Question

I’m writing some code to interface with a piece of hardware. The hardware connects to the PC via a USB with a USB-to-Serial converter inside the device (it shows up as a COM port device in Windows).

I’m having issues with the Win32 API ReadFile system call. I can’t seem to get it to work as advertised. I’ve setup the COMMTIMEOUTS structure as so:

COMMTIMEOUTS ct;
ct.ReadIntervalTimeout = MAXDWORD;
ct.ReadTotalTimeoutconstant = 0;
ct.ReadTotalTimeoutMultiplier = 0;
ct.WriteTotalTimeoutConstant = 0;
ct.WriteTotalTimeoutMultiplier = 0;

if(SetCommTimeouts(device_id_, &ct) == 0)
{
     return ERROR; // this is never hit.
}

Which according to the Win32 API documentation, says:

ReadIntervalTimeout

The maximum time
allowed to elapse between the arrival
of two bytes on the communications
line, in milliseconds. During a
ReadFile operation, the time period
begins when the first byte is
received. If the interval between the
arrival of any two bytes exceeds this
amount, the ReadFile operation is
completed and any buffered data is
returned. A value of zero indicates
that interval time-outs are not used.

A value of MAXDWORD, combined with
zero values for both the
ReadTotalTimeoutConstant and
ReadTotalTimeoutMultiplier members,
specifies that the read operation is
to return immediately with the bytes
that have already been received, even
if no bytes have been received.

The command I’m sending is supposed to return a single byte integer. Most of the time, the command is received by the device and it returns the appropriate value. Sometimes, however, it doesn’t seem to return a value and ReadFile() blocks until more bytes are recieved (eg. by pressing buttons on the device). Once a button is hit, the initial integer response I was expecting is received along with the button press code. While this isn’t the behavior I’m expecting from the device itself, I’m more concerned with ReadFile() blocking when it shouldn’t be, according to the MSDN documentation. Is there a remedy for ReadFile() blocking here?

Answer 1

D’oh! Turns out ReadFile blocking was just a symptom, not the problem. The hardware device in question only has a 4MHz processor in it. Splitting up the 3 character command written to the device and sending them individually with a 1ms pause between characters fixes the issue.