ReadFile () reads Junk characters when tried to read from COM1 Port

pandit84

Hi , I have created sample application which opens the serial COM Port and tried to write and Read the data. I have used CreateFile () method to open the COM1.

hCom = CreateFile (pcCommPort,GENERIC_READ | GENERIC_WRITE,0,NULL, OPEN_EXISTING, 0,NULL );

Then I set the COM state

 GetCommState( hCom, &lpCC.dcb); 
 lpCC.dcb.BaudRate = CBR\_9600;     //  baud rate
 lpCC.dcb.StopBits = ONESTOPBIT;    //  stop bit	
 lpCC.dcb.ByteSize = 8;             //  data size, xmit and rcv		 
 lpCC.dcb.Parity   = NOPARITY;      //  parity bit
 lpCC.dcb.fBinary = TRUE ;
 lpCC.dcb.fParity = TRUE ;
      SetCommState( hCom, &lpCC.dcb );

I am trying to write the data using

WriteFile (hCom,(LPCVOID)ptr,20,&byteswritten,NULL);

and trying to read the data using

ReadFile (hCom, (LPVOID)ptr, 20, &dwBytesTransferred, 0);

ptr is pointer to the BYTE array. The above code of "ReadFiles() returns true and Reads Junk characters. I have written exact 20 characters through WriteFile() and it returned TRUE . I am not able to figure out the exact problem . Please provide any input in this regards. Thanks

David Crow

pandit84 wrote:

...and Reads Junk characters.

How are you verifying this? Do you know that data is available to read before calling ReadFile()?

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pandit84

I am printing the ptr using printf statement.

barneyman

if you're doing this between two computers, you are using a null-modem cable?

David Crow

And that looks like?

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"Fireproof doesn't mean the fire will never come. It means when the fire comes that you will be able to withstand it." - Michael Simmons

"Some people are making such thorough preparation for rainy days that they aren't enjoying today's sunshine." - William Feather

pandit84

I have not yet tried this on two computers. I am using only one computer to test.

pandit84

"═══════════════════════════════════════²²²²"

barneyman

I doubt very much the COM redirector understands how to loopback i.e. You can't read and write on the same COM port on the same computer, and if you have a machine with two COM ports, you still need a null modem cable between them

modified on Wednesday, May 18, 2011 4:31 AM

David Crow

That's what your printf() statement looks like?

"One man's wage rise is another man's price increase." - Harold Wilson

"Fireproof doesn't mean the fire will never come. It means when the fire comes that you will be able to withstand it." - Michael Simmons

"Some people are making such thorough preparation for rainy days that they aren't enjoying today's sunshine." - William Feather

pandit84

oops I am sorry

printf("The date %s", ptr) ;

David Crow

Ok, so are you using communications events?

"One man's wage rise is another man's price increase." - Harold Wilson

"Fireproof doesn't mean the fire will never come. It means when the fire comes that you will be able to withstand it." - Michael Simmons

"Some people are making such thorough preparation for rainy days that they aren't enjoying today's sunshine." - William Feather

pandit84

No I am not using communication event. I have tried using

::WaitCommEvent

. But it never get any event and it goes into continuous wait state. So I have removed that and tried to read the Port using ::ReadFile () without using any event.

kakan

You just got to use overlapped I/O when you are using COM ports. There are some classes/samples here. Just search for a suitable article.

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David Crow

pandit84 wrote:

. But it never get any event and it goes into continuous wait state.

Which means nothing is available to read.

pandit84 wrote:

So I have removed that and tried to read the Port using ::ReadFile () without using any event.

So reading what's not available (to read) seemed like a better idea?

"One man's wage rise is another man's price increase." - Harold Wilson

"Fireproof doesn't mean the fire will never come. It means when the fire comes that you will be able to withstand it." - Michael Simmons

"Some people are making such thorough preparation for rainy days that they aren't enjoying today's sunshine." - William Feather

pandit84

Finally this solution works for me. What I did is 1. In ReadFile() - Read from different COM port. E.G Writing is done on COM1 and Reading is Done on COM2. As I was having only 1 COM Port . I installed Virtual Port Software on my machine and Paired COM1 and COM2 2. ReadFile() provided a buffer of exact size which I used in WriteFile () method.