procedure TMainForm.WMDEVICECHANGE(var Msg: TWMDeviceChange);
var
 lpdb  : PDevBroadcastHdr;
 lpdbpr: PDevBroadCastPort;
 S: AnsiString;
begin
{Заголовок сообщения}
lpdb := PDevBroadcastHdr(Msg.dwData);
case Msg.Event of
  DBT_DEVICEARRIVAL:
  begin {Добавление}
    if lpdb^.dbch_devicetype = DBT_DEVTYP_PORT {DBT_DEVTYP_DEVICEINTERFACE} then begin
      lpdbpr:= PDevBroadCastPort(Msg.dwData);
      S := StrPas(PWideChar(@lpdbpr.dbcp_name));

      GetSystemController.Init(S);
    end;
  end;
  DBT_DEVICEREMOVECOMPLETE:
  begin {Удаление}
    if lpdb^.dbch_devicetype = DBT_DEVTYP_PORT then begin
      lpdbpr:= PDevBroadCastPort(Msg.dwData);
      S := StrPas(PWideChar(@lpdbpr.dbcp_name));
      GetSystemController.ProcessDisconnect(S);
    end;
  end;
end;
end;

Msg.Event

DBT_DEVNODES_CHANGED

DBT_DEVNODES_CHANGED

RegisterDeviceNotification

There is no additional information about which device has been added to or removed from the system. Applications that require more information should register for device notification using the RegisterDeviceNotification function.

RegisterDeviceNotification

The DBT_DEVICEARRIVAL and DBT_DEVICEREMOVECOMPLETE events are automatically broadcast to all top-level windows for port devices. Therefore, it is not necessary to call RegisterDeviceNotification for ports, and the function fails if the dbch_devicetype member is DBT_DEVTYP_PORT.