Scale. These allow you to scale Modbus Holding Register and Input Register values when converting them to and from SNMP values. The scaling uses the formulae:
SNMP_value = (Modbus_value + offset) / scale
Modbus_value = SNMP_value * scale - offset