В исходниках терминалки есть файлик serail.c — в нем и есть код работы с ком портом, только он уж очень корявый.
Исходники блокнота даже не смотрел — судя по всему это и не надо.
Вот код программы (проверить не могу — нет термопечи и вообще нет ком порта)

#include <windows.h>
#define MESS_SERIAL (WM_USER+1)

LRESULT CALLBACK WndProc(HWND hWnd, UINT uMsg, WPARAM wParam, LPARAM lParam);
BOOL OpenPort(int port,int baud,int HwFc, HWND handle);

int APIENTRY WinMain(HINSTANCE hInst, HINSTANCE hPInst, LPSTR lpCmd, int nCmd)
{
    WNDCLASSW             wc;
    HWND                 hWnd;
    MSG                     msg;
    wchar_t                *szWndCls = L"STCLS";
    memset(&wc, 0, sizeof(wc));
    wc.style            = CS_HREDRAW | CS_VREDRAW;
    wc.lpfnWndProc        = (WNDPROC)WndProc;
    wc.hInstance        = hInst;
    wc.hCursor            = LoadCursorW(0, IDC_ARROW);
    wc.lpszClassName    = szWndCls;
    if (!RegisterClassW(&wc))
        return 1;
    hWnd = CreateWindowW(szWndCls, L"Simple Terminal", WS_OVERLAPPEDWINDOW, CW_USEDEFAULT, 0, CW_USEDEFAULT, 0, 0, 0, hInst, 0);
    ShowWindow(hWnd, nCmd);
    UpdateWindow(hWnd);
    while (GetMessage(&msg, 0, 0, 0)) {
        TranslateMessage(&msg);
        DispatchMessage(&msg);
    }
    return 0;
}

LRESULT CALLBACK WndProc(HWND hWnd, UINT uMsg, WPARAM wParam, LPARAM lParam)
{
    static HWND                 hEdit;
    switch (uMsg) {
        case WM_CREATE: {
            CREATESTRUCT    *CS;
            CS = (CREATESTRUCT *)lParam;
            hEdit = CreateWindowExW(WS_EX_CLIENTEDGE, L"EDIT", 0, WS_CHILD | WS_VISIBLE | WS_VSCROLL | ES_AUTOVSCROLL | ES_MULTILINE | ES_READONLY, 0, 0, 0, 0, hWnd, 0, CS->hInstance, 0);
            SendMessage(hEdit, WM_SETFONT, (WPARAM)CreateFontA(14, 0, 0, 0, FW_NORMAL, 0, 0, 0, DEFAULT_CHARSET, OUT_DEFAULT_PRECIS, CLIP_DEFAULT_PRECIS, CLEARTYPE_QUALITY, DEFAULT_PITCH, "dejavu sans mono"), 0);
            OpenPort(1, 9600, 0, hWnd);
            return 0;
        }
        case WM_SIZE: {
            SetWindowPos(hEdit, 0, 0, 0, LOWORD(lParam), HIWORD(lParam), SWP_NOZORDER);
            return DefWindowProc(hWnd, uMsg, wParam, lParam);
        }
        case WM_CLOSE: {
            PostQuitMessage(0);
            return 0;
        }
        case MESS_SERIAL: {
            int         iLength;
            char    *szTmp;

            szTmp = (char *)lParam;
            szTmp[wParam] = 0;
            iLength = GetWindowTextLengthW(hEdit);
            SendMessage(hEdit, EM_SETSEL, iLength, iLength);
            SendMessageA(hEdit, EM_REPLACESEL, 0, (LPARAM)szTmp);
            return 0;
        }
    }
    return DefWindowProc(hWnd, uMsg, wParam, lParam);
}


/* From serial.c */
#include <stdio.h>

// Functions:
HANDLE StartCommThread(void);
DWORD WINAPI ThreadProc(void *p);

// Variables:
HANDLE SerialPort=NULL;
HANDLE Thread;
HWND handle=NULL;
int StopThread=0;
int FlowControl=0;

void ShowLastError(void)
{
    LPVOID lpMsgBuf;
    FormatMessage(FORMAT_MESSAGE_ALLOCATE_BUFFER | FORMAT_MESSAGE_FROM_SYSTEM, NULL, GetLastError(), MAKELANGID(LANG_NEUTRAL, SUBLANG_DEFAULT), (LPTSTR) &lpMsgBuf, 0, NULL);
    MessageBox(NULL, (LPTSTR)lpMsgBuf, TEXT("Error"), MB_OK|MB_ICONINFORMATION );
    LocalFree( lpMsgBuf );
}
BOOL OpenPort(int port,int baud,int HwFc, HWND hwnd)
{
  HANDLE Comport;
  DCB myDCB;
  COMMTIMEOUTS CTout;
  char str[100];
  
  FlowControl = HwFc;
  
  if (port > 9)
    wsprintfA(str,"\\\\.\\COM%d",port);
  else
    wsprintfA(str,"COM%d",port);
  Comport = CreateFileA(str,GENERIC_READ|GENERIC_WRITE,0,NULL,OPEN_EXISTING,0,NULL);
  if (Comport == INVALID_HANDLE_VALUE)
    return FALSE;
  if (!SetupComm(Comport,350,20))
    return FALSE;
  if (!GetCommState(Comport,&myDCB))
    return FALSE;
  myDCB.fInX = FALSE;
  myDCB.fOutX = FALSE;
  myDCB.fOutxDsrFlow = FALSE;
  if (HwFc)
    {
      myDCB.fOutxCtsFlow = TRUE;
      myDCB.fRtsControl = RTS_CONTROL_HANDSHAKE;
    }
  else
    {
      myDCB.fOutxCtsFlow = FALSE;
    }
  myDCB.BaudRate = baud;
  myDCB.DCBlength = sizeof(DCB);
  myDCB.fBinary = 1;
  myDCB.fParity = 0;
  myDCB.fDtrControl = DTR_CONTROL_DISABLE;
  myDCB.fDsrSensitivity = 0;
  myDCB.fTXContinueOnXoff = 1;
  myDCB.fNull = 0;
  myDCB.fRtsControl = RTS_CONTROL_DISABLE;
  myDCB.fDummy2 = 0;
  myDCB.wReserved = 0;
  myDCB.Parity = NOPARITY;
  myDCB.StopBits = ONESTOPBIT;
  myDCB.wReserved1 = 0;
  myDCB.ByteSize = 8;

  if (!SetCommState(Comport,&myDCB)) {
      ShowLastError();
      return FALSE;
  }

  CTout.ReadIntervalTimeout = 0xffffffff;
  CTout.ReadTotalTimeoutMultiplier = 0;
  CTout.ReadTotalTimeoutConstant = 0;
  CTout.WriteTotalTimeoutMultiplier = 0;
  CTout.WriteTotalTimeoutConstant = 5000;
  SetCommTimeouts(Comport,&CTout);
  EscapeCommFunction(Comport,SETDTR);
  PurgeComm(Comport,PURGE_TXCLEAR | PURGE_RXCLEAR);
  handle = hwnd;
  SerialPort = Comport;
  StartCommThread();
  return TRUE;
}
void CloseSerialPort(void)
{
  if (!SerialPort) return;
  if (Thread)
    {
      StopThread = TRUE;
      WaitForSingleObject(Thread,2000);
      CloseHandle(Thread);
      Thread = NULL;
      StopThread = FALSE;
    }
  PurgeComm(SerialPort,PURGE_TXCLEAR | PURGE_RXCLEAR);
  CloseHandle(SerialPort);
  SerialPort = NULL;
}
void PutSerialChar(int c)
{
  int Cnt;
  DWORD ModemStat;
  DWORD ticks;
  int Cts=1;
  ticks = GetTickCount();
  if (FlowControl)
    {
      while (Cts)
        {
          if (!GetCommModemStatus(SerialPort, &ModemStat))
            {
              ShowLastError();
              return;
            }
          Cts = !(ModemStat & MS_CTS_ON);
          if (GetTickCount() > ticks + 1000)
            break;
        }
    }
  WriteFile(SerialPort,&c,1,(LPDWORD)&Cnt,NULL);
}
HANDLE StartCommThread(void)
{
  int ThreadID;
  StopThread = FALSE;
  Thread = CreateThread(NULL,4096,ThreadProc,SerialPort,0,(LPDWORD)&ThreadID);
  return Thread;
}
DWORD WINAPI ThreadProc(void *p)
{
  int Cnt;
  char buf[256];
  if (!handle)
    return 0;
  for(;;)
    {
      if (ReadFile(SerialPort,&buf,255,(LPDWORD)&Cnt,NULL) && Cnt)
        {
          SendMessage(handle,MESS_SERIAL,(unsigned int)Cnt,(unsigned long)buf);
        }
      else
        Sleep(50);
      
      if (StopThread)
        break;
    }
  return 0;
}
int SerialPortIsOpen(void)
{
  if (SerialPort)
    return TRUE;
  return FALSE;
}
BOOL SerialIsChar(void)
{
  int Cnt,Avail,Remain;
  int buf;
  PeekNamedPipe(SerialPort, (LPVOID)&buf, (DWORD) 1, (LPDWORD)&Cnt,(LPDWORD)&Avail, (LPDWORD)&Remain);
  if (Avail)
    return TRUE;
  return FALSE;
}
int SerialGetChar(void)
{
  char ch;
  int Cnt;
  if (!SerialPort)
    return EOF;
  ReadFile(SerialPort,&ch,1,(LPDWORD)&Cnt,NULL);
  if (!Cnt)
    return EOF;
  return (int) ch;
}