Communication; Introduction To Modbus; Register Read And Write - ABB ACS 400 User Manual

Communication

This chapter describes the Modbus communication on ACS 400 drives.

Introduction to Modbus

Modbus is a serial, asynchronous protocol. The Modbus protocol does not specify the physical
interface. Typical physical interface is RS485.
Modbus is designed for integration with Modicon PLCs or other automation devices, and the
services closely correspond to the PLC architecture. The ACS 400 drive 'looks like' a Modicon PLC
on the network.
If detailed information regarding the Modicon Modbus protocol is required, contact your ABB
supplier for a copy of Modbus Protocol Guide.

Register Read and Write

The ACS 400 has all drive parameter, control and status information mapped into a 4xxxx register
area. This holding register area can be read from an external device, and an external device can
modify the register values by writing to them.
There are no setup parameters for mapping the data to the 4xxxx register. The mapping is
predefined and corresponds directly to the ACS 400 parameter grouping.
All parameters are available for both reading and writing. The parameter writes are verified for
correct value, and for valid register addresses. Some parameters never allow writes (including
Group 1 actual values), some allow only zero write (including Group 1 fault memories), some
parameters allow write only when the drive is stopped (including Group 99 setup variables), and
some can be modified at any time (including e.g. Group 22 acceleration and deceleration ramp
times).
Note! Parameter writes through Channel 1 (Standard Modbus) are always volatile i.e. modified
values are not automatically stored to permanent memory. Parameter 1607 . can be
PARAM SAVE
used to save all altered values.
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