Control Logix UDP Data Transfer

[whussain6]

Hi,

I am trying to design Modbus UDP driver. I have already designed and tested Modbus TCP and RTU drivers. Now the requirement is to send the Modbus request using UDP/IP in addition to TCP/IP.

I have changed the Socket create message parameter 'Type' to 2 i.e. UDP (1 for TCP) but it does not seem to work. The socket is created and connected successfully , but my read/write messages not working. Write message showing no error but not sure where it is writing to? Read message showing error 0xff with extended error code 16#0046 which seem like if both read/write are triggered at the same time. It is definitely not the case as I have used not read/write enable to trigger the next message.

I am also not sure if I am testing it correctly. I have modbus simulator downloaded which I have used for Modbus TCP testing and works fine. Not sure if it allows Modbus RTU so I am open to test if using two EN2T modules in the same rack if some one please guide me how. Thanks

 

[Contr_Conn]

Logix Socket UDP Read does not work the same way as TCP Read on all modules (except EWEB):

TCP read is getting DN bit when Read buffer is empty, while UDP Read is getting error that should be handled.

Saying this, Modbus UDP is not something standard and common, so before you invest your time, take a look to make sure there is no TCP implementation on your server.

Also please take a look at new Logix Modbus TCP AOIs recently released in the Sample Code library - these are officially supported by Rockwell.

 

[whussain6]

Hi Conn,
Thanks for reply.

My TCP AOI is actually based in the sample code provided by Rockwell.

I have tested TCP using Modbus Simulator downloaded on my PC on port 502.

I am now trying to add facility to select UDP or TCP mode. I assumed that just by changing the Type to 2 would work keeping everything else the same. Is it not just sending the same data over UDP rather than TCP?

 

[Contr_Conn]

I am very familiar with new AOI code as well as earlier version 1.02.

No, it will not work the way that you are expecting, there is no easy way to convert from TCP to UDP.

This is not about changing Type from 1 to 2 like you think.

First, you will not use "Connect" or "Accept" messages with UDP.
Then, "Read" difference between UDP and TCP that I have mentioned earlier, you will have to separate valid errors from "Empty buffer" error.

Also, TCP is a stream, and code assembles this stream, then parses the data.
UDP is a just a single packet. Every time it comes, it wipes the earlier data from the buffer.
So basically, you will have to re-write whole socket portion of the code that is 60% of the it...

 

[drbitboy]

So, it is recommended h that, in light of the COVID19 outbreak, that all TCP-based protocols be switched over to use UDP to cut down on handshaking.