while <условие> do begin
  Read_USB_Device_Buffer(1);
  Signal := FT_In_Buffer[0] and $10 <> 0; // поступил сигнал
    if Signal then begin
     работа
    .............
    end;
end

00010000
00010000
00000000
00010000
00010000
00010000
00010000

while <условие> do begin
  Read_USB_Device_Buffer(1);
  Signal := FT_In_Buffer[0] and $10 <> 0; // поступил сигнал
    if Signal then begin
     работа
    .............
    end;
end

CL>>while <условие> do begin
CL>>  Read_USB_Device_Buffer(1);
CL>>  Signal := FT_In_Buffer[0] and $10 <> 0; // поступил сигнал
CL>>    if Signal then begin
CL>>     работа
CL>>    .............
CL>>    end;
CL>>end
CL>>

FT_STATUS FT_SetEventNotification (FT_HANDLE ftHandle, DWORD dwEventMask, PVOID pvArg)
Sets conditions for event notification.

Parameters
ftHandle - Handle of the device.
dwEventMask - Conditions that cause the event to be set.
pvArg - Interpreted as the handle of an event.

hEvent := CreateEvent(0,false,false,'');
FT_SetEventNotification(h,FT_EVENT_RXCHAR,@hEvent);
WaitForSingleObject(hEvent,Infinite);
FT_GetStatus(h,@RxBytes,@TxBytes,@EventDWord);
if RxBytes > 0 then begin
  FT_Read(h,@Buf,1,@Count);
  WriteLn('buf = ',IntToBin(buf));
end;

type
  TUSBThread = class(TThread)
  private
    FOnSignal: TNotifyEvent;
  protected
    constructor Create(CreateSuspended: Boolean);
    procedure Execute; override;
    destructor Destroy; override;
  published
    property OnSignal: TNotifyEvent read FOnSignal write FOnSignal;
  end;

implementation

uses D2XXUnit;

constructor TUSBThread.Create(CreateSuspended: Boolean);
begin
  inherited Create(CreateSuspended);
  Open_USB_Device;
  Set_USB_Device_LatencyTimer(2);
  Set_USB_Device_BitMode($e3,1);
end;

destructor TUSBThread.Destroy;
begin
  Set_USB_Device_BitMode($e3,0);
  Close_USB_Device;
  inherited;
end;

procedure TUSBThread.Execute;
begin
  while not Terminated do begin
    Read_USB_Device_Buffer(1);
    if FT_In_Buffer[0] and $10 <> 0 then
      if Assigned(OnSignal) then OnSignal(Self);
  end;
end;

procedure TForm1.FormCreate(Sender: TObject);
begin
  UsbThread := TUsbThread.Create(True);
  UsbThread.OnSignal := Signal;
  UsbThread.Resume;
end;

procedure TForm1.FormDestroy(Sender: TObject);
begin
  UsbThread.Free;
end;

procedure TForm1.Signal(Sender: TObject);
var
  i: Integer;
begin
//  реакция на сигнал
end;

CL>>>>hEvent := CreateEvent(0,false,false,'');
CL>>>>FT_SetEventNotification(h,FT_EVENT_RXCHAR,@hEvent);
CL>>>>WaitForSingleObject(hEvent,Infinite);
CL>>>>FT_GetStatus(h,@RxBytes,@TxBytes,@EventDWord);
CL>>>>if RxBytes > 0 then begin
CL>>>>  FT_Read(h,@Buf,1,@Count);
CL>>>>  WriteLn('buf = ',IntToBin(buf));
CL>>>>end;
CL>>>>

FT_STATUS FT_SetEventNotification (FT_HANDLE ftHandle, DWORD dwEventMask, PVOID pvArg)
Sets conditions for event notification.

Parameters
ftHandle — Handle of the device.
dwEventMask — Conditions that cause the event to be set.
pvArg — Interpreted as the handle of an event.

Return Value
FT_OK if successful, otherwise the return value is an FT error code.

Remarks
An application can use this function to setup conditions which allow a thread to block until one of
the conditions is met. Typically, an application will create an event, call this function, then block on
the event. When the conditions are met, the event is set, and the application thread unblocked.
dwEventMask is a bit-map that describes the events the application is interested in. pvArg is
interpreted as the handle of an event which has been created by the application. If one of the
event conditions is met, the event is set.
If FT_EVENT_RXCHAR is set in dwEventMask, the event will be set when a character has been
received by the device. If FT_EVENT_MODEM_STATUS is set in dwEventMask, the event will be
set when a change in the modem signals has been detected by the device.

This example shows how to wait for a character to be received or a change in modem status.
First, create the event and call FT_SetEventNotification.

FT_HANDLE ftHandle; // handle of an open device
FT_STATUS ftStatus;
HANDLE hEvent;
DWORD EventMask;
hEvent = CreateEvent(NULL,false,false,"");
EventMask = FT_EVENT_RXCHAR | FT_EVENT_MODEM_STATUS;
ftStatus = FT_SetEventNotification(ftHandle,EventMask,hEvent);

Sometime later, block the application thread by waiting on the event, then when the event has
occurred, determine the condition that caused the event, and process it accordingly.

WaitForSingleObject(hEvent,INFINITE);

DWORD EventDWord;
DWORD RxBytes;
DWORD TxBytes;

FT_GetStatus(ftHandle,&RxBytes,&TxBytes,&EventDWord);
if (EventDWord & FT_EVENT_MODEM_STATUS) {
  // modem status event detected, so get current modem status
  FT_GetModemStatus(ftHandle,&Status);
  if (Status & 0x00000010) {
    // CTS is high
  }
  else {
    // CTS is low
  }
  if (Status & 0x00000020) {
    // DSR is high
  }
  else {
    // DSR is low
  }
}
if (RxBytes > 0) {
// call FT_Read() to get received data from device
}