B&B Electronics MES1A - Manual (Page 39)

Configuring the MES1A/B
Manual Documentation Number: MES1A/MES1B-2106m PN7138-rev001
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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 RS-485 bus of an MES1B there must be a way to
address them individually.
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 RS-485 interfaces to an MES1B.