Interfacing to the PC Serial Port

In almost all cases, when interfacing to the serial port of a PC, the tools that come with the PC are adequate. There are cases, however, when HyperTerminal isnt sufficient. For example, if you need to observe binary data on a serial line or if you need to implement a custom serial protocol on the target, you will find it quite handy to be able to manipulate the UART on the PC directly. Listing A.3 is a base for a home-grown serial port interface that you can use in place of the default Windows tools when necessary.

Listing A.3: Base for a Home-Grown Serial Port Interface.

 HANDLE comOpen(int portnum, int baud) {     DCB dcb;     HANDLE hCom;     char portname[16];     sprintf(portname,"COM%d",portnum);     hCom = CreateFile(portname,         GENERIC_READ  GENERIC_WRITE,   // Open for read/write.         0,                              // Sharing flags.         NULL,                           // Security attributes.         OPEN_EXISTING,                  // Required for serial port.         0,                              // File attributes (NA).         NULL);                          // Required for serial port.     if (hCom == INVALID_HANDLE_VALUE) {         ShowLastError("comOpen() CreatFile()");         return(INVALID_HANDLE_VALUE);     }     // Fill in the DCB...     if (!GetCommState(hCom,&dcb)) {         ShowLastError("comOpen() GetCommState()");         return(INVALID_HANDLE_VALUE);     }     dcb.BaudRate = baud;     dcb.ByteSize = 8;     dcb.Parity = NOPARITY;     dcb.StopBits = ONESTOPBIT;     dcb.fDtrControl = DTR_CONTROL_ENABLE;     dcb.fDsrSensitivity = FALSE;     dcb.fOutX = FALSE;     dcb.fInX = FALSE;     dcb.fNull = FALSE;     if (!SetCommState(hCom,&dcb)) {         ShowLastError("comOpen() SetCommState()");         return(INVALID_HANDLE_VALUE);     }     return(hCom); } void comClose(HANDLE hCom) {     CloseHandle(hCom); } int comRead(HANDLE hCom,char *buf,int count) {     DWORD   bytesread;     DWORD   tot;     tot = (DWORD)count;     while(tot) {         if (ReadFile(hCom,buf,(DWORD)tot,             &bytesread,NULL) != TRUE) {             ShowClearCommError(hCom);             ShowLastError("comread ReadFile()");             return(-1);         }         tot -= bytesread;         buf += bytesread;     }     return(count); } int comWrite(HANDLE hCom, char *buffer,int count) {     DWORD   byteswritten;          if (WriteFile(hCom,buffer,(DWORD)count,         &byteswritten,NULL) != TRUE)             return(-1);     return((int)byteswritten); } 

 

The code in Listing A.3 depends on the Windows services: CreateFile() , GetCommState() , SetCommState() , ReadFile() , and WriteFile() . For details on these system calls, refer to the Visual C++ CD documentation. The comOpen() function is a wrapper for CreateFile() , GetCommState() , and SetCommState() to open a com port that creates a convenient hook to configure baud, parity, and other serial-port parameters. The comRead() and comWrite() functions create convenient wrappers for the ReadFile() and WriteFile() system calls, which are used to transmit to and receive from the serial port.