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reading/writing to COM1 using MFC

Posted on 1998-09-23
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Last Modified: 2013-11-19
I need to read/write to COM1 using Visual C++, I'm talking
to an external board. The board is connected to a video
camera, essentially the whole thing is a vision system for
little robots (robotic soccer). So the external vision board
sends data on the location of the soccer ball, which I must
read in off of COM1. I will also need to write data once in
awhile. thanks!

Anthony Nicholson
nicholso@ukans.edu
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Question by:nicholso
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Accepted Solution

by:
dataman earned 100 total points
ID: 1322530
You can find an object which handles serial communication on the following site
(including the source code)
www.codeguru.com

I did not use that specific object but I used other objects from this site and they
are very good.

Hope this helps


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Expert Comment

by:snoegler
ID: 1322531
Dataman, you seem to have answered before i did :) But i'll post this as a comment, though.
The COM port is treated like a file in the Windows environment. To open the COM1 port, you
can use something like this:

HANDLE hComPort=CreateFile("COM1:",
                                                         GENERIC_READ|GENERIC_WRITE,
                                                         0, // dwShareMode
                                                         NULL, //lpSecurityAttributes
                                                         OPEN_EXISTING,
                                                         FILE_FLAG_WRITE_THROUGH,
                                                         NULL);  // hTemplateFile

Important: You have to use OPEN_EXISTING(dwCreationDistribution).

Then, you'll have to set the communication parameters(the example below is for
9600,n,8,1, CTS/RTS flow control)

a) "complex" version (see "easy" version below)

DCB  commDCB;

ZeroMemory(&commDCB,sizeof(commDCB)); // set all unused values to zero
commDCB.DCBlength=sizeof(DCB);
commDCB.BaudRate=9600;
commDCB.fBinary=TRUE; // no eof check
commDCB.fParity=FALSE;// no parity checking (see also commDCB.Parity)
commDCB.fOutxCtsFlow=TRUE; // enable CTS
commDCB.fOutxDsrFlow=FALSE; // no DSR
commDCB.fDtrControl=DTR_CONTROL_DISABLE; // no DTR
commDCB.fDsrSensitivity=FALSE;
commDCB.fNull=FALSE; // do not omit '\0' characters
commDCB.fRtsControl=RTS_CONTROL_HANDSHAKE; // RTS handshake enabled
commDCB.fAbortOnError=FALSE; // no aborting if error occurs
commDCB.ByteSize=8; // 8 Bits
commDCB.Parity=NOPARITY;
commDCB.StopBits=ONESTOPBIT;
commDCB.fOutX=FALSE; // no XON/XOFF handshaking on sending
commDCB.fInX=FALSE; // no XON/XOFF handshaking on receiving
SetCommState(hComFile,&commDCB);

b) "easy" version:

DCB commDCB;
BuildCommDCB("baud=9600 parity=N data=8 stop=1");
// Pls. see the online help on 'BuildCommDCB' for further details, as it is a quite large
// subject

At last, you should the the timeouts(the example sets them to 'none' - so ReadFile() and
WriteFile() return immediately if nothing was received/sent).

COMMTIMEOUTS cto;
cto.ReadIntervalTimeout=MAXDWORD;
cto.ReadTotalTimeoutMultiplier=0;
cto.ReadTotalTimeoutConstant=0;
cto.WriteTotalTimeoutMultiplier=0;
cto.WriteTotalTimeoutConstant=0;
SetCommTimeouts(commFile,&cto);

Now, you can read and send data:

BYTE readBuffer[16];
DWORD bytesRead=0;
BOOL success;
success=ReadFile(commFile,(LPVOID)readBuffer,16,&bytesRead,NULL);
if(!success)
  // some error occured - this does *not* happen if there is no input to read!!!

if(bytesRead==0)
  // no data available

// Data received, number of bytes in bytesRead


'WriteFile' sends data in the same manner as above.

You can also use an OVERLAPPED structure if don't want to poll if there is data to read.
But if your application isn't that time critical you could possibly use a WM_TIMER message
to check if there is data available. OVERLAPPED offers event-driven reading, so you
can come closer to realtime, if necessary. If you want, i can explain.

Hope this helped :)
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