🚀 go-pugleaf

I'm have an application that communicates with an embedded controller via 
the serial port, either directly with a serial cable or via modem.

When I'm connected directly with a serial cable the communications work 
fine.  At a baudrate of 19200 I can send/receive 100 messages per sec.  But 
when I connect with a modem at 19200 the communications slow down 
significantly.  I'm only able to send/receive about 8 messages per sec.  Of 
course I didn't expect to match the speed of a direct connection, but I'm 
off by more than a factor of 10.

I am using overlapped communications.  The delay seems to be in the 
GetOverlappedResut( ) of my GetChar() routine.  I added some debug 
statements,  and the GetOverlappedResult( ) functions takes about 150 ms to 
return when I use a modem.  Can anyone advise me in how to speed this up.

Here's the relevent code:

overlapped.Offset = 0;
overlapped.OffsetHigh = 0;
overlapped.hEvent = NULL;

ibytes = 0;
ierr = ERROR_IO_PENDING;
if (!ReadFile(hcomm, xbyte, 1, &ibytes, &overlapped))
    {
    ierr = GetLastError();
    }

while ((!ibytes) &&
            (clock() < (int)timeout) &&
            ((ierr == ERROR_IO_PENDING) ||
            (ierr == ERROR_IO_INCOMPLETE)))
    {
    if (!GetOverlappedResult(hcomm, &overlapped, &ibytes, true))  //  about 
150ms to return
         {
        ierr = GetLastError();
        }
    }

return(ibytes != 0);


Thanks in advance for your help,
Phil