Hi,

After a succesful communication between
Siemen gsm with hyperterminal as Mr Jan Waclawek
suggested.

I tried to send an 'AT' command to the siemen gsm,
as has been ok with the hyperperterminal, but
when the microcontroller sent the command to the gsm
the gsm did not respon. The baud rate and other comm
settings has been set exactly as has been done with the
hyperterminal.

I have tried to code the AT89c51 to send and receive
a char to emulate the hyperterminal,
eg. first send char A
receive char
send char T
receive char
send CR
receive char
send LF
receive char

receive char ---> to expect the O
receive char ---> to expect the K

When connected to the gsm,
uC did send 'A' but the the gsm did not reply any char
but with hyperterminal the gsm replied with 'A'.

I have monitored the communication with the hyperterminal
to make sure the communication is done, by tabbing
(one to pc and one to the gsm)
the pin Txd from uC to PC. At the next time from the
gsm txd pin to the hyperterminal.

All connection from the gsm to pc is at the 3 volt level,
a 3 volt zener is connected to the Txd of the uC to suppress
the 5v level from the uC Txd pin to 3 volt to the gsm.

May be there is a trick to do that,
any suggestion please.

Thank you in advance.
Kurnia Brahmana