Question : Problem: Modem answers ERROR after an ATZ request

I am creating an application in C language and I faced with a problem of modem. When I need to init the modem (with "ATZ\r" string) while the application is running, I get an ERROR answer from the modem if it answered BUSY the last time. But, when I launch the application and send the "ATZ\r" the first time, all is well.
I'm a beginner with communications with modem, so the error may be quite simple.

Thank you.

Answer : Problem: Modem answers ERROR after an ATZ request

if the modem is not connected , You were in command mode.
Send port 'ATZ' + CR code(#13 - Enter char).Certainly you must get ok.

if you dont get ok, try this...
You can loop from com1 to com2 for check ports. send a char com1, listen a char com2.
if you get right chars, ports are ok.
(Please check hex char)