ModbusTCP/UDP protocol
The Ethernet protocol is addressed by means of the MAC-ID. The user does not normally need to be concerned about this address. The IP number has a length of 4 bytes, and must be parameterized by the user on the Bus Coupler and in the application. In ModbusTCP, the TCP port is set to 502. The UNIT can be freely selected under ModbusTCP, and does not have to be configured by the user.
ModbusUDP
Modbus UDP can also be used instead of ModbusTCP with some products. The port number usually remains the same. Very little changes for the user.
The advantages of ModbusUDP are that it is in most cases simpler to use and usually somewhat faster than a TCP/IP connection.
The advantages of ModbusTCP are the more secure data transmission with repetitions already built in as well as further typical advantages that a TCP/IP connection features.
TCP port number
The TCP port number for ModbusTCP has been standardised to 502.
Modbus-Unit
The unit is returned by the slave.
ModbusTCP Protocol
|
Byte |
Name |
Description |
|---|---|---|
|
0 |
Transaction identifier |
Is returned by the slave |
|
1 |
Transaction identifier |
Is returned by the slave |
|
2 |
Protocol identifier |
always 0 |
|
3 |
Protocol identifier |
always 0 |
|
4 |
Length field |
0 (if the message is less than 256 bytes in length) |
|
5 |
Length field |
Number of following bytes |
|
6 |
UNIT identifier |
returned by the slave |
|
7 |
Modbus |
Modbus protocol with the function follows |