There are two primary functions that these unit perform.
First, due to difference in the RTU and Modbus/TCP protocols, they convert the message formats. Primary differences here are RTU address field replaced with a Modbus/TCP header, and the CRC is dropped with Modbus/TCP since message integrity is handled by TCP/IP. Also, this conversion generally handles both master and slave serial devices.
The second function is queuing of requests on the Ethernet interface. This requires that the unit queue requests while waiting for a response from a specific serial slave, permitting multiple Ethernet hosts.
Paul Wacker
Product Manager
Industrial Communications
Advantech eAutomation
Modbus/TCP and Modbus/RTU are essentially the same protocol. The difference is that with RTU a checksum is added to the end of the message to detect errors. For TCP, it simply relies on the Ethernet checksum. The functions (commands), data, exceptions, etc. are however the same in both cases.
For Mobus/RTU on RS-485, the address of the field device is in the "unit id" byte. There is also a unit id in the TCP version, but it's not needed to deliver the message (the TCP/IP address takes care of that) so a normal field device just ignores it. However, it's still there, so it's available to use for RTU messages.
modbus TCP access the slave by ip address of that slave and modbus rtu access data by slave id. there would be several slave connected via rs 485 physical links. converter would be accessed by ip address after that to reach up to particular slave, how does it know which slave has been called?
Modbus/TCP protocol includes a "slave address" field in the header. So, there is no need to use a separate IP address for each RTU device on the RS-485 serial side of a Modbus Gateway.
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