reserved for Modbus. If the connection request is accepted by the server, the client can
start communicating with the server unit.
IEDs can usually accept several simultaneous Modbus TCP/IP client connections
even though the number of connections is limited. It is possible to configure the IED
to only accept socket connection requests from known client IP addresses.
2.1.3
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.
2.1.4
Terms and definitions
Modbus data appear in different memory areas in the Modbus device. The four most
common areas are coils, digital inputs, input registers and holding registers. These are
also referred to as 0X, 1X, 3X and 4X areas respectively.
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. Addresses start from 1.
Refer also to the Modbus protocol standard documentation that can be found for free at
www.modbus.org
.
2.1.5
Documentation
Address information concerning Modbus bits, registers and register structures stated
in this document is similar in all 620 series IEDs. The rest of the Modbus application
data are 620 series configuration-dependent. This means that the Modbus data
Section 2
1MRS757645 B
Modbus overview
10
620 series
Communication Protocol Manual
