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and i use this one before usb-ttl converter to connect with the module but it's not working now
i can't tell if the module or the usb-ttl is broken.
that's why i will use max232 to check which one of my GSM(sim900 or t290i) is working
Floating or unconnected CMOS logic pins can cause oscillations. Those oscillations can cause the chip to overheat. Because leakage currents are involved and ground and +V are low impedance it causes a large current to flow. Think in terms of the limit as R approaches zero of I=V/R. You need a resistor to absorb the voltage generated from a tiny current. So look at I= 0.001 V/0.01 Ohms. It's a big number. i= 0.001 V/10,000 is a small number.
Can you post the link to your USB to TTL converter? Is it 5V or 3.3 V? I need to some more research here. But do check the maximum output of your USB to serial converter at the serial pins at idle.
The power supply HAS to be a battery from 3.4 to 4.5 V and it expects 2A bursts from the battery.
I actually read the datasheet as it does not support 3.3 V for the maximum input to the serial inputs. TTL inputs are usually INVERTING but that usually comes out in the wash.
A 3.3 to 3V LEVEL SHIFTER MAY be required. Not sure at this point.
With RS232, there is a DTE and a DCE and it has always been a "reall mess" to interface.
There were two signals called DTR and DSR which basically said the "terminal" had power and the "modem" had power respectively.
RI or Ring indicator was used to indicate the the phone was ringing - In this product it has like 3 uses.
RTS/CTS = Request to send and Clear to Send were partly used to Answer the phone and to tell the modem that it's ready to accept a call.
So it's something like: The phone rings (RI); There is RTS or (Answer the phone); then there is a wait for DCD (Data carrier detect) and then there is a CTS or Clear to send.
Hardware handshaking was at one time a real nightmare. RTS/CTS is preferred, but some used DTR and DCR and some got it backwards.
There was also PG or protective ground.
As you can generally see, Rx/Tx and SG (Signal ground) are the minimum signals required.
The voltage levels for true RS232 was from -3 to 25V and +3 to 25V. Even that got messed up. +-12 became common and then +-5 became common and now TTL inverted has become common.
Software handshaking comes in two major flavors:
^S/^Q for stop/resume from the old paper tape readers
and ^S/resume on any character
I did not in this little blurb indicate directions To/From DTE/DCE but they are essential to the understanding.
"Null Modems" removed the requirement that the devices had to be connected to a modem.
And, it's entirely POSSIBLE that if 3.3V was used, the modem is toast, but as I said, RS232 or TTL RS232 is tough to get to work.
Baud has to be right. Handshaking has to be right etc. Parity, data, stop etc all have to be right.
You can disable handshaking by playing with CTS and RTS and DSR and DTR. Looping CTS to RTS usually works. I have to look at the datasheet again to figure out if DTR or DSR is used and if it's an input or an output. Since this is a modem, it should be DSR and it would be an output.
So, connecting CTS and RTS looped at the modem and Rx connected to Tx and Tx connected to Rx and handshaking disabled with Hyperterminal, you should be able to find the baud rate by trial and error. Don't forget SG or signal ground.
Then you have to get handshaking to work for real data transmission to occur reliably.
and i use this one before usb-ttl converter to connect with the module but it's not working now
i can't tell if the module or the usb-ttl is broken.
that's why i will use max232 to check which one of my GSM(sim900 or t290i) is working
There was a link to the manual for the USB to TTL converter, but I could not open it.
I do think this device wants < 3.0 V on the serial pins. If Rxd had internal pull-ups AND the USB-TTL converter had Open Collector outputs you would be OK, but then there is CTS/RTS and the others.
I do believe that most USB to TTL is 3.3 or 5V based and mostly 3.3.
The inputs COULD be TOLERENT of 3.3 with a series resistor.
I made my originals assumptions based on the wrong manual, so first thing is having the correct manual. Secondly if there is a RS-232 port available then I would interface directly to that to remove one more dongle that can add to the mix.
And to the OP, the MAX232 is a 5v part, based on what your manual says, you may need a low voltage equivalent such as the one I mentioned earlier.
RS-232 has always led to confusion as what is the DCE and DTE, null modem or straight through. Fortunately the EIA standard defines the pinouts on the PC RS port, so all we need are the pinouts of the module that you are using.
For simplicity you should test with minimal devices such as...
[DCE 900D}-------{MAX3222}----------{DTE PC RS-232]
Once you get this simple arrangement working, then you can move to the USB converter, that is how I would do it.
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