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<p align="center"><font face="Verdana" size="2"><b>[<u> </u></b></font><font size="2" face="Verdana"><b><u>Serial Communication in Windows</u></b></font><font face="Verdana" size="2"><b><u>
</u>]<br>
</b><a href="mailto:ashishdhar@hotmail.com"><i>ashish dhar</i></a></font></p>
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<p><font size="2" face="Verdana"><a href="../Source.zip">Source code</a><b><br>
<br>
</b>This article is meant to give you a jump start on doing serial communication
in Windows (NT family). The article will provide a class called
CSerialCommHelper which you can use directly to do serial communication in your
application. The class that is provided here with this article does uses
overlapped IO. You donot need to know much about serial communication or
overlapped IO for this article. However, you need to know some about the
synchronization objects like Events and some Windows APIs like
WaitForSingleObject and WaitForMultipleObject etc. Also some basic understanding
of windows threads is required - like thread creation and <i>termination</i>.</font></p>
<p><font size="2" face="Verdana"><b>Introduction<br>
</b>In order for your computer to be able to do serial communication, computer
has to have a serial port. Most of the computers have at least one serial port
also known as COM port ( communication port ) and are generally called COM1 COM2
etc. Then there are the device drivers for the serial ports. If you think it
over, all you that you need to do in serial communication is either send data or
receive data. In other words, you are doing input/output (IO) to the serial
port. The same IO is done with disk based files. Hence there is no surprise that
the APIs for reading and writing to a file apply to serial ports as well. When
you send data to the serial port its in terms of bytes but when it leaves the
serial port it is in the form of bits. Similarly, when the data arrives at the
serial port, its in bit format and when you get data you get it in bytes. <br>
Without any further discussion lets get started.<br>
<br>
<b>Opening the COM port<br>
</b>The first and the foremost step in doing a serial communication is to open
the desired port. Lets say you have your device hooked to COM1 you can open the
COM port using following API:</font></p>
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<pre><font size="2">HANDLE m_hCommPort = ::<font color="#0000FF">CreateFile</font>( szPortName,
GENERIC_READ|GENERIC_WRITE,//access ( read and write)
0, //(share) 0:cannot share the COM port
0, //security (None)
OPEN_EXISTING,// creation : open_existing
FILE_FLAG_OVERLAPPED,// we want overlapped operation
0// no templates file for COM port...
);</font></pre>
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<p><font size="2" face="Verdana">The third fifth and seventh parameters have to be what they
are in the above example by law. We want to open the file ( the COM port ) in an
overlapped fashion - that's why the sixth param is FILE_FLAG_OVERLAPPED. We will
get into the details of overlapped IO a bit later. As you must have guessed from
the name , CreateFile() API can be used to create a file (disk based) and also
it can be used to open an existing file. <br>
To Windows a serial port or a disk based file both are IO devices . So, in order
to open an existing file ( serial port ) all we need to know the name of the
device ( COM1) and pass the creation flags as OPEN_EXISTING.<br>
If a COM port is opened successfully, the API returns handle to the com port
just like a handle to a file. However, if the system could not open the COM
port, it would return INVALID_HANDLE_VALUE . And you can get the reason by
calling GetLastError(). One of the common errors while opening a COM port is
that the COM port is already opened by some other application and in that case
you would get ERROR_ACCESS_DENIED (5). Similarly if you by mistake opened a COM
port that doesnot exist , you would get ERROR_FILE_NOT_FOUND as the last
error.<br>
<u>Note: Remember not to do make any function calls (like ASSERT) before calling
GetLastError() or you would get 0</u>.<u><br>
</u>Once you have opened the com port all you need to do now is to start using
it.<br>
<br>
<b>Reading and Writing<br>
</b>Now, once you have a com port open, you may want to send some data to
the connected device. For example, lets say you want to send "Hello"
to the device(e.g., another PC). When you want to send the data across the
serial port, you need to write to the serial port just like you would write to a
file. You would use following API:<br>
</font></p>
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<td width="100%" bgcolor="#C0C0C0"><font size="2" face="Verdana">iRet = WriteFile (m_hCommPort,data,dwSize,&dwBytesWritten ,&ov); </font></td>
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<p><font size="2" face="Verdana">where data contains "Hello" . <br>
Lets say in response to your "Hello" , the device sends you
"Hi" . So you need to read the data. Again ,you would use
following API:</font></p>
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<td width="100%" bgcolor="#C0C0C0"><font size="2" face="Verdana">abRet = ::ReadFile(m_hCommPort,szTmp
,sizeof(szTmp ),&dwBytesRead,&ovRead) ;</font></td>
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<p><font size="2" face="Verdana"><i>For now do not try to understand everything.We will get to
all this later.<br>
</i>All this sounds very simple. Right? <br>
Now lets start digging into issues. </font></p>
<p><font size="2" face="Verdana"><b>Issues with serial communication</b><br>
Just now I said, in response to your "Hello", the device may send you
"Hi" back and you would like to read that. But the problem here is
that you don't know when the device is going to respond? Or will it ever
respond? When should you start to read from the port. One option is that as soon
as you made call to WriteFile, you make call to ReadFile . If no data is there
you need to make read again later on. This leads to what is called polling. You
keep polling the port for data. This model does not really seem to be a
good one. It would be nice if somehow you were notified by the system when data
has arrived and only then would you make call to ReadFile. This is event driven
approach and fits well into Windows programming. And good news is that such a
model is possible . <br>
<br>
<br>
<br>
Another issue with the serial communication is that since it always occurs
between two devices, the two devices need to agree on how they talk to each
other. Each side needs to follow certain protocols to conduct business. Since
its the serial port that actually carries out the communication, we need to
configure the serial port. There is an API available for exact same purpose.
Following is the API:</font></p>
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<td width="100%"><font size="2" face="Verdana">SetCommState ( HANDLE hFile, LPDCB lpDCB)</font></td>
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<p><font size="2" face="Verdana">The first parameter is the handle to COM port and the second
paramter is what is called device control block (DCB) . The DCB is a struct
defined in winbase.h and has 28 data members. For example, we need to specify
baud rate at which the COM port operates, you need to set the <b>BaudRate </b>member
of the struct . Baud rate is usual 9600 (bps) . But the two devices have to use
the same baud rate to conduct business. Similarly if you want to use parity you
need to set <b>Parity</b> member of the struct. Again the two devices have to
use same parity. Some of the data members are reserved and have to be 0. I have
found it easier to get the current DCB struct and then set those members which
we are interested in changing. Following code gets the current dcb and sets some
of the fields:<br>
<br>
</font></p>
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<pre><font size="2" face="Verdana">DCB dcb = {0};
dcb.DCBlength = sizeof(DCB);
if (!::<font color="#0000FF">GetCommState </font>(m_hCommPort,&dcb))
{
TRACE ( "CSerialCommHelper : Failed to Get Comm State Reason: %d",GetLastError());
return E_FAIL;
}
dcb.BaudRate = dwBaudRate;
dcb.ByteSize = byByteSize;
dcb.Parity = byParity;
if ( byStopBits == 1 )
dcb.StopBits = ONESTOPBIT;
else if (byStopBits == 2 )
dcb.StopBits = TWOSTOPBITS;
else
dcb.StopBits = ONE5STOPBITS;
if (!::<font color="#0000FF">SetCommState</font> (m_hCommPort,&dcb))
{
ASSERT(0);
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