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I am using p89c51Rd2 chip serial port is connected to modem port after dialling connection is established but ,didn't receive "CONNECTED 9600" string to progarmm is problem with speed of serial port? I have treid using getchar and GetByte routine
"I am Using 3 lines for modem TXD,RXD and DTR ... i need to toggle DTR line But for some reason it's not working" Of course it's not - you just said you don't have a DTR line!!! "can u tell me how to hang modem when only 3 lines are used?" You need to escape to Command mode - see your modem's Manual
The standard method of getting back to command mode is: 1 second pause then three "+" characters rapidly then another 1 second pause. Toggling DTR is a much quicker way to get there, but you have to send a specific command to the modem to enable this behaviour, see the manual.
"Toggling DTR is a much quicker way" and a lot easier - none of that critical timing! "but you have to send a specific command to the modem to enable this behaviour" The "Guard Time" (1 second before & after, in your example) and the "Escape Character" ('+' in your example) will also all be programmable via S-Registers; possibly also the time between "Escape Characters" "see the manual" what else?!