Configuring the MES1A/B
Slave Address / Unit ID Source
All devices connected to a TCP/IP network require a unique IP address. Each MES1A/B Converter has a unique IP address on the network, allowing messages to be directed to it. However, when multiple Modbus/RTU devices are connected to the
Standard Modbus/RTU fields are encapsulated inside Modbus/TCP messages. Modbus/RTU’s Slave Address field is called the Unit ID in Modbus/TCP. The Unit ID is used to specify a particular Modbus/RTU device on the serial side of the MES1A/B. When the MES1A/B receives a message (sent to its IP address), it looks at the Unit ID field of the message and passes the message to the specified slave.
0 for Auto
Setting the Slave Address field to 0 allows the MES1A/B to use the Unit ID received from a Modbus master to direct messages to a specific slave address. The MES1A/B receives the Unit ID number on the Modbus/TCP side and passes it along as the Slave Address on the serial side of the converter.
1…255
Unfortunately, some early Modbus/TCP products assumed that each IP address would have only one slave. These products always set their Unit ID to 0 (relying on the IP address only to address the slave). The lack of an appropriate Unit ID presents a problem for the MES1B. It must include a Slave Address in its messages to Modbus/RTU slaves.
To overcome this problem, the MES1A/B Slave Address field can be set to the address of one Modbus/RTU slave (any number in the range of 1 to 255). This limits the number of slaves that can be connected to the MES1A/B to one, but ensures that when any message for the MES1A/B’s IP address is received (regardless of its Unit ID), it is passed along to the slave.
To configure the Slave Address field:
•If Modbus TCP masters on your system use appropriate Unit ID addresses, select 0. You will be able to connect multiple Modbus/RTU slaves with
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