M
Marian___
Guest
I am trying to control RTS signal on my USB-UART device on Windows 10 and found that the usbser.sys driver does not generate any USB SET_CONTROL_LINE_STATE requests for EscapeCommFunction(hndl, SETRTS) or EscapeCommFunction(hndl, CLRRTS) calls as expected. As results, RST signal is being unchanged on UART device.
My example is based on Configuring a Communications Resource - Win32 apps as following:
<code>
#include <windows.h>
#include <tchar.h>
#include <stdio.h>
int _tmain(int argc, TCHAR* argv[])
{
DCB dcb;
HANDLE hCom;
BOOL fSuccess;
TCHAR *pcCommPort = (TCHAR*)TEXT("COM9");
// Open a handle to the specified com port.
hCom = CreateFile(pcCommPort,
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);
}
// Initialize the DCB structure.
SecureZeroMemory(&dcb, sizeof(DCB));
dcb.DCBlength = sizeof(DCB);
// Build on the current configuration by first retrieving all current
// settings.
fSuccess = GetCommState(hCom, &dcb);
if (!fSuccess)
{
// Handle the error.
printf("GetCommState failed with error %d.\n", GetLastError());
return (2);
}
// Fill in some DCB values and set the com state:
// 115,200 bps, 8 data bits, no parity, and 1 stop bit.
dcb.BaudRate = CBR_115200; // baud rate
dcb.ByteSize = 8; // data size, xmit and rcv
dcb.Parity = NOPARITY; // parity bit
dcb.StopBits = ONESTOPBIT; // stop bit
dcb.fDtrControl = DTR_CONTROL_DISABLE;
dcb.fRtsControl = RTS_CONTROL_HANDSHAKE;
fSuccess = SetCommState(hCom, &dcb);
if (!fSuccess)
{
// Handle the error.
printf("SetCommState failed with error %d.\n", GetLastError());
return (3);
}
// Set RTS
fSuccess = EscapeCommFunction(hCom, SETRTS); // SET_CONTROL_LINE_STATE request is expected here on the USB bus, but nothing happens.
if (!fSuccess)
{
// Handle the error.
printf("GetCommState failed with error %d.\n", GetLastError());
return (2);
}
_tprintf(TEXT("RTS is set\n"));
Sleep(1000); // Sleep for one second
// Clear RTS
fSuccess = EscapeCommFunction(hCom, CLRRTS); // SET_CONTROL_LINE_STATE request is expected here on the USB bus, but nothing happens.
if (!fSuccess)
{
// Handle the error.
printf("GetCommState failed with error %d.\n", GetLastError());
return (2);
}
_tprintf(TEXT("RTS is clear\n"));
Sleep(1000); // Sleep for one second
_tprintf(TEXT("Serial port %s successfully reconfigured.\n"), pcCommPort);
return (0);
}
</code>
Expected Result: After EscapeCommFunction(hndl, SETRTS) or EscapeCommFunction(hndl, CLRRTS) calls on the USB bus, SET_CONTROL_LINE_STATE request appears which reflects the RTS change.
Actual Result: After EscapeCommFunction(hndl, SETRTS) or EscapeCommFunction(hndl, CLRRTS) calls, no SET_CONTROL_LINE_STATE request appears on the USB bus at all. As results, RST signal is being unchanged on UART device. Note: that RST can be changed if enforce the updating DTR signal right after that.
A similar question was asked several years ago, but there was not answered:
Virtual serial port (USBSER.SYS) is not sending RTS/CTS via SET_CONTROL_LINE_STATE packets.
Could you please clarify whether this is an expected behavior of usbser.sys?
Thanks,
Marian
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