Register Addressing; Function Codes - ABB FSCA-01 User Manual

70 Communication protocol

Register addressing

The address field of Modbus requests for accessing holding
registers is 16 bits. This allows the Modbus protocol to support
addressing of 65536 holding registers.
Historically, Modbus master devices used 5-digit decimal
addresses from 40001 to 49999 to represent holding register
addresses. 5-digit decimal addressing limited to 9999 the number
of holding registers that could be addressed.
Modern Modbus master devices typically provide a means to
access the full range of 65536 Modbus holding registers. One of
these methods is to use 6-digit decimal addresses from 400001 to
465536. This manual uses 6-digit decimal addressing to represent
Modbus holding register addresses.
Modbus master devices that are limited to 5-digit decimal
addressing may still access registers 400001 to 409999 by using
5-digit decimal addresses 40001 to 49999. Registers 410000-
465536 are inaccessible to these masters.
Note: Register addresses of the 32-bit parameters cannot be
accessed by using 5-digit register numbers.

Function codes

The adapter module supports the Modbus function codes shown
below.
Function
code
03h Read Holding Registers
06h Write Single Register
Name Description
Reads the contents of a
contiguous block of holding
registers in a server device.
Writes a single holding register in
a server device.