Modbus RTU over TCP vs. Modbus TCP

[Tim Ganz]

If a device has Modbus RTU over serial and Modbus RTO over TCP and Modbus TCP then there is a difference between Modbus RTU over TCP vs Modbus TCP correct?

I am trying to get data into a controllogix using the modbus TCP AOI but the device I want to ise has Modbus RTU over 485 as well as Modbus RTU over TCP but doesn’t list Modbus TCP as a protocol and I am thinking this is different and will an issue?

 

[Corsair]

It depends on how different people define the terms ...

True Modbus TCP uses a data packet that is very similiar but not identical to a Modbus RTU serial packet. The TCP version has some extra bytes at the beginning (I think 6) and no checksum at the end. Serial RTU has a 2-byte checksum at the end. This is not needed in the TCP version since the TCP layer has it's own checksum stuff. Somebody could describe true Modbus TCP as 'Modbus RTU over TCP' but in my mind that is not correct.

It is possible to send the same packet as serial RTU over TCP. That may be what someone is talking about when they talk about a 'Modbus RTU over TCP' that is different from 'Modbus TCP'. Someone may think this is desirable when there is a generic Ethernet to Serial converter at the end of the line instead of a true Modbus Ethernet to Serial gateway. I'm not sure how well this concept would work since the converter would not know about the packet structure and timing rules that are part of the Modbus RTU serial specification. I'd be much more comfortable with a true Modbus-aware converter that plays by the rules.

My guess is that the device you are using is playing by the proper Modbus over TCP rules and they are making a poor choice of wording by calling it 'Modbus RTU over TCP'. It probably will work. If it is the other way the AOI will not work.

Modbus UDP is another discussion . Modbus purists insist there is no such thing. It has at least 2 variations....

I wish you well with your project.

 

[lfe]

Modbus RTU over TCP is not a standard protocol and it is normal that ControlLogix and most devices do not support it.

The recommended solution is that you insert a Modbus RTU to Modbus TCP converter, they are not usually expensive devices.

Regarding UDP, it should be remembered that with UDP it is not ensured that the data packet reaches the other side, nor does the sender have any means of knowing if it arrived.
On the other hand, with TCP, if there is no error, it is ensured that the other side received the data completely.

 

[Mark-]

RTU over TCP but doesn’t list Modbus TCP as a protocol and I am thinking this is different and will an issue?

MODBUS RTU is a regular MODBUS RTU exchange (request/response) over TCP. No extra data is used.

In other words, the data packets sent and received are MODBUS RTU packets/frames using TCP to handle packet transfer.

MODBUS RTU over TCP and MODBUS TCP are two different protocols, the packets/frames are not the same.

 

[Contr_Conn]

Modbus TCP AOI will not work with Modbus RTU over TCP because of checksum bytes in the RTU packet that is not in the Modbus TCP.

 

[lfe]

Modbus TCP AOI will not work with Modbus RTU over TCP because of checksum bytes in the RTU packet that is not in the Modbus TCP.

It is not just the checksum, the byte sequence of Modbus TCP is different that of Modbus RTU.

For example, TCP incorporates a transmission identifier and the total length in bytes, things that RTU does not transmit.

 

[jschulze]

Test your device with a computer and Modbus Poll (Modbus Master Simulator) to determine which variation of Modbus your device uses. Modbus Poll supports pretty much all variations of the Modbus protocol, including both true Modbus/TCP and Modbus RTU Over TCP/IP.

 

[Tim Ganz]

Thank you all for clearing that up. I think we will have to choose a more compatible device.