IOCTL_SERIAL_IMMEDIATE_CHAR control code
The IOCTL_SERIAL_IMMEDIATE_CHAR request causes a specified character to be transmitted as soon as possible. The immediate character request completes immediately after any other write that might be in progress. Only one immediate character request can be pending at a time.
The AssociatedIrp.SystemBuffer member points to the UCHAR value to transmit immediately.
The Parameters.DeviceIoControl.InputBufferLength member is set to the size, in bytes, of a UCHAR.
If the request is successful, the Information member is set to the size, in bytes, of a UCHAR. Otherwise, Information is set to zero.
The Status member is set to one of the Generic Status Values for Serial Device Control Requests. A status of STATUS_INVALID_PARAMETER indicates that a previous immediate character request is pending.