Communication methodology, Modbus/rtu and modbus, Ascii to-slaves protocol interface – Comtrol Modbus TCP User Guide User Manual
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DeviceMaster UP Modbus/TCP User Guide: 2000447 Rev. I
Chapter 2. Programming Interface - 29
Modbus/RTU and Modbus/ASCII To-Slaves Protocol Interface
2.4. Modbus/RTU and Modbus/ASCII To-Slaves Protocol Interface
The DeviceMaster UP provides access to serial Modbus/RTU and Modbus/ASCII slave devices via Modbus/
TCP, serial Modbus/RTU masters, and serial Modbus/ASCII masters. Modbus master messages are
translated to Modbus/RTU or Modbus/ASCII messages, devices are automatically located, and appropriate
Modbus responses are returned to the Modbus masters.
2.4.1. Communication Methodology
The DeviceMaster UP translates Modbus master messages into Modbus/RTU or Modbus/ASCII messages and
forwards them to slave devices attached to the Modbus/RTU or Modbus/ASCII slave serial ports. Each
Modbus message is transmitted and a response is expected. The DeviceMaster UP times out the Modbus/RTU
or Modbus/ASCII messages if there is no response returned within the configured timeout period.
The following diagram displays the Modbus message transfer.
The following apply to Modbus slaves serial ports.
•
All valid Modbus master messages are translated to Modbus slave messages for serial port transmission.
•
Modbus slave devices are automatically located on a DeviceMaster UP 2-port or 4-port.
•
Messages are timed out if no response is returned within the configured timeout period.
•
Appropriate Modbus responses are returned to the Modbus master.
•
Broadcast Modbus messages, those with a unit identifier of zero, are transmitted out all Modbus slave
serial ports on the DeviceMaster UP.
The following restrictions apply to the Modbus slave interface:
•
The DeviceMaster UP serves as a slave Modbus/TCP device, a master on Modbus To-Slaves serial ports,
and a slave on Modbus To-master serial ports.
•
All Modbus slave devices attached to a DeviceMaster UP gateway (1, 2, or 4-port) must have unique Unit
Identifiers. Valid Unit Identifiers are 1 to 247 and the Broadcast Identifier is zero.
To communicate to Modbus slave device(s) through a DeviceMaster UP, perform the following steps.
1.
Using the embedded web page, select the appropriate Port.
2.
Under Serial Configuration, configure the serial port parameters such as the Mode, Baud rate, Data Bits,
and so forth.
3.
Under General Protocol Settings, set the Select Serial Port Protocol to Modbus slave.
4.
Under Modbus Slave Protocol Settings, set the Device Response Timeout to the desired value.
Note: 2- and 4-Port only: set the Lost Device Search Enable setting. For a discussion on this setting, see