Firtsly, I'd like to say thank you for
all the participant's respons on my question
on the forum,

Please help me for this connection problem.

Gsm Txd is connected to the Rxd Hyperterminal, and
AT89c51 Txd is connected to the GSM Rxd.
When uC send AT, the GSM respon OK to the Hyperterminal.

But when the GSM Txd is connected to the Rxd of the uC
the uC it seems did not receive any char sent by the GSM.

my C51 on uC:
unsigned char idata respon[4]={0};
unsinged char ind;

sendAT();
for (ind=0x00; ind<0x03; ind++)
{
respon[ind]=readcharfromgsm(); // cr,lf,'O','K',lf,cr from gsm
}

for (ind=0x00; ind<0x03; ind++)
{
P0=respon[ind]; // to show what char has been received in P0
delay_500ms();
}


Why do the hyperterminal can receive, but the uC not?
What should be added?.

Again, thank you very much.
Kurnia Brahmana.