SerialSend – a Windows program to send a short text string via serial port

NOTE (18-12-2012): Since posting this, I have updated SerialSend to add some additional useful features and make more robust. The updated version is available from the SerialSend page.

When I’m building robots, I sometimes find it useful to send short commands from the PC to a microcontroller (usually a dsPIC30F4011) in real-time via a USB-to-serial adapter. At the moment, one of the engineers in my Robotics module (Anthony Gaule) is trying to send commands from a machine vision program running on his laptop to a miniature SCARA arm that he has built (controlled by a dsPIC30F4011). Here’s the little Windows program of mine that he’s using. It just sends one word of text to an external device via a user-specified COM port. Binary download (from my github page):

SerialSend.exe (18kB, 6-12-2011)

Here’s the full source code:

//
// SerialSend.c - This program sends text via serial port
// Written by Ted Burke - last updated 6-12-2011
//
// Command line arguments are used to specify the text
// to send and the serial port to use. The baud rate used
// is currently always 38400 baud. The text cannot contain
// any spaces.
//
//		argv[1] = device name
//		argv[2] = text to send
//
// To compile with MinGW:
//
//		gcc -o SerialSend.exe SerialSend.c
//
// To compile with cl, the Microsoft compiler:
//
//		cl SerialSend.c
//
// To run (this example sends the characters "S365" via COM1):
//
//		SerialSend COM1 S356
//
// References:
//
//	Robertson Bayer, "Windows Serial Port Programming", March 30, 2008
//	(I used code from Bayer's serial port tutorial as my starting point)
//

#include <windows.h>
#include <stdio.h>

int main(int argc, char *argv[])
{
	// Declare variables and structures
	HANDLE hSerial;
	DCB dcbSerialParams = {0};
	COMMTIMEOUTS timeouts = {0};
	DWORD dwBytesWritten = 0;

	char dev_name[MAX_PATH];
	char text_to_send[MAX_PATH];

	// Parse command line arguments
	if (argc < 3)
	{
		fprintf(stderr, "Usage:\n\n\tSerialSend DEVICE_NAME TEXT_TO_SEND\n");
		return 1;
	}
	strcpy(dev_name, argv[1]);
	strcpy(text_to_send, argv[2]);

	// Open the specified serial port (first command line argument)
	fprintf(stderr, "Opening serial port %s...", dev_name);
	hSerial = CreateFile(argv[1], GENERIC_READ|GENERIC_WRITE, 0, 0,
							OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, 0);
	if (hSerial==INVALID_HANDLE_VALUE)
	{
		fprintf(stderr, "Error\n");
		return 1;
	}
	fprintf(stderr, "OK\n");

	// Set device parameters (38400 baud, 1 start bit,
	// 1 stop bit, no parity)
	dcbSerialParams.DCBlength = sizeof(dcbSerialParams);
	if (GetCommState(hSerial, &dcbSerialParams) == 0)
	{
		fprintf(stderr, "Error getting device state\n");
		CloseHandle(hSerial);
		return 1;
	}
	dcbSerialParams.BaudRate = CBR_38400;
	dcbSerialParams.ByteSize = 8;
	dcbSerialParams.StopBits = ONESTOPBIT;
	dcbSerialParams.Parity = NOPARITY;
	if(SetCommState(hSerial, &dcbSerialParams) == 0)
	{
		fprintf(stderr, "Error setting device parameters\n");
		CloseHandle(hSerial);
		return 1;
	}

	// Set COM port timeout settings
	timeouts.ReadIntervalTimeout = 50;
	timeouts.ReadTotalTimeoutConstant = 50;
	timeouts.ReadTotalTimeoutMultiplier = 10;
	timeouts.WriteTotalTimeoutConstant = 50;
	timeouts.WriteTotalTimeoutMultiplier = 10;
	if(SetCommTimeouts(hSerial, &timeouts) == 0)
	{
		fprintf(stderr, "Error setting timeouts\n");
		CloseHandle(hSerial);
		return 1;
	}

	// Send specified text (second command line argument;
	// cannot contain spaces)
	fprintf(stderr, "Sending text: %s\n", text_to_send);
	if(!WriteFile(hSerial, text_to_send, strlen(text_to_send),
					&dwBytesWritten, NULL))
	{
		fprintf(stderr, "Error writing text to %s\n", dev_name);
	}
	else
	{
		fprintf(stderr, "%d bytes written to %s\n",
				dwBytesWritten, dev_name);
	}

	// Close serial port
	fprintf(stderr, "Closing serial port...");
	if (CloseHandle(hSerial) == 0)
	{
		fprintf(stderr, "Error\n", dev_name);
		return 1;
	}
	fprintf(stderr, "OK\n");

	// exit normally
	return 0;
}

I’m planning to develop this program a little further to make it more flexible so that it can be used in a wider variety of applications. Features I’m thinking of adding include:

• Send longer strings, including ones that contain spaces.
• Select baud rate.
• Specify a time delay between transmitted bytes.
• List available serial ports.
• Default to first serial port.
• Default to highest numbered serial port.
• Send the contents of a file.