Moacon Modbus TCP Slave (Server)

[Mike]

This is an implementation of the a Modbus TCP slave (server) for the Moacon. It supports function codes 1, 2, 3, 4, 5, 6, 15, & 16; basically reading and writing bits and words.

Attached to this post is the entire source code (fn_modbusTcp.c) and a sample illustrating it's use (ModbusTcpTest.c). We have made an effort to make it's usage similar to that of the Moacon RTU slave.

The function that does the vast majority of work is processModbusTcpRequest() in fn_modbusTcp.c. This function will attempt to read a Modbus TCP frame from the Ethernet module's receive buffer, parse it, read or write to the coil and register buffers, and generate a response. It takes a socket, a pointer to register memory, and a pointer to coil memory.

Code: Select all

void processModbusTcpRequest(u8 socket, u16* registerMemory, u8* coilMemory)

Because connections in TCP/IP are more virtual than physical, some kind of connection management needs to be performed in software. In the attached example you can see this connection management in doModbusTcp() in ModbusTcpTest.c.

Code: Select all

void doModbusTcp(u8 socket, u16 port, u16* registerMemory, u8* coilMemory)
{      
   switch(netStatus(socket))
   {
      case SOCK_CLOSED:
         //Open socket if closed
         socketOpen(socket, port);
         break;
         
      case SOCK_INIT:
         //Start listening if socket is open
         listen(socket);
         break;
         
      case SOCK_ESTABLISHED:   
         //process any modbus TCP requests if client is connected
         processModbusTcpRequest(socket, registerMemory, coilMemory);
         break;
   }   
}

The Ethernet module does not provide a receive interrupt to listen for, so you'll have to service the Modbus TCP slave in a timer, or in a never-ending loop. In the attached example we have chosen the latter.

Code: Select all

void cmain(void)
{
   //Network Settings
   u8 gatewayIP[]={192,168,0,2};
   u8 subnetMask[]={255,255,255,0};
   u8 macAdr[]={0,0,34,53,12,0};
   u8 moaconIp[]={192,168,0,12};
   netBegin(gatewayIP, subnetMask, macAdr, moaconIp);
   
   //Modbus Settings
   u8 socket = 0;
   u16 port = 502;
   static u8 coilMemory[8];
   static u16 registerMemory[8];
   
   //make sure the socket is closed before starting
   disConnect(socket);
   socketClose(socket);

   //Continually poll the socket to process modbus TCP requests
   while(1)
   {
      doModbusTcp(socket, port, registerMemory, coilMemory);
   }
}

To use, choose "Project" --> "Add Files to Project..." in Moacon Studio, and add fn_modbusTcp.c to your project.