如何获取串行 COM 端口的状态

Question

What I mean is that, when I code a project, I need to communicate with the serial port like COM1 , COM2 ... but sometimes there is no device connected and I also can use the function CreateFile to get the COM port handle.

When I use the WriteFile function to send a string to the COM port the software blocks.

After I dig into the problem I find another function GetCommModemStatus which can get status of the COM port but when I use the usb-rs232 transition line, the second parameter always returns 0 .

How can I get the COM port status so that I can check if is there some devices connected to the computer?

Answer 1

If I understand correctly, you want to detect if a device is connected to your COM port and ready to accept packets. If that's the case, you need to check control signals (DTR/DSR and CTS/RTS) before sending data, assuming your device is aware of them and sets the appropriate PIN on your DB-9 or DB-25 connector. Some devices rely on software handshaking instead (XON/XOFF) and do not set control signals. Your best bet would be to consult documentation of your device.

I have been using ComPort Library version 4.10 by Dejan Crnila. It does support both hardware and software handshakings, so you can focus on your own code instead of reinventing the wheel .

Answer 2

As several people have already pointed out, it is not a good idea to try to "re-invent the wheel." Except for "quick and dirty" testing, your code will have to handle the com port in a separate thread and the available solutions all make this much easier.

BUT, if you Really want to do it, I'll give you some pointers.

If you are using "WriteFile" then you have probably already figured out the "CreateFile" part of the procedure and how complicated things can get depends upon what kind of IO you specified in that procedure, Overlapped or not. Overlapped IO is more complicated but does let the OS handle some of the burden.

You mentioned that your call to "WriteFile" hangs. You should look up the "SetCommTimeOuts" function. Setting both the WriteTotalTimeoutMultiplier and WriteTotalTimeoutConstant members of TCommTimeouts to zero will tell the OS to return immediately. You may also need to "SetCommMask" if your target uses handshaking.

What happens next really depends on what your target is supposed to do. The GetCommMask function can be used to check the status of the handshake lines.