ABB Relion 620 Series Protocol Manual

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reserved for Modbus. If the connection request is accepted by the server, the client canstart communicating with the server unit.IEDs can usually accept several simultaneous Modbus TCP/IP client connectionseven though the number of connections is limited. It is possible to configure the IEDto only accept socket connection requests from known client IP addresses.2.1.3 Application data implementationThis IED is designed to operate with a wide range of different Modbus masters andclients. The Modbus memory map offers the possibility to view IED's internal processdata in a simple I/O map style which is mainly aimed at PLC masters and other processautomation devices. Time-tagged, chronological event lists and fault records can beread over the Modbus interface. These data are more suitable for SCADA type ofModbus masters.The Modbus standard defines four main memory areas for mapping IED's processdata. Due to its open nature, the Modbus standard does not define exactly what typeof data should be mapped to each memory area. The Modbus mapping approach of theIED ensures that the same process data are readable from as many Modbus memoryareas as possible. The users may then choose the memory areas that are most suitablefor their Modbus master systems.2.1.4 Terms and definitionsModbus data appear in different memory areas in the Modbus device. The four mostcommon areas are coils, digital inputs, input registers and holding registers. These arealso referred to as 0X, 1X, 3X and 4X areas respectively.Modbus defines addressing in two ways: PLC addressing starts from address 1 andregular Modbus data addressing starts from 0. For example, a holding register at PLCaddress 234 can be referred to either as 4X register 234 or as 40234. The regularModbus addressing, that is the PLC address decremented by one, is shown whenanalyzing the Modbus traffic on the physical network.Listings and references to the Modbus data in this documentationfollow the PLC addressing scheme. Addresses start from 1.Refer also to the Modbus protocol standard documentation that can be found for free atwww.modbus.org.2.1.5 DocumentationAddress information concerning Modbus bits, registers and register structures statedin this document is similar in all 620 series IEDs. The rest of the Modbus applicationdata are 620 series configuration-dependent. This means that the Modbus dataSection 2 1MRS757645 BModbus overview10 620 seriesCommunication Protocol Manual