Comtrol Modbus TCP User Guide - Manual (Page 19)

DeviceMaster UP Modbus/TCP User Guide: 2000447 Rev. I Chapter 2. Programming Interface - 19
What is Modbus/TCP?
2.1.4. What is Modbus/TCP?
Modbus/TCP is an Ethernet network based protocol that contains a Modbus/RTU message, with the exception
of the 2 byte CRC. The Modbus/TCP message contains a header with information designed to provide message
identification and routing information. The format is as follows:
Where:
• The terms Master or Client are used to identify the sender of the message.
• The terms Slave or Server are used to identify the devices responding to the message.
• Modbus TCP messages are typically sent to and received on a defined Ethernet TCP/IP socket of 502.
• Modbus TCP implementations provide more capability, but also require more processing than simpler
Modbus/RTU implementations.
Modbus TCP is used for connecting advanced Ethernet based devices, such as PLCs, HMIs, SCADA Systems,
and most OPC Servers to:
• Other Ethernet devices supporting Modbus TCP.
• Serial Modbus/RTU and/or Modbus/ASCII devices through gateways (such as the DeviceMaster UP
running the Modbus/TCP or Modbus Router applications).
• Serial or Ethernet TCP/IP raw/ASCII devices (barcode scanners, printers, RFID readers, visions systems,
etc) through a gateway (such as the DeviceMaster UP running the Modbus/TCP application).
2.2. Raw Data Interface
This subsection contains the following topics:
• Supported Modbus Messages on Page 19
• Serial Port Raw/ASCII Interface on Page 20
• Ethernet Device Raw/ASCII Interface on Page 21
• Receive Data Message (Raw Data) on Page 23
• Transmit Data Message (Raw Data) on Page 25
• Sequence Number Messages (Raw Data) on Page 27
2.2.1. Supported Modbus Messages
DeviceMaster UP supports the following Modbus messages over Modbus/TCP for raw data transfer.
Note: Your PLC programming software may not allow maximum size serial packets.
Message Type Function Code Maximum Message Size Maximum Serial Packet Size
Read Holding Registers 3 250 BYTEs (125 WORDs) 246 BYTEs (123 WORDs)
Write Multiple Registers 16 (10 hex) 240 BYTEs (120 WORDs) 236 BYTEs (118 WORDs)
Read/Write Multiple
Registers
23 (17 hex) 236 BYTEs (118 WORDs) 232 BYTEs (116 WORDs)