Communication; Overview; Register Addressing - ABB FSCA-01 Manual

Communication

Overview

This chapter describes the Modbus communication protocol for
the FSCA-01 RS-485 Adapter Module. For detailed information on
Modbus communication, refer to Modbus Application Protocol
Specification v1.1b.

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. The 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 the 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.
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Communication