C++ Configure DCB before opening serial port

Wim Sturkenboom 0

In C# it's reasonably easy to create a serial port object, configure it and open the serial port. I'm looking for a way to do the same in C++. Opening the serial port with CreateFile asserts DTR and RTS which I would like to prevent. For this, I have the below code but it does not behave as expected. It retrieves the default COMMCONFIG, modifies it and sets it with the modified values. However the default does not change and opening the serial port still results in DTR being asserted. Tested with an SparkFun RedBoard with FT232 chip.

Advice appreciated.

// SerialConsoleCpp_DTR_RTS.cpp : This file contains the 'main' function. Program execution begins and ends there.

Wim Sturkenboom 0

Looks like the code is incomplete

Hmmm, sorry for the mess. Not sure what I'm doing wrong.

// SerialConsoleCpp_DTR_RTS.cpp : This file contains the 'main' function. Program execution begins and ends there.
//
// https://learn.microsoft.com/en-us/windows/win32/api/winbase/ns-winbase-dcb
// see https://stackoverflow.com/questions/61485127/including-windows-h-causes-unknown-attributeno-init-all-error
#define no_init_all deprecated  
#include <iostream> 
#include <Windows.h> 
#include <winbase.h>   
void printDCB(DCB &dcb) 
{ 	
printf("DCB\n"); 	
printf("%-20s = %u\n", "DCBlength", dcb.DCBlength); 	printf("%-20s = %d\n", "BaudRate", dcb.BaudRate); 	
printf("%-20s = %d\n", "fBinary", dcb.fBinary); 	
printf("%-20s = %d\n", "fParity", dcb.fParity); 	
printf("%-20s = %d\n", "fOutxCtsFlow", dcb.fOutxCtsFlow); 	printf("%-20s = %d\n", "fOutxDsrFlow", dcb.fOutxDsrFlow); 	printf("%-20s = %d\n", "fDtrControl", dcb.fDtrControl); 	printf("%-20s = %d\n", "fDsrSensitivity", dcb.fDsrSensitivity); 	printf("%-20s = %d\n", "fTXContinueOnXoff", dcb.fTXContinueOnXoff); 	printf("%-20s = %d\n", "fOutX", dcb.fOutX); 	
printf("%-20s = %d\n", "fInX", dcb.fInX); 	
printf("%-20s = %d\n", "fErrorChar", dcb.fErrorChar); 	printf("%-20s = %d\n", "fNull", dcb.fNull); 	
printf("%-20s = %d\n", "fRtsControl", dcb.fRtsControl); 	printf("%-20s = %d\n", "fAbortOnError", dcb.fAbortOnError); 	printf("%-20s = %d\n", "fDummy2", dcb.fDummy2); 	
printf("%-20s = %d\n", "wReserved", dcb.wReserved); 	printf("%-20s = %d\n", "XonLim", dcb.XonLim); 	
printf("%-20s = %d\n", "XoffLim", dcb.XoffLim); 	
printf("%-20s = %d\n", "ByteSize", dcb.ByteSize); 	
printf("%-20s = %d\n", "Parity", dcb.Parity); 	
printf("%-20s = %d\n", "StopBits", dcb.StopBits); 	
printf("%-20s = %02X\n", "XonChar", dcb.XonChar); 	
printf("%-20s = %02X\n", "XoffChar", dcb.XoffChar); 	printf("%-20s = %02X\n", "ErrorChar", dcb.ErrorChar); 	printf("%-20s = %02X\n", "EofChar", dcb.EofChar); 	
printf("%-20s = %02X\n", "EvtChar", dcb.EvtChar); 	
printf("%-20s = %d\n", "wReserved1", dcb.wReserved1); }  

void printCommConfig(COMMCONFIG &cfg) { 	
printf("COMMCONFIG\n"); 	
printf("%-20s = %d\n", "dwSize", cfg.dwSize); 	
printf("%-20s = %d\n", "wVersion", cfg.wVersion); 	
printf("%-20s = %d\n", "wReserved", cfg.wReserved); 	printf("%-20s = %d\n", "dwProviderSubType", cfg.dwProviderSubType); 	printf("%-20s = %d\n", "dwProviderOffset", cfg.dwProviderOffset); 	printf("%-20s = %d\n", "dwProviderSize", cfg.dwProviderSize); 	printf("%-20s = %02X (%d)\n", "wcProviderData[1]", cfg.wcProviderData, cfg.wcProviderData);  	
printDCB(cfg.dcb); 
}     
int main() 
{ 	
wchar_t portName[] = TEXT("COM7");
    	DCB dcb;
 	COMMCONFIG cfg;
 	DWORD size = sizeof(COMMCONFIG);
  	BOOL x = GetDefaultCommConfig(portName, &cfg, &size);
 	if (x == false)
 	{
 		printf("Error retrieving default config");
 		return (1);
 	}
 	else
 	{
 		printf("At start\n");
 		printCommConfig(cfg);
 	}
  	cfg.dcb.fDtrControl = DTR_CONTROL_DISABLE;
 	cfg.dcb.fRtsControl = RTS_CONTROL_DISABLE;
   	x = SetDefaultCommConfig(portName, &cfg, size);
 	if (x == false)
 	{
 		printf("Error setting default config");
 		return (1);
 	}
  	printf("\n\n");
 	x = GetDefaultCommConfig(portName, &cfg, &size);
 	if (x == false)
 	{
 		printf("Error retrieving modified config");
 		return (1);
 	}
 	else
 	{
 		printf("After reconfig\n");
 		printCommConfig(cfg);
 	}
   	printf("Opening port\n");
  	HANDLE hCom = CreateFile(portName, 		GENERIC_READ | GENERIC_WRITE, 		0,      //  must be opened with exclusive-access 		NULL,   //  default security attributes 		OPEN_EXISTING, //  must use OPEN_EXISTING 		0,      //  not overlapped I/O 		NULL); //  hTemplate must be NULL for comm devices
  	if (hCom == INVALID_HANDLE_VALUE)
 	{ 		//  Handle the error.
 		printf("CreateFile failed with error %d.\n", GetLastError());
 		return (1);
 	}
   	// get the dcb
 	bool fSuccess = GetCommState(hCom, &dcb);
  	if (!fSuccess)
 	{ 		//  Handle the error.
 		printf("GetCommState failed with error %d.\n", GetLastError());
 		return (2);
 	}
  	printf("\n\n");
 	printDCB(dcb);
   	printf("Sleeping\n");
 	Sleep(10000);
  	printf("Closing port\n");
 	CloseHandle(hCom);
 	return 0;
 }

David Lowndes 2,535 Reputation points MVP

2024-02-04T19:57:39.6+00:00

Searching around this seems to be a fairly common issue (possibly caused by some serial drivers not playing by the rules?).
If you immediately call Get/SetCommState after CreateFile to disable those pins, does that work fast enough to eliminate the problem that you're presumably encountering because of this behaviour?
Wim Sturkenboom 0 Reputation points

2024-02-05T07:14:55.9466667+00:00

@David Lowndes Thanks for the reply.

(possibly caused by some serial drivers not playing by the rules?)

The driver in question is Microsoft's usbser.sys.

If you immediately call Get/SetCommState after CreateFile to disable those pins, does that work fast enough to eliminate the problem

No; that is the standard approach in all examples that I have found. You probably did not run my example (possibly because you don't have microcontroller boards) so you could not see it. If I do a GetDefaultCommConfig() after a SetDefaultCommConfig(), the settings did not change.

There is however something fishy going on as it now works as expected but I can't reverse it :(
Viorel 116.7K Reputation points

2024-02-05T08:18:24.6533333+00:00

Did you try it in C#? Maybe it would be reasonable to reuse the C# classes in your C++ program.

Wim Sturkenboom 0

This is a very basic C# code that works as expected.

using System;

using System.IO.Ports;

namespace SerialPortConsole_Csharp_DTR_RTS
{
    class Program
    {
        static SerialPort sp = new SerialPort();

        static void Main(string[] args)
        {
            sp.BaudRate = 115200;
            sp.PortName = "COM3";
            sp.DtrEnable = false;
            sp.RtsEnable = false;
            //sp.DtrEnable = true;
            //sp.RtsEnable = true;

            Console.WriteLine("Opening port");
            sp.Open();
            Console.WriteLine("Sleeping");
            System.Threading.Thread.Sleep(10000);

            Console.WriteLine("Closing port");
            sp.Close();
            Console.ReadKey();

        }
    }
}

I did give the conversion to C++ a try (based on some sample code) but it did not compile (can't remember why); as I'm not a Windows C++ programmer I gave up, rather wanted to get the 'native' C++ working.

David Lowndes 2,535 Reputation points MVP

2024-02-06T14:09:15.1966667+00:00

Looking at the underlying code that SerialPort uses it only appears to use the underlying Win32 APIs. So, assuming the .NET code works, duplicating it by calling the Win32 APIs should also work.

1 answer

k s 0 Reputation points

2024-09-11T12:13:24.6333333+00:00
Dear friend, have you found the solution or workaround? I had the same issue with CH340 chip. I was planning to use RTS# line to reset my MCU. The problem was that after PC reboot or USB replug COM settings resets to defaults with RTS# & DTR# enabled. Then I have undesired pulse at first port opening with CreateFile() until DCB init with SetCommState(). With WCH VCP driver I have option DisableModemHandShake (see picture). With this option checked, the problem was solved, RTS# & DTR# lines not asserts, even after USB replug, DCB reading gives fRtsControl, fDtrControl = 0. However, I still can use these signals with EscapeCommFunction() method. FTDI VCP driver gives similar option (disable modem signals at startup), but unfortunately it doesn't work same way :( Did you solve the problem with FT232 chip?

HANDLE hComPort; char szComPortName[20]; // ... /* Try open */ hComPort = CreateFile(szComPortName, (GENERIC_READ | GENERIC_WRITE), 0, nullptr, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, nullptr); // RTS# & DTR# signals asserted now if (hComPort == INVALID_HANDLE_VALUE) { GetLastErrorMessage(); return false; } /* Set params */ DCB dcb {}; dcb.DCBlength = sizeof(DCB); GetCommState(hComPort, &dcb); dcb.BaudRate = (DWORD)baud; dcb.fBinary = 1; dcb.ByteSize = 8; dcb.Parity = NOPARITY; dcb.StopBits = ONESTOPBIT; dcb.fRtsControl = RTS_CONTROL_DISABLE; dcb.fDtrControl = DTR_CONTROL_DISABLE; if (!SetCommState(hComPort, &dcb)) { GetLastErrorMessage(); Close(); return false; } // RTS# & DTR# signals disabled after SetCommState() success.
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C++ Configure DCB before opening serial port

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