通过 tcp 的 Modbus 通信不会向所有设备发送数据

Question

I have a gateway connected to a computer which runs a C# program. I have a Modbus slave-master relation between computer and the gateway. Here are the gateway configurations (model EKI-1221-BE):


The set up I have is the same as in this picture but I have 6 devices connected instead of 2 and will later have 12:


I can send a Modbus command to any of the 6 devices without any problems but the issue happens when I try to send multiple commands in quick interval. In the following code, only the device 1,2 and 6 receive the command.

PumpsComm.SendMessage(1, 3099, 2000);
PumpsComm.ClientSocket.Receive(TempBuffer);
PumpsComm.SendMessage(2, 3099, 3000);
PumpsComm.ClientSocket.Receive(TempBuffer);
PumpsComm.SendMessage(3, 3099, 4000);
PumpsComm.ClientSocket.Receive(TempBuffer);
PumpsComm.SendMessage(4, 3099, 6000);
PumpsComm.ClientSocket.Receive(TempBuffer);
PumpsComm.SendMessage(5, 3099, 8000);
PumpsComm.ClientSocket.Receive(TempBuffer);
PumpsComm.SendMessage(6, 3099, 10000);
PumpsComm.ClientSocket.Receive(TempBuffer);

The function SendMessage is as follows, where ClientSocket is a TCP Socket that is connected to the gateway / Modbus slave.

public void SendMessage(int pumpID, int register, int command)
    {
        byte[] message = new byte[12];

        message[0] = 0;
        message[1] = 0;     //Message number
        message[2] = 0;
        message[3] = 0;
        message[4] = 0;
        message[5] = 6;     //Message length
        message[6] = (byte)ToHexInDec(pumpID, 'L');      //Pump ID
        message[7] = 6;
        message[8] = (byte)ToHexInDec(register, 'H');     //Register high
        message[9] = (byte)ToHexInDec(register, 'L');     //Register low
        message[10] = (byte)ToHexInDec(command, 'H');    //Command high
        message[11] = (byte)ToHexInDec(command, 'L');    //Command low

        try
        {
            ClientSocket.Send(message);
        }
        catch
        {
            return;
        }
    }

If I make the Thread sleep for 30ms between each send, every command works but this is not an option in my case as I need to be able to update every device in a very short amount of time.
I looked at the packets that I was sending and receiving to and from the gateway with Wireshark and I think I found where the
This is what the output of Wireshark was (the Ip that ends with 200 is the computer and the one that ends with 107 is the gateway): It looks as though the communication for the device 3,4,5 and 6 where put in the same TCP packet and for some reason, the only Modbus communication that was read from that packet is the last one.

I tried adding MBAP transaction numbers for every communication in the MBAP header of the Modbus communication but it didn't change anything. I then tried to add a blocking receive that makes the program wait for the Modbus response before sending the next communication but I am not receiving anything (even though we can see that the gateway is sending a Modbus response back when the communication was successfull)
I don't know what I can do anymore and was looking to see if anyone with more experience with TCP modbus communications could help. I can provide more details if necessary. Sorry if this is not clear enough, english is not my first language.

Thanks!

Answer 1

As per the spec

Several MODBUS transactions can be activated simultaneously on the same TCP Connection.
Remark: If this is done then the MODBUS transaction identifier must be used to uniquely identify the matching requests and responses

However you are sending messages with the transaction identifier set to 0000 :

message[0] = 0;
message[1] = 0;     //Message number

(I'm guessing that the gateway is detecting the duplication and assigning a new identifier because of this).

A better approach would be to send all of your requests (each with a different identifier; it does not matter what this is as long as it's unique) and then wait for the responses to come in (matching the response to the request using the transaction identifier in each response; responses may be in a different order to the requests).

Note that you should not assume that each response will come in a separate TCP packet (or that it will be in a single packet); the protocol takes this into account:

When MODBUS is carried over TCP, additional length information is carried in the MBAP header to allow the recipient to recognize message boundaries even if the message has been split into multiple packets for transmission. The existence of explicit and implicit length rules, and use of a CRC-32 error check code (on Ethernet) results in an infinitesimal chance of undetected corruption to a request or response message.