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I configure a connection between my app and the COM1 port of a PC using the following piece of code:
hComm = CreateFile(("COM1"),
GENERIC_READ | GENERIC_WRITE,
0,
0,
OPEN_EXISTING,
FILE_FLAG_OVERLAPPED,
0);
then using the GetCommState() function for reading the settings of the port and SetCommState() for setting the desired value's and trying to read the inputbuffer with ReadFile() I stumbled on a problem:
When starting my app this reports no error at all but it doesn't detect any input on the serial port (in my case a barcode scanner) and stops the reading function on a time_out and the handle is closed. If I start now (even when my app is still running) the Windows HyperTerminal with the appropriate settings for the scanner communication and quit the HyperTerminal again (even if I didn't scan any code, just establishing the connection and terminating it) and I start my app again the scanned codes are shown. If I check the settings of the DCB they are the same, when I run my app and then HyperTerminal.
Do I have to define something, somewhere regarding the "COM1" string used in the CreateFile section, and if so how is it done?
Frank Peeters
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FPeeters wrote: then using the GetCommState() function for reading the settings of the port and SetCommState() for setting the desired value's
What are you setting with SetCommState ? The baudrate ?
Make sure that you specify the correct parity, stop bits and byte size. Does your barcode scanner use some kind of handshaking ? That might be the problem.
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If I look in HyperTerminal the dataflow control is set to NONE and I will try to set the DCB also to NONE.
But, how is it possible that the same app runs well when het Hyperterminal is started and stopped before the app starts to communicate and doesn't communicate when I start after rebooting Windows and not opening the hyperterminal?
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Did you check return values (you cannot have COM1 open for both Hyperterminal and your application)?
If the Lord God Almighty had consulted me before embarking upon the Creation, I would have recommended something simpler.
-- Alfonso the Wise, 13th Century King of Castile.
This is going on my arrogant assumptions. You may have a superb reason why I'm completely wrong.
-- Iain Clarke
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The Com1 port is closed in my app when I receive a Time_Out event (CloseHandle). So when opening the Hyperterminal after this event shouldn't generate a problem (otherwise HyperTerminal creates an error message). Also, as far as I know, if I try to create a handle to an open COM port, C++ generates an errorcode. But, please correct me if I am wrong.
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FPeeters wrote: if I try to create a handle to an open COM port, C++ generates an errorcode
What do you mean with that ?
In fact, if you are trying to open a port which is already opened, the CreateFile function will return an invalid handle (check the documentation of this function). So, before trying to read on the port, you should verify that your handle is valid (handle != INVALID_HANDLE_VALUE).
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Cedric Moonen wrote: In fact, if you are trying to open a port which is already opened, the CreateFile function will return an invalid handle (check the documentation of this function). So, before trying to read on the port, you should verify that your handle is valid (handle != INVALID_HANDLE_VALUE).
Sorry for the confusion, but this is what I was trying to say...
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When I look at an article written by Konchat (Creating a Serial Communication on WIN32) he shows a screenshot (figure 2) where he sets portName as "COM1", but in the presented watch screen that portname has a value (0x00421a2c "COM1"). I assume that the hex code is in fact the real adress of the port. I haven't declared anything like this is my app, and I don't know what to declare and where.
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Did you check all return values?
CreateFile returns a valid handle?
GetCommState and SetCommState return TRUE?
If the Lord God Almighty had consulted me before embarking upon the Creation, I would have recommended something simpler.
-- Alfonso the Wise, 13th Century King of Castile.
This is going on my arrogant assumptions. You may have a superb reason why I'm completely wrong.
-- Iain Clarke
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As far as I can tell CreateFile returns a valid Handle and GetCommState and SetCommState are true.
Here is in pseudo-code how I did it:
hComm=CreateFile(("COM1"), GENERIC_READ|GERERIC_WRITE,0,0,OPEN_EXISTING,FILE_FLAG_OVERLAPPED,0);
if (hComm == INVALID_HANDLE_VALUE)
//ERROR MESSAGE TO LISTBOX
else
//OK MESSAGE TO LISTBOX
if (!GetCommState(hComm,&dcb))
//ERROR MESSAGE TO LISTBOX
else
//OK MESSAGE TO LISTBOX
if (!SetCommState(hComm, &dcb))
//ERROR MESSAGE TO LISTBOX
else
//OK MESSAGE TO LISTBOX
if(osReader.hEvent == NULL)
//ERROR MESSAGE TOLISTBOX
else
while (ReadActive)
if(!ReadFile(hComm,lpBuff,sizeof(lpBuff),&dwRead,&osReader))
//ERROR MESSAGE TO LISTBOX
else
dwResult = WaitForSingleObject(osReader.hEvent, READ_TIMEOUT);
switch(dwResult)
case WAIT_OBJECT_0
case WAIT_TIMEOUT
default
So if I get an INVALID_HANDLE_VALUE the rest of the code is skipped and an errormessage is written in a ListBox, also if either the GetCommState() and SetCommState() function restuns FALSE I should receive an error message in this ListBox. But I don't receive an error message and the app reports a Time-out on the Listbox.
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Maybe the read operation was completed before the wait starts. Why don't you call GetOverlappedResult?
If the Lord God Almighty had consulted me before embarking upon the Creation, I would have recommended something simpler.
-- Alfonso the Wise, 13th Century King of Castile.
This is going on my arrogant assumptions. You may have a superb reason why I'm completely wrong.
-- Iain Clarke
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My question is:
Why is he opening a bar code scanner with FILE_FLAG_OVERLAPPED in the first place. I have never seen a bar code scanner which required overlapped I/O.
Best Wishes,
David Delaune
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Maybe his application requires it.
If the Lord God Almighty had consulted me before embarking upon the Creation, I would have recommended something simpler.
-- Alfonso the Wise, 13th Century King of Castile.
This is going on my arrogant assumptions. You may have a superb reason why I'm completely wrong.
-- Iain Clarke
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No the reason why I used this kind of IO is quit simple: I couldn't find any explanation on how to run the COM port otherwise... So if there is any suggestion on how to work with a barcode scanner I would like to know.
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You can simply use COM synchronously.
If the Lord God Almighty had consulted me before embarking upon the Creation, I would have recommended something simpler.
-- Alfonso the Wise, 13th Century King of Castile.
This is going on my arrogant assumptions. You may have a superb reason why I'm completely wrong.
-- Iain Clarke
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In all the articles I've found everyone says you can use com synchronously but no-one explains it.
I am a starter on C++ (formerly programming PLC's and slowly changing over to PC control for some machines).
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If you configure a COM for synchronous operations (see for instance [^]) then each call to ReadFile becomes a blocking one (until timeout expires, see [^]).
If the Lord God Almighty had consulted me before embarking upon the Creation, I would have recommended something simpler.
-- Alfonso the Wise, 13th Century King of Castile.
This is going on my arrogant assumptions. You may have a superb reason why I'm completely wrong.
-- Iain Clarke
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Thanks for the info, but to go back to my original problem: Does this solve it, or is it a better way to communicate for me with the com port?
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At least, you can try.
If the Lord God Almighty had consulted me before embarking upon the Creation, I would have recommended something simpler.
-- Alfonso the Wise, 13th Century King of Castile.
This is going on my arrogant assumptions. You may have a superb reason why I'm completely wrong.
-- Iain Clarke
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OK I deserved that one, ofcourse I can try, and I will..
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Okay, I've tried the synchrounous approach but there is no change in the situation.
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FPeeters wrote: I configure a connection between my app and the COM1 port of a PC using the following piece of code:
hComm = CreateFile(("COM1"),
Shouldn't this be:
CreateFile("\\\\.\\COM1", ...);
"Love people and use things, not love things and use people." - Unknown
"To have a respect for ourselves guides our morals; to have deference for others governs our manners." - Laurence Sterne
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Both methods are correct. You could consider "COM1" as a mapping to "\\\\.\\COM1" such as is created by DefineDosDevice[^]
Best Wishes,
-David Delaune
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DavidCrow wrote: Shouldn't this be:
CreateFile("\\\\.\\COM1", ...);
This is, sadly enough, not the solution to my problem. But I'm curious: what is the meaning of "\\\\.\\"
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FPeeters wrote: But I'm curious: what is the meaning of "\\\\.\\"
It's just a format that some arguments to CreateFile() must have. COM1 must not be one of those. Sorry 'bout that.
"Love people and use things, not love things and use people." - Unknown
"To have a respect for ourselves guides our morals; to have deference for others governs our manners." - Laurence Sterne
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