Serial port, WriteFile affects ReadFile - c

Question

I'm having a problem with the serial port code.
I just do:
opencomm();
send();
closecomm();
and the ClearCommError() (inside recv()) returns in
comstat.cbInQue the same amount that was sent.
So, if sizeof (sendbuff) is 100,
I get 100 in comstat.cbInQue.
After reading one byte with ReadFile, comstat.cbInQue decrements (after subsequent ClearCommError(), of course).
The values read are not the ones that were written.
There is no device connected to the port.
The strangest thing is this code used to work, but not anymore.
WORD sendbuff[128];
static HANDLE hComm;
static void opencomm (void)
{
static COMMTIMEOUTS timeouts = {0,0,0,0,0};
static DCB dcb = {
sizeof (DCB), // DCBlength
115200, // * BaudRate
1, // fBinary
0, // * fParity
0, // fOutxCtsFlow
0, // fOutxDsrFlow
0, // fDtrControl
0, // fDsrSensitivity
1, // fTXContinueOnXoff
0, // fOutX
0, // fInX
0, // fErrorChar
0, // fNull
0, // fRtsControl
0, // fAbortOnError
0, // fDummy2
0, // wReserved
8*k, // XonLim
2*k, // XoffLim
8, // * ByteSize
0, // * Parity
0, // * StopBits
0, // XonChar
1, // XoffChar
0, // ErrorChar
0, // EofChar
0, // EvtChar
0 // wReserved1
};
hComm = CreateFile("COM1", GENERIC_READ | GENERIC_WRITE, 0, NULL, OPEN_EXISTING, 0, NULL);
if (hComm != INVALID_HANDLE_VALUE) {
SetupComm(hComm, 16*k, 16*k);
SetCommState(hComm, &dcb);
SetCommTimeouts(hComm, &timeouts);
}
}
static void closecomm (void)
{
CloseHandle(hComm);
}
static BYTE recv (void)
{
BYTE text;
DWORD temp;
COMSTAT comstat;
while (1) {
ClearCommError(hComm, &temp, &comstat);
if (comstat.cbInQue != 0) break;
Sleep(1);
}
ReadFile(hComm, &text, 1, &temp, NULL);
return text;
}
static void send (void)
{
DWORD temp;
// send to other comp
WriteFile(hComm, sendbuff, sizeof (sendbuff), &temp, NULL);
// check other comp done
if (recv() != 0xAA) {
Beep(1000, 100);
quit(); // comm error
}
}

Answer 1

It was the cable. Not properly shielded and too long.