Siemens SIMATIC S7-200 System Manual page 377

Accumulator Usage
The accumulators (AC0, AC1, AC2, AC3) are utilized by the Modbus Master instructions and
appear in the Cross Reference listing. The values in the accumulators are saved and restored by
the Modbus Master instructions. All user data in the accumulators is preserved while executing
the Modbus Master instructions.
Holding Registers Addresses Greater Than 9999
Modbus holding addresses are generally within the range of 40001 through 49999. This range is
adequate for most applications but there are some Modbus slave devices with data mapped into
holding registers with addresses greater than 9999. These devices do not fit the normal Modbus
addressing scheme.
The Modbus Master instructions support addressing holding registers greater than 9999 via an
alternate addressing method. The MBUS_MSG instruction allows an additional range for the
parameter Addr to support an extended range of holding register addresses.
400001 to 465536 for holding registers
For example: to access holding register 16768, the Addr parameter of MBUS_MSG should be set
to 416768.
The extended addressing allows access to the full range of 65536 possible addresses supported
by the Modbus protocol. This extended addressing is only supported for holding registers.
Using the Modbus Protocol Library Chapter 12
363