Comtrol Modbus Server - Manual (Page 6)

6 - Modbus Server Application Overview DeviceMaster Modbus Server User Guide: 2000535 Rev. C
Terms and Definitions
Terms and Definitions
This table provides Modbus Server definitions.
What is Modbus?
Modbus/RTU
(Supported by
Modbus Server) Modbus/RTU is native Modbus in hexadecimal format. These are the base Modbus
messages that contain simple read and write requests. 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/RTU is used for:
• Serial port connectivity. RS-485 is the most common, but RS-232 and RS-422
are also used.
• Ethernet TCP/IP socket connections. This is not the same as Modbus/TCP
(please see next section), but does provide a very simple method of interfacing
to remote devices. It is used by many applications and some OPC servers.
Note: This communication method is not used by PLCs.
Term Definition
Master (or Client) Mode
The method of operation when a DeviceMaster or an
application is operating as a Master or the message
originator.
Slave (or Server)
The method of operation when a DeviceMaster or an
application is operating as a Slave or the message
receiver.
Modbus/RTU
The standard Modbus messages, in hexadecimal
format, that are typically transmitted over serial lines
but can also be transmitted over other communication
methods such as wireless or Ethernet TCP/IP socket
connections.
Note: Modbus/RTU over Ethernet TCP/IP is not the
same as Modbus/TCP.
Polling
The process where an application requests data on a
continual basis. In this operation the Master sends the
request messages while the Slave responds to the
messages.
Sockets
The method used to communicate between devices
while communicating over Ethernet TCP/IP.
DeviceMaster DeviceMaster RTS or DeviceMaster UP.