I have a few question that need clarification and help here. Please take note that i am trying to send an AT Command to a conventional external 56k modem to instruct it to dial a telephone number :

1. How does a Pin 3.1 which is also called as TxD differs from other port if i want to use it to transfer a series of bytes data out from the serial port to let's say a modem?

2. If i am to use ASM to send out a byte of data out from the TxD (P3.1) to a modem.For e.g : i want to send the character "A" to a modem, does the command needed to be sent this looks like this?

MOV SBUF, #A

If yes, how does the C Code counterpart of this line looks like? I can't think of it no matter how hard i tried, i already even googled for few hours but it's no avail.

3. If i were able to send out a byte of character successfully to the modem, let's advance the problem to next stage. This time i need to send a string of command to the modem to instruct it to dial a number instead of a single character. For e.g : "ATD1234567" where 1234567 is the telephone number. Can i use the similiar line:

MOV SBUF, #ATD1234567 ?

Again, if yes, how do it implement this using C Code? Of course, a sample of code would be much appreciated, but even slight hints and tips would be greatly appreciated.

4. What would be the appropriate baud value suitable to be configured (TH1) to be used with a modem?


P/S - The code shown are based purely on the dialing command only, other command to configure/resetting modem is assumed done beforehand.

That is all i need to know for now to kick off with my coding. Please help! Thank you very much!