Practical Industrial Data Networks:Design, Installation and ...

Appendix D – Miscellaneous industrial protocols overview 405The real advantage of MODBUS/TCP is in how fast it can be implemented. Anyorganization with the following three resources can implement MODBUS/TCP in a fewdays: source code for an existing MODBUS/RTU serial driver, a TCP socket library anda competent programmer.Thus, vendors can now have a MODBUS/TCP driver almost instantly, plus all theapplication programs that already support the data access paradigm of MODBUS/RTUcan also support MODBUS/TCP instantly. In addition, direct MODBUS/TCP accessoffers good performance.Some of the various network protocols supported by most Modbus/TCP complianthardware, that also form a part of the TCP/IP suite of protocols for networkcommunication are: ARP, UDP, TCP, ICMP, Telnet, TFTP, DHCP, and SNMP. Fortransparent connections, TCP/IP (binary stream) or Telnet protocols are used. Firmwareupgrades can be made with the TFTP protocol.The IP protocol defines addressing, routing and data block handling over the network.The TCP (transmission control protocol) assures that no data is lost or duplicated, andthat everything sent into the connection on one side arrives at the target exactly as it wassent.For typical datagram applications where devices interact with others withoutmaintaining a point-to-point connection, UDP datagram is used.IP addressing forms an important part of any standard TCP/IP network and thereforemust also be considered when using Modbus/TCP. Every device connected to the TCP/IPnetwork including the multi-serial device servers must have a unique IP address. Whenmultiple Modbus devices share a single IP, then Modbus/TCP includes an additionaladdress called the unit ID.When the multi-serial device server is receiving Modbus/TCP messages from remotemasters, the unit ID is converted to use in the Modbus/RTU message as the slave address.When the multi-serial device server is receiving Modbus/RTU messages from localserial masters, a user-defined lookup table is used to match the 8-bit Modbus slaveaddress to a remote IP address. The Modbus slave address received is used as the unit ID.While the Modbus/TCP standard specification requires Modbus/TCP masters/clients toonly issue 1 poll at a time, the full-duplex flow-controlled nature of TCP/IP allows themto issue more than one at a time and the TCP socket will happily buffer them. TheModbus bridge will fetch them 1 at a time and answer each in turn.