GE PACSystems RXi User Manual

Chapter 8. Gigabit Ethernet (GbE) Interface Overview and Operation146 PACSystems* RXi Distributed IO Controller User Manual GFK-2816F8.5 Modbus TCP Server OperationThe Modbus TCP Server supports up to 16 simultaneous connections. These connectionsare not shared with any other applications. Other TCP-based application protocols such asSRTP Server use a different set of TCP connections.8.5.1 Modbus Conformance ClassesPACSystems Modbus TCP Server supports Modbus Conformance classes 0, 1, and 2.The RXi GbE Ethernet interface has been certified by the Modbus TCP Conformance TestLaboratory to be in conformance with Conformance Test Policy Version 2.1.8.5.2 Server Protocol ServicesThe Modbus TCP Server responds to incoming Request Connection, Terminate Connectionand Request Service messages. The client with which the server is interacting should wait forthe server’s response before issuing the next Request Service, otherwise requests couldbe lost.There is no inactivity timeout in the server. If a client opens a connection, that connectionstays open until the connection is terminated.8.5.3 Station Manager SupportThe Modbus TCP Server supports the standard Station Manager monitor mode commands:STAT, TALLY, and TRACE. The Modbus TCP Server task letter is “o”.8.5.4 Reference MappingThe Modbus protocol’s reference table definition is different from the internal structure of thePACSystems reference tables. Modbus refers to Holding Register, Input Register, InputDiscrete and Coil tables; PACSystems uses Discrete Input (%I), Discrete Output (%Q),Analog Input (%AI), Register (%R), and Word (%W) reference tables for Modbus data. Thefollowing table shows how each Modbus table is mapped to the PACSystems referencetables.Modbus Reference TablesModbus FileAccess(6xxxx)Modbus HoldingRegister Table(4xxxx)Modbus InputRegister Table(3xxxx)Modbus InputDiscrete Table(1xxxx)ModbusCoil Table(0xxxx)PACSystemsReferenceTables--- --- --- 1 – 32768(bits)--- %I1 – 32768(bits)--- --- 1 – 32640(16-bit words)--- --- %AI1 – 32640(16-bit words)--- --- --- --- 1 – 32768(bits)%Q1 – 32768(bits)--- 1 – 32640(16-bit words)--- --- --- %R1 – 32640(16-bit words)F1,R1 –F525,R2880(16-bit words)--- --- --- --- %W1 –5,242,880(16-bit words)Modbus File Access TableThe Modbus File Access table is mapped exclusively to PACSystems %W memory.