Comtrol Modbus Server - Manual (Page 7)

DeviceMaster Modbus Server User Guide: 2000535 Rev. C Modbus Server Application Overview - 7
Modbus/TCP (Not supported by Modbus Server)
Modbus/TCP (Not
supported by Modbus
Server) 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.
• Remote serial Modbus/RTU devices through gateways (such as the
DeviceMaster UP).
• Remote serial or Ethernet TCP/IP ASCII devices through a gateway (such as
the DeviceMaster UP).
Note: Refer to the DeviceMaster UP for Modbus/TCP functionality.
Modbus Server Functionality
The Modbus Server application provides the following functionality:
• Supports Modbus/RTU over Ethernet TCP/IP connections to the
corresponding serial port through intelligent Modbus message handling and
routing.
• Supports only Modbus/RTU over Ethernet TCP/IP connections to a serial port.
Note: For Modbus/TCP functionality, refer the DeviceMaster UP .
• Supports up to six Ethernet TCP/IP connections to each serial port.
- One TCP/IP connection can be created with the Connect To connection
method.
- The Listen connection method accepts up to five or six connections,
depending if the Connect To connection is active.
• Supports up to 255 Modbus devices per port. Both valid, (1-247), and reserved,
(248-255), device Ids are supported.
• Modbus/RTU specific message handling:
- CRC verification of all messages received on the TCP/IP and serial
interfaces.
- Timing out of responses from slave Modbus/RTU devices.
- Broadcast message handling on connected port only.