Hi,
I am interfacing GSM module SIM900 with my MSP430FG4618 Dev. Board. My observations are as follows:
1. When I connect the MSP with the PCs HyperTerminal it transmits and receives everything neatly.
2. When I connect my GSM module with PCs HyperTerminal it transmits and receives everything neatly.
3. When I connect MSP with GSM module the module receives nothing (no data).
So I took a DSO and checked the TXD pin of the MSP and here are the observations:
a. When connected to MSP to PC: the voltage varies between -6V to +6V on the TXD and +7V to -7V on RXD which enables it to communicate perfectly.
b. When connected to GSM module to PC: the voltage varies between -7.5V to +7.5V on the TXD and +9V to -9V on RXD which enables it to communicate perfectly.
c. but when I interconnect the GSM module and MSP together the voltage on TXD pin of the MSP drops between +4V and +6V which doesn't allow the GSM module to recognize the characters but at the same time the bit pattern remains the same just it has a attenuation and a DC shift. When I checked the second time the voltage at the TXD was between -1V to -5V.
NOTE: I have checked the connections more than a dozen times.
Please help.
Thank you.