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Section 2
Modbus overview
2.1.3
2.2
2.3
8
Application data implementation
This IED is designed to operate with a wide range of different Modbus masters and clients.
The Modbus memory map offers the possibility to view IED's internal process data in a
simple I/O map style which is mainly aimed at PLC masters and other process automation
devices. Time-tagged, chronological event lists and fault records can be read over the
Modbus interface. These data are more suitable for SCADA type of Modbus masters.
The Modbus standard defines four main memory areas for mapping IED's process data.
Due to its open nature, the Modbus standard does not define exactly what type of data
should be mapped to each memory area. The Modbus mapping approach of the IED
ensures that the same process data are readable from as many Modbus memory areas as
possible. The users may then choose the memory areas that are most suitable for their
Modbus master systems.
All Modbus data in the IEDs can be accessed using command functions belonging to
Modbus conformance classes 0 and 1. This means that most master systems are able to
communicate with the IED.
Terms and definitions
Modbus data appears in different memory areas in the Modbus device. The four most
common areas are coils, digital input, input register and holding register. These are also
referred to as 0X, 1X, 3X and 4X areas respectively.
Data within these four areas are addressed from 1 onwards. Modbus defines addressing in
two ways: PLC addressing starts from address 1 and regular Modbus data addressing starts
from 0. For example, a holding register at PLC address 234 can be referred to either as 4X
register 234 or as 40234. The regular Modbus addressing, that is the PLC address
decremented by one, is shown when analyzing the Modbus traffic on the physical network.
Listings and references to the Modbus data in this documentation follow the PLC
addressing scheme. Refer also to the Modbus protocol standard documentation that can be
found for free at www.modbus.org.
Documentation
The ANSI Modbus point list manuals cover all the 615/620 series variants.
A newer SW version of the same 615/620 series configuration may contain
additional Modbus points.
1MAC052634-MB D
615/620 series ANSI
Communication Protocol Manual
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