In this tutorial we will learn How to communicate with an external device like a microcontroller board or modem using the Serial port of a windows PC(Windows XP,7). The software is written using C language and communicates with the Serial Port using Win32 API.

In Windows ,Serial ports are named as COM1,COM2 ,COM3.. etc .COM1 and COM2 usually refer to the hardware serial ports present in the PC while COM numbers in double digits like COM32,COM54,COM24.. etc are given to USB to Serial Converters or PCI serial port extenders.

If your PC does not have any hardware serial ports (RS232 DB9 ports), you can use USB to Serial Converter's like USB2SERIAL.

If you are interested in setting up an RS485 network controlled by your PC to control a robot or a sensor network ,you can use USB2SERIAL board (buy here).

The Board can also be used as an FT232 development board.

Sourcecodes

All the C sourefiles used in this tutorial can be downloaded from our GitHub Page.

Please note that the sourcecodes on the website show only the relevant sections to highlight the process of programming the serial port.

Please use the complete sourcecodes from our github repo when building your own program.

If you are new to Github ​Check this article to download code .

Compilers and IDE's used

To Compile the C file you can use either Visual Studio Community edition from Microsoft or MinGW (Windows port of GCC).

Visual Studio 2013 Express Edition for Desktop can be freely downloaded (its a large download) from Microsoft's website.

If you don’t want to download the Visual Studio and want something simpler you can use MinGW, which is an open source port of GCC for windows environment. After you have installed the MinGW, package make sure to add gcc to system path so that you can call it from anywhere.

Finding out your COM port Number

To find out the COM number corresponding to your serial port, Open Device Manager by right clicking on My Computer icon and selecting Manage → Device Manager.Under Ports(COM & LPT) you can see the parallel and serial ports (COM) detected by your system.

If your PC has any hardware ports, it will be shown either as COM1 or COM2 under the Ports Section. I am using a FTDI based USB to Serial Converter (USB2SERIAL)which is recognized as COM24 (this may be different under your system). If you double click on COM24,you can see the details of the corresponding port.

Opening and Closing a Serial Port

In Windows we use the CreateFile() function to open a serial port.

CreateFile() is a Win 32 function which is used to create or open a file, stream or an IO device like serial port.On success CreateFile() will return a handle which is then used to refer the connection in all subsequent operations.

After opening a serial port using the CreateFile() function you should close it with CloseHandle() function, otherwise port will become unavailable to other programs.

Now let's write a small program to open and close a serial port on Windows. Open a text editor like notepad or Notepad++ and type the below code and save it as “serial.c”.If you are using IDE like VS Express, use the one integrated with it.

#include<windows.h>

#include<stdio.h>

int main()

{

HANDLE hComm; hComm = CreateFile(“\\\\.\\COM24”, //port name

GENERIC_READ | GENERIC_WRITE, //Read/Write

0, // No Sharing

NULL, // No Security

OPEN_EXISTING,// Open existing port only

0, // Non Overlapped I/O

NULL); // Null for Comm Devices if (hComm == INVALID_HANDLE_VALUE)

printf(“Error in opening serial port”);

else

printf(“opening serial port successful”); CloseHandle(hComm);//Closing the Serial Port return 0;

}

Find out the COM port corresponding to your system and substitute in CreateFile() instead of COM24.

Now compile and run the program by pressing F5 in Visual Studio

or

by running the following command for gcc (MingW).Please make sure that gcc is added to you system path.

D:\> gcc -o serial serial.c

Now let me explain the code ,

windows.h header file contain all the definitions, function prototypes and constants required by the program.

In Windows everything is controlled by using handles.In the first line

HANDLE hComm;

we declare a handle hcomm to access and control the serial port.

Next we open a connection to serial port using CreateFile() function. The CreateFile() function on success, returns a valid handle to the hComm variable.

CreateFile() function takes 7 arguments,

1. Name of the serial port to be opened here \\\\.\\COM24.

2. Mode of access, here Read and Write

3. Sharing options, Serial ports can't be shared so 0

4. NULL for Serial ports, used for File operations

5. Open the existing port, OPEN_EXISTING

6. Overlapped IO or Non overlapped IO, here 0 means we are using NonOverlapped IO. Overlapped IO is used for multithreaded programs where

several threads can interact with the port simultaneously.

7. NULL for Serial port, used for file operations

If the function succeeds in opening the serial port, it returns a valid handle to hcomm which is then used for error checking.

After that the connection to the port is closed using

CloseHandle(hComm);

Please note that in Windows, COM port numbers from COM1 to COM9 are reserved by the system. If you are using a serial port whose COM port number falls in that range, you don’t need the back slashes (\\\\.\\)shown in the above code.

You can access the serial port like this,

hComm = CreateFile(“COM1”, // for COM1—COM9 only

GENERIC_READ | GENERIC_WRITE, //Read/Write

0, // No Sharing

NULL, // No Security

OPEN_EXISTING, // Open existing port only

0, // Non Overlapped I/O

NULL);

Configuring the DCB Structure

In Windows ,settings like Baud rate ,Number of start/Stop bits,data formats etc for the serial port are controlled by the DCB structure.

To Configure the DCB structure we use two functions,

GetCommState() function which retrieves the current control settings of the serial port and

SetCommState() function which configures the serial port with the new values in DCB structure provided by us.

First you declare a new structure of type DCB and initializes it.

DCB dcbSerialParams = { 0 }; // Initializing DCB structure

dcbSerialParams.DCBlength = sizeof(dcbSerialParams);

After that retrieve the current settings of the serial port using the GetCommState() function.

Status = GetCommState(hComm, &dcbSerialParams);

and set the values for Baud rate, Byte size, Number of start/Stop bits etc.

dcbSerialParams.BaudRate = CBR_9600; // Setting BaudRate = 9600

dcbSerialParams.ByteSize = 8; // Setting ByteSize = 8

dcbSerialParams.StopBits = ONESTOPBIT;// Setting StopBits = 1

dcbSerialParams.Parity = NOPARITY; // Setting Parity = None

if you want to change the Baud rate values prefix standard values with CBR like this CBR_4800,CBR_9600,CBR_192600 etc.

Number of Stop bits can be changed to ONESTOPBIT or TWOSTOPBITS.

Parity can also be changed to EVENPARITY,ODDPARITY,NOPARITY etc.

Now its time to configure the serial port according to the DCB structure using SetCommState() function.

SetCommState(hComm, &dcbSerialParams);

Setting Timeouts

Timeouts helps to prevent your program from waiting endlessly till data arrives. It helps the read or write calls to return after a set time period has elapsed.



COMMTIMEOUTS timeouts = { 0 };

timeouts.ReadIntervalTimeout = 50; // in milliseconds

timeouts.ReadTotalTimeoutConstant = 50; // in milliseconds

timeouts.ReadTotalTimeoutMultiplier = 10; // in milliseconds

timeouts.WriteTotalTimeoutConstant = 50; // in milliseconds

timeouts.WriteTotalTimeoutMultiplier = 10; // in milliseconds



All values are in milliseconds.

ReadIntervalTimeout Specifies the maximum time interval between arrival of two bytes. If the arrival time exceeds these limits the ReadFile() function returns.

ReadTotalTimeoutConstant is used to calculate the total time-out period for read operations. For each read operation, this value is added to the product of the ReadTotalTimeoutMultiplier member and the requested number of bytes.

ReadTotalTimeoutMultiplier is used to calculate the total time-out period for read operations. For each read operation, this value is multiplied by the requested number of bytes to be read.

WriteTotalTimeoutConstant similar to ReadTotalTimeoutConstant but for write operation.

WriteTotalTimeoutMultiplier similar to ReadTotalTimeoutMultiplier but for write operation.

After this you have to set the values using SetCommTimeouts() function.

Writing Data to Serial Port

Writing data to the opened serial port is accomplished by the WriteFile() function. WriteFile() function can be used to write both into the files and I/O ports.

char lpBuffer[] = "A";

DWORD dNoOFBytestoWrite; // No of bytes to write into the port

DWORD dNoOfBytesWritten = 0; // No of bytes written to the port

dNoOFBytestoWrite = sizeof(lpBuffer); Status = WriteFile(hComm, // Handle to the Serial port

lpBuffer, // Data to be written to the port

dNoOFBytestoWrite, //No of bytes to write

&dNoOfBytesWritten, //Bytes written

NULL);

hComm Handle of the serial port to write.

lpBuffer[] character array containing the data to write into the serial port.

dNoOFBytestoWrite is the total number of bytes to be written into the serial port.Here we are using the sizeof() operator to find out that .

dNoOFBytestoWrite = sizeof(lpBuffer);

dNoOfBytesWritten is the total number of bytes written successfully to the port by the WriteFile() operation.

Now in the Zip File containing the source codes you can find “USB2SERIAL_Write_W32.c” which contains the complete code for writing into serial port. You can compile the code using Visual Studio Express or GCC.

If your PC does not have any hardware serial ports you can use any USB to Serial Converters(I am using USB2SERIAL).

I have interfaced a microcontroller board(MSP430G2553 on Launch Pad) to the serial port using a null modem cable like this

You can use any microcontroller of your choice like 8051,AVR or ARM(LPC2148).The Controller waits for a character to be received and lights up the corresponding LED. The code for MSP430 is included in the zip file.If you want to know how to configure the MSP430 controller UART you can check this tutorial.



Please note that if you are using a DB9 RS232 Serial Port of your PC, you will have to build a RS232 signal level converter at the microcontroller side to decode the RS232 signal.

Directly connecting the PC's RS232 Serial port to MSP430 's pins will damage the chip.

Here is the screen shot of the Program writing into serial port.

Reading from the Serial Port

Reading from the serial port is accomplished by the ReadFile() function.

One way to do that is to use polling where the ReadFile() continuously reads from the serial port and checks for any received characters.

Other way is to setup an event and let windows notify us when a character is received.

We are going to use the second method here, following are the steps.

1. Create an Event for a particular action like character reception, change in modem lines etc using SetCommMask() function .

2. Ask windows to wait for the event set by SetCommMask() function using WaitCommEvent() and notify us when the condition happens.

3. Call ReadFile () to read the received data from the Serial port.

Functions used are

SetCommMask() is used to set the events to be monitored for a communication device. Here we are going to set the event as character received (EV_RXCHAR).The function takes two arguments, Handle of the serial port (hComm) and the code for the event (EV_RXCHAR) to be monitored.

Status = SetCommMask(hComm, EV_RXCHAR);

WaitCommEvent() is used to wait for the events set by SetCommMask() to happen, in this case reception of a character. The flow of execution of the program stops and the program waits until a character is received.

DWORD dwEventMask;

Status = WaitCommEvent(hComm, &dwEventMask, NULL);

dwEventMask contains a hex value indicating the event, which has caused WaitCommEvent() to return.

After WaitCommEvent() has returned, call ReadFile() function to read the received characters from the Serial Port Buffer.

char TempChar; //Temporary character used for reading

char SerialBuffer[256];//Buffer for storing Rxed Data

DWORD NoBytesRead;

int i = 0; do

{

ReadFile( hComm, //Handle of the Serial port

&TempChar, //Temporary character

sizeof(TempChar),//Size of TempChar

&NoBytesRead, //Number of bytes read

NULL); SerialBuffer[i] = TempChar;// Store Tempchar into buffer

i++;

} while (NoBytesRead > 0);

ReadFile() function is similar to the WriteFile() function we had seen earlier,instead of writing we are reading from the serial port.

&TempChar Temporary variable used to store the byte read from serial port buffer.

sizeof(TempChar) used to calculate the number of bytes to read.

&NoBytesRead Bytes successfully read by the ReadFile().

Now in the Zip File containing the source codes you can find “USB2SERIAL_Read_W32.c” which contains the complete code for writing into serial port. You can compile the code using Visual Studio Express or GCC.

On running “USB2SERIAL_Read_W32.exe” ,The code will wait for the characters to be transmitted by the microcontroller.

Reset the Micro controller to transmit the string “Hello from MSP430”.

Check out our next section, If you want to know how to control the RTS and DTR pins of the serial port .

Back to Serial Programming Tutorials