Tera Term

I am using Teraterm extensively to automate some interactions with the serial port. Teraterm effectively waits for a specific string then exits via a TTL macro.

I'm seeing some instability where sometimes the serial port has already sent the string I'm looking for and the macro just times out. However, if I send an enter key to the terminal window; the string is received and the macro completes successfully.

For reasons that I won't go into; I don't want to just automatically send the enter key from the macro as it can lead to other issues.

I have teraterm being launched from a python script which then looks at the log file when teraterm exits to determine where the macro was successful. What I want to do now is when I detect this rare 2% condition; I want to be able to send an enter key to the teraterm window for the "workaround" to trigger. I thought this would be easy as I've done it in the past scripts. However, I must be doing something wrong but can't figure out what it might be.

I've proven that if I manually interact with the Teraterm window and press a key; the intended action occurs.
However, if I use the example script below... I never see either the Enter key or the A key (used as a sanity check).

#
import pprint, time
import win32gui, win32con, win32api, win32process
from win32gui import SendMessage, GetWindowText, PostMessage, GetWindow

def get_hwnds_for_pid (pid):
print("in get_hwnds_for_pid(%d)" % pid)
def callback (hwnd, hwnds):
if win32gui.IsWindowVisible (hwnd) and win32gui.IsWindowEnabled (hwnd):
_, found_pid = win32process.GetWindowThreadProcessId (hwnd)
if found_pid == pid:
hwnds.append (hwnd)
return True

hwnds = []
win32gui.EnumWindows (callback, hwnds)
print(" returns: ", end='')
pprint.pprint(hwnds)
return hwnds

pid=12728
hwnd = get_hwnds_for_pid( pid )
for ahwnd in hwnd:
fval = win32api.SendMessage(ahwnd, win32con.WM_KEYDOWN, win32con.VK_RETURN, 0)
time.sleep(0.1)
fval2 = win32api.SendMessage(ahwnd, win32con.WM_KEYUP, win32con.VK_RETURN, 0)
print('Sent VK_RETURN(%x) to %x calculated from %x: %x %x' % (win32con.VK_RETURN, ahwnd, pid, fval, fval2))
win32api.PostMessage(ahwnd, win32con.WM_CHAR, 'A', 0)
print('Sent %x,A to %x calculated from %x: %x %x' % (win32con.WM_CHAR, ahwnd, pid, fval, fval2))



What am I missing? Why isn't the SendMessage Windows API successful in sending the keystroke to TeraTerm?