What is modbus/tcp, Raw data interface, Supported modbus messages – Comtrol Modbus TCP User Guide User 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:
•
•
Serial Port Raw/ASCII Interface
•
Ethernet Device Raw/ASCII Interface
•
Receive Data Message (Raw Data)
•
Transmit Data Message (Raw Data)
•
Sequence Number Messages (Raw Data)
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)